# Is the RSGC4 (Alicante 8) cluster a real star cluster?: peculiar radial
velocities of red supergiant stars

Sang-Hyun Chun Korea Astronomy and Space Science Institute, 776 Daedeokdae-ro,
Yuseong-gu, Daejeon 34055, Republic of Korea  GyuChul Myeong Institute of
Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
Jae-Joon Lee Korea Astronomy and Space Science Institute, 776 Daedeokdae-ro,
Yuseong-gu, Daejeon 34055, Republic of Korea  Heeyoung Oh Korea Astronomy and
Space Science Institute, 776 Daedeokdae-ro, Yuseong-gu, Daejeon 34055,
Republic of Korea

###### Abstract

Young massive star clusters, like the six red supergiant clusters in the
Scutum complex, provide valuable insights into star-formation and galaxy
structures. We investigated the high-resolution near-infrared spectra of 60
RSG candidates in these clusters using the Immersion Grating Infrared
Spectrograph. Among the candidates in RSGC4, we found significant scattering
in radial velocity ($-64$ km/s to $115$ km/s), unlike other clusters with
velocities of $\sim$100 km/s. Most candidates in RSGC4 have $Q_{GK_{s}}$
values larger than 1.7, suggesting that they could be early AGB stars. Four
candidates in RSGC4 exhibit infrared excess and distinct absorption features
absent in other candidates. Two of these stars exhibit absorption lines
resembling those of D-type symbiotic stars, showing radial velocity changes in
multi-epoch observations. Analysis of relative proper motions revealed no
runaway/walkaway stars in RSGC4. The dynamic properties of RSGC4 and RSGC1
differ from the disk-like motions of other clusters: RSGC4 has low normalized
horizontal action $J_{\mathrm{hor}}=J_{\mathrm{\phi}}/J_{\mathrm{tot}}$ and
vertical action
$J_{\mathrm{ver}}=(J_{\mathrm{z}}-J_{\mathrm{R}})/J_{\mathrm{tot}}$ values and
high eccentricities, while RSGC1 has vertical motions with high
$J_{\mathrm{ver}}$ values and inclinations. We propose that RSGC4 may not be a
genuine star cluster but rather a composite of RSGs and AGBs distributed along
the line of sight at similar distances, possibly originating from various
environments. Our results suggest a complex and hierarchical secular evolution
of star clusters in the Scutum complex, emphasizing the importance of
considering factors beyond density crowding when identifying star clusters in
the bulge regions.

Star clusters (1567), Young massive clusters (2049), Red supergiant stars
(1375), Near infrared astronomy (1093), High resolution spectroscopy (2096)

##  1 Introduction

Young massive star clusters (YMCs) containing high-mass stars are ideal
laboratories for investigating the various astrophysical aspects of galaxies.
YMCs are intrinsically bright and are considered the basic building blocks of
galaxies. Most stars form within star clusters (Lada & Lada, 2003), and YMCs,
in particular, contain substantial populations of massive stars (Zinnecker &
Yorke, 2007). YMCs also lose members to galaxy field through dissolution
processes, such as dynamical evolution (e.g. Poveda et al., 1967; Gies &
Bolton, 1986; Fujii & Portegies Zwart, 2011; Oh et al., 2015) or tidal
stripping (e.g. Tutukov, 1978; Kroupa et al., 2001; Baumgardt et al., 2008).
Therefore, YMCs are powerful tracers of star formation, evolutionary history,
and parent galaxy structures.

YMCs also play a distinct role as probes for studying stellar evolution and
dynamics in star clusters. YMCs contain large populations of massive, coeval
stars (Zinnecker & Yorke, 2007) with similar chemical compositions, from which
we can trace recent massive star formations and study their stellar evolution.

Many massive stars are found in multiple systems and undergo interactions with
companion stars throughout their lifetimes (Sana et al., 2012; Kobulnicky et
al., 2014; Dunstall et al., 2015; Moe & Di Stefano, 2017). Entire stellar
evolutionary paths can be changed by binary interactions. Processes such as
binary supernova ejection (Blaauw, 1961; Renzo et al., 2019) or dynamical
encounters (Poveda et al., 1967; Banerjee et al., 2012; Perets & Šubr, 2012;
Oh & Kroupa, 2016) in multiple systems can transfer stars from their parent
clusters to galaxy fields. Thus, the binary systems in clusters significantly
influence the dynamical evolution of the clusters. A deeper understanding of
the properties of binary systems in YMCs is crucial for comprehending their
binary effects on massive stellar evolution and the dynamic evolution of
clusters.

Despite their significant contributions to star formation, galaxy evolution,
and cluster dynamics, our understanding of the properties of YMCs remains
incomplete. This is because YMCs with an $M_{\mathrm{cluster}}>10^{4}$
$M_{\sun}$ are very scarce in the Milky Way, and massive stars in the clusters
are also rare due to their fast evolution. Fortunately, recent near-infrared
all-sky surveys, such as 2MASS and WISE, have proven invaluable for
discovering many obscured star clusters in the Milky Way (Ivanov et al., 2002;
Camargo et al., 2016; Ryu & Lee, 2018).

The discovery of six red supergiant clusters (RSGCs) at the intersection of
the Scutum-Crux arm and the Galactic bar tip (hereafter, referred to as the
Scutum complex region) in the Milky Way ($l\sim 24$–29$\arcdeg$) is extremely
interesting. These clusters include RSGC1 (Figer et al., 2006), RSGC2 (Davies
et al., 2007), RSGC3 (Clark et al., 2009), RSGC4 (Alicante 8; Negueruela et
al., 2010), RSGC5 (Alicante 7; Negueruela et al., 2011), and Alicante 10
(González-Fernández & Negueruela, 2012). These discoveries not only allow a
detailed investigation of massive star evolution but also raise many
intriguing questions about star formation in this region.

RSGCs all have large populations of RSGs, with 10–25 RSGs in each cluster and
an age range of 12–20 Myr. The presence of such large numbers of RSGs
indicates that the initial mass range needed for clusters to harbor such stars
is in the range of 2–4$\times 10^{4}M_{\sun}$. The close proximity of
clusters, with projected separation ranges of 31–400 pc at almost identical
distances of 5–6 kpc, and the presence of a diffuse field of RSGs (Garzón et
al., 1997; López-Corredoira et al., 1999) in the Scutum complex region suggest
a recent starburst resulting from interactions between the Galactic bar and
the spiral arm (Garzón et al., 1997; Davies et al., 2007). Physical
associations between clusters, which are similar to those observed in the
extended cluster complexes of external galaxies (e.g., M51, Bastian et al.,
2005), are also expected to exist in the Scutum complex region.

Interestingly, subsolar metallicities have been reported for these clusters
(Davies et al., 2009; Origlia et al., 2016, 2019; Chun, 2020), indicating the
presence of low-metallicity gas fueling into the inner disks (Magrini et al.,
2009; Bono et al., 2013). This is inconsistent with the high metallicity
properties of this area, and is not considered in current Galactic evolution
models. Therefore, it is important to characterize the cluster parameters
through follow-up observations.

The recent starburst, potential cluster associations, and subsolar
metallicities observed in these clusters motivated us to obtain high-
resolution spectra for studying the dynamical properties of RSGs in six RSGCs
in the Scutum complex. In this study, we refer to RSGs as RSG candidates, as
their confirmation requires high-resolution spectroscopy. We note that high-
resolution spectroscopic studies have not been conducted solely for RSGC4.
Recently, Asa’d et al. (2023) conducted a study on RSGC4 using spectroscopic
data (R$\sim$5600) from VLT/X-shooter. In Section 2, we introduce the sample
selection, observation, and data reduction. In Section 3, we describe the
peculiar radial velocities of the RSG candidates in RSGC4 and those in the
other RSGCs. In Section 4, we explore various potential explanations for the
peculiar radial velocities of RSG candidates in RSGC4. Finally, we present our
conclusions and a summary in Section 5.

##  2 Target selection, Observation, and data reduction

Table 1: The observation log, radial velocities of observed RSG candidates and
proper motions from $Gaia$ DR3

ID | RA | DEC | Exp. time (s) | S/N |  $V_{Helio}$ (km s-1) |  $V_{LSR}$ (km s-1) |  $\mu_{\alpha}$cos$\delta$ (mas yr-1) |  $\mu_{\delta}$ (mas yr-1)
---|---|---|---|---|---|---|---|---
RSGC4 (12) |  |  |  |  |  $\overline{V}_{Helio}$=7.3$\pm 65.4$ |  $\overline{V}_{LSR}$=22.6$\pm 65.6$ |  $\overline{\mu_{\alpha}cos\delta}$=-1.731$\pm 1.757$ |  $\overline{\mu_{\delta}}$=-4.942$\pm 1.927$
RSGC4-1 | 18:34:58.40 | -07:14:24.8 | 640 | 260 |  -28.7$\pm$0.7 |  -12.9$\pm$0.7 |  -0.662$\pm$0.269 |  -1.986$\pm$0.229
RSGC4-1aaRadial velocities are estimated from the spectra obtained in 2021. |  |  | 160 | 200 |  -25.4$\pm$0.9 |  -10.2$\pm$0.9 |  |
RSGC4-1bbRadial velocities from Asa’d et al. (2023). |  |  |  | 174 |  -33$\pm$0.5 |  |  |
RSGC4-2aaRadial velocities are estimated from the spectra obtained in 2021. | 18:34:55.12 | -07:15:10.8 | 200 | 210 |  73.7$\pm$0.3 |  88.9$\pm$0.3 |  -1.854$\pm$0.351 |  -2.695$\pm$0.300
RSGC4-2bbRadial velocities from Asa’d et al. (2023). |  |  |  | 92 |  71$\pm$0.6 |  |  |
RSGC4-3 | 18:34:50.00 | -07:14:26.2 | 240 | 380 |  22.0$\pm$0.3 |  37.8$\pm$0.3 |  -0.964$\pm$0.277 |  -5.726$\pm$0.232
RSGC4-3aaRadial velocities are estimated from the spectra obtained in 2021. |  |  | 200 | 410 |  20.6$\pm$0.3 |  35.9$\pm$0.3 |  |
RSGC4-3bbRadial velocities from Asa’d et al. (2023). |  |  |  | 146 |  37$\pm$0.5 |  |  |
RSGC4-4aaRadial velocities are estimated from the spectra obtained in 2021. | 18:34:51.02 | -07:14:00.5 | 200 | 380 |  90.8$\pm$0.4 |  106.1$\pm$0.4 |  -1.032$\pm$0.815 |  -5.108$\pm$0.706
RSGC4-4bbRadial velocities from Asa’d et al. (2023). |  |  |  | 43 |  88$\pm$0.7 |  |  |
RSGC4-5 | 18:34:51.33 | -07:13:16.3 | 200 | 300 |  -10.8$\pm$0.4 |  5.0$\pm$0.4 |  -3.276$\pm$0.297 |  -7.537$\pm$0.253
RSGC4-5aaRadial velocities are estimated from the spectra obtained in 2021. |  |  | 200 | 270 |  -12.3$\pm$0.4 |  3.1$\pm$0.4 |  |
RSGC4-5bbRadial velocities from Asa’d et al. (2023). |  |  |  | 102 |  -9$\pm$0.5 |  |  |
RSGC4-6 | 18:34:41.55 | -07:11:38.8 | 960 | 190 |  114.9$\pm$0.4 |  130.7$\pm$0.4 |  -2.127$\pm$0.462 |  -6.021$\pm$0.398
RSGC4-6aaRadial velocities are estimated from the spectra obtained in 2021. |  |  | 200 | 310 |  113.3$\pm$0.2 |  128.6$\pm$0.2 |  |
RSGC4-6bbRadial velocities from Asa’d et al. (2023). |  |  |  | 112 |  111$\pm$0.6 |  |  |
RSGC4-7 | 18:34:43.56 | -07:13:29.7 | 320 | 320 |  -2.0$\pm$0.2 |  13.3$\pm$0.2 |  -3.831$\pm$0.288 |  -6.550$\pm$0.246
RSGC4-7bbRadial velocities from Asa’d et al. (2023). |  |  |  | 110 |  -4$\pm$0.6 |  |  |
RSGC4-8 | 18:34:44.51 | -07:14:15.3 | 640 | 260 |  -0.1$\pm$0.7 |  15.7$\pm$0.7 |  -0.676$\pm$0.263 |  -2.859$\pm$0.230
RSGC4-8aaRadial velocities are estimated from the spectra obtained in 2021. |  |  | 200 | 150 |  -5.0$\pm$0.3 |  10.2$\pm$0.3 |  |
RSGC4-8aaRadial velocities are estimated from the spectra obtained in 2021. |  |  | 52 | 173 |  -1.5$\pm$0.3 |  13.7$\pm$0.3 |  |
RSGC4-9 | 18:34:45.81 | -07:18:36.2 | 960 | 240 |  -64.4$\pm$0.7 |  -48.6$\pm$0.7 |  1.116$\pm$0.273 |  -6.562$\pm$0.217
RSGC4-9aaRadial velocities are estimated from the spectra obtained in 2021. |  |  | 320 | 360 |  -81.1$\pm$0.2 |  -65.9$\pm$0.2 |  |
RSGC4-10 | 18:34:26.81 | -07:15:27.9 | 192 | 210 |  1.0$\pm$0.6 |  16.2$\pm$0.6 |  0.324$\pm$0.889 |  -1.270$\pm$0.721
RSGC4-11 | 18:35:00.32 | -07:07:37.4 | 200 | 280 |  44.8$\pm$0.4 |  60.6$\pm$0.4 |  -1.636$\pm$0.249 |  -3.933$\pm$0.212
RSGC4-11aaRadial velocities are estimated from the spectra obtained in 2021. |  |  | 400 | 310 |  45.8$\pm$0.6 |  61.1$\pm$0.6 |  |
RSGC4-12aaRadial velocities are estimated from the spectra obtained in 2021. | 18:35:16.88 | -07:13:26.9 | 240 | 330 |  -13.5$\pm$1.2 |  1.7$\pm$1.2 |  -5.325$\pm$0.309 |  -6.766$\pm$0.260
RSGC1 (14) |  |  |  |  |  $\overline{V}_{Helio}$=105.9$\pm 2.6$ |  $\overline{V}_{LSR}$=122.0$\pm 3.2$ |  $\overline{\mu_{\alpha}cos\delta}$=-3.961$\pm 0.478$ |  $\overline{\mu_{\delta}}$=-4.832$\pm 0.800$
RSGC1-1 | 18:37:56.29 | -06:52:32.2 | 300 | 430 |  107.2$\pm$1.0 |  122.5$\pm$1.0 |  -4.569$\pm$0.717 |  -5.074$\pm$0.603
RSGC1-2 | 18:37:55.28 | -06:52:48.4 | 200 | 490 |  100.5$\pm$0.7 |  115.7$\pm$0.7 |  -4.065$\pm$0.792 |  -6.729$\pm$0.658
RSGC1-3 | 18:37:59.72 | -06:53:49.4 | 280 | 590 |  110.8$\pm$0.7 |  126.1$\pm$0.7 |  -2.945$\pm$0.728 |  -4.865$\pm$0.596
RSGC2 (11) |  |  |  |  |  $\overline{V}_{Helio}$=90.3$\pm 3.0$ |  $\overline{V}_{LSR}$=106.9$\pm 3.1$ |  $\overline{\mu_{\alpha}cos\delta}$=-2.181$\pm 0.267$ |  $\overline{\mu_{\delta}}$=-4.627$\pm 0.219$
RSGC2-13 | 18:39:17.7 | -06:04:02.5 | 120 | 390 |  93.0$\pm$0.5 |  110.0$\pm$0.5 |  -2.101$\pm$0.227 |  -4.739$\pm$0.196
RSGC2-16 | 18:39:24.0 | -06:03:07.3 | 120 | 390 |  90.5$\pm$0.5 |  107.6$\pm$0.5 |  -2.015$\pm$0.124 |  -4.269$\pm$0.101
RSGC2-17 | 18:39:15.1 | -06:05:19.1 | 120 | 350 |  89.7$\pm$0.5 |  106.7$\pm$0.5 |  -2.052$\pm$0.206 |  -4.695$\pm$0.175
RSGC3 (9) |  |  |  |  |  $\overline{V}_{Helio}$=91.0$\pm 12.5$ |  $\overline{V}_{LSR}$=107.5$\pm 12.5$ |  $\overline{\mu_{\alpha}cos\delta}$=-2.348$\pm 0.641$ |  $\overline{\mu_{\delta}}$=-5.016$\pm 0.812$
RSGC3-1 | 18:45:23.60 | -03:24:13.9 | 150 | 310 |  92.3$\pm$0.4 |  108.7$\pm$0.4 |  -2.012$\pm$0.214 |  -5.903$\pm$0.166
RSGC3-6 | 18:45:19.39 | -03:24:48.3 | 240 | 320 |  106.3$\pm$0.4 |  122.7$\pm$0.4 |  -2.590$\pm$0.278 |  -5.934$\pm$0.225
RSGC3-8 | 18:45:20.06 | -03:22:47.1 | 320 | 320 |  89.8$\pm$0.5 |  106.2$\pm$0.5 |  -2.561$\pm$0.230 |  -5.385$\pm$0.188
RSGC5 (6) |  |  |  |  |  $\overline{V}_{Helio}$=77.0$\pm 23.0$ |  $\overline{V}_{LSR}$=93.4$\pm 23.0$ |  $\overline{\mu_{\alpha}cos\delta}$=-1.824$\pm 0.340$ |  $\overline{\mu_{\delta}}$=-4.458$\pm 0.902$
RSGC5-1 | 18:44:20.53 | -03:28:44.6 | 320 | 410 |  79.6$\pm$0.5 |  96.1$\pm$0.5 |  -1.686$\pm$0.175 |  -4.166$\pm$0.145
RSGC5-4 | 18:44:27.96 | -03:29:42.5 | 100 | 370 |  86.0$\pm$0.5 |  103.4$\pm$0.5 |  -2.360$\pm$0.219 |  -4.749$\pm$0.185
RSGC5-5 | 18:44:29.45 | -03:30:02.4 | 80 | 310 |  84.1$\pm$0.5 |  100.5$\pm$0.5 |  -1.493$\pm$0.213 |  -4.191$\pm$0.189
Alicante10 (8) |  |  |  |  |  $\overline{V}_{Helio}$=70.8$\pm 11.1$ |  $\overline{V}_{LSR}$=87.2$\pm 11.1$ |  $\overline{\mu_{\alpha}cos\delta}$=-2.269$\pm 0.860$ |  $\overline{\mu_{\delta}}$=-5.081$\pm 1.052$
Alican10-1 | 18:45:07.94 | -03:41:24.0 | 360 | 380 |  66.6$\pm$0.4 |  83.0$\pm$0.4 |  -0.747$\pm$0.354 |  -4.769$\pm$0.313
Alican10-2 | 18:45:01.85 | -03:42:06.5 | 300 | 260 |  76.9$\pm$0.6 |  93.3$\pm$0.6 |  -2.998$\pm$0.469 |  -5.628$\pm$0.346
Alican10-3 | 18:45:17.60 | -03:43:05.1 | 180 | 310 |  80.3$\pm$0.5 |  96.7$\pm$0.5 |  -3.106$\pm$0.290 |  -6.142$\pm$0.234

Full Table 1 is published in the machine-readable format. A portion is shown
here for guidance regarding its form and content. Redundant information is
eliminated. The first header of each cluster shows average radial velocity and
proper motions. The numbers in parentheses represent the counts of observed
RSG candidates in each cluster.

We selected RSG candidates in six young massive star clusters from several
published catalogs: 16 stars in RSGC (Davies et al., 2008), 26 stars in RSGC2
(Davies et al., 2007), 15 stars in RSGC3 (Clark et al., 2009), 13 stars in
RSGC4 (Negueruela et al., 2010), 9 stars in RSGC5 (Negueruela et al., 2011),
and 8 stars in Alicante 10 (González-Fernández & Negueruela, 2012). In the
case of RSGC2, the stars whose chemical abundances were not investigated by
Davies et al. (2009) were selected as the observation targets. The
distribution of RSG candidates in the clusters in $(J-K_{s},K_{s})$ color-
magnitude diagram was examined, and we preselected as many target stars as
possible.

High-resolution near-infrared spectra of the RSG candidates in the six star
clusters were obtained using the Immersion Grating Infrared Spectrometer
(IGRINS) installed as a visitor instrument on the Gemini South telescope. The
IGRINS simultaneously provides high-resolution (R$\sim$45,000) spectra of the
$H$ and $K$ bands, covering a wavelength range from 14,800 to 24,800
$\mathrm{\AA}$, for which we used a $0.34^{\prime\prime}\times
5^{\prime\prime}$ slit. Detailed and extensive explanations of IGRINS have
been provided by Yuk et al. (2010), Park et al. (2014), and Mace et al.
(2018).

Observations were conducted by two poor weather programs in service mode in
June and July 2018 (Program ID: GS-2018A-Q-418 for RSGC2, RSGC3, RSGC4, RSGC5,
and Alicante 10) and in July and August 2021 (Program ID: GS-2021A-Q-417 for
RSGC1 and RSGC4) because our targets are so bright. Most of RSG candidates
were secured using the ABBA nod sequence, while some stars were observed using
the ON-OFF sequence, depending on weather conditions. Individual exposure
times were varied between 20 and 180 s to obtain high signal-to-noise ratios
(median S/N$>$100 over the entire spectra of the $H$ and $K$ bands in the
final reduced spectra). In addition to the scientific targets, we obtained A0V
telluric standard stars at air masses close to those of the scientific targets
during the same night to remove the telluric absorption lines. The observation
logs are listed in Table 1.

The data reduction process was performed using the IGRINS reduction pipeline
package (Lee et al., 2017). This pipeline facilitates standard reduction
processes, including bad pixel correction, flat fielding, geometric
transformation, background removal, wavelength calibration, and order
extraction. The resulting spectra were divided by the A0V spectra to remove
the telluric features and then multiplied by a model spectrum of Vega. The
final S/N ratios per pixel of the reduced spectra, representing the median S/N
across the entire spectrum, are provided in Table 1.

##  3 Peculiar radial velocities of red supergiant stars in RSGC4

Figure 1: Left: IGRINS $K$-band spectra with only heliocentric correction for
all observed stars in RSGC4 in the region around the CO bandhead. The ID
numbers of the stars correspond with those of the stars in RSGC4 by Negueruela
et al. (2010). Right: $K$-band spectra resulting with only heliocentric
correction for observed stars in RSGC2 in this study. The ID numbers for RSGC2
follow those of Davies et al. (2007). The dotted vertical line indicates the
rest wavelength position of the blue edge of the CO bandhead.

Figure 1 shows the IGRINS spectra for the region around the CO bandhead in the
$K$-band of the observed stars in RSGC4 (left panel) and RSGC2 (right panel).
Only heliocentric correction was applied to the spectra. The ID numbers of the
stars in both panels correspond to those of RSGC4 of Negueruela et al. (2010)
and RSGC2 of Davies et al. (2007). All spectra exhibit strong absorption
features and deep CO bandhead absorption, which are characteristic of late-
type stars. The amount of Doppler shift for stars is shown qualitatively with
respect to the rest wavelength of the blue edge of the CO bandhead (dotted
line) in Figure 1. In the left panel of Figure 1, we find blue-shifted or red-
shifted blue edges of the CO bandhead in the RSGC4 spectra. The extent of the
shift is also inconsistent. In contrast, the wavelengths at the CO bandhead of
the RSGC2 spectra are nearly identical. This discrepancy suggests that the RSG
candidates in RSGC4 do not have consistent radial velocities.

To accurately determine the radial velocities of the stars, the radial
velocities for all observed RSG candidates in the six clusters were estimated
by cross-correlation of the spectra with synthetic RSG spectra. In the high-
resolution IGRINS spectra, several absorption lines of the RSG candidates
enable us to conduct precise radial velocity estimations. We selected several
spectral segments containing prominent absorption lines, such as Ti, Mg and CO
bands in the $H$ and $K$ bands to compare the synthetic RSG spectra.

Synthetic spectra for the $H$ and $K$ bands were calculated using local
thermodynamic equilibrium (LTE) line analysis and synthetic spectrum code MOOG
(Sneden, 1973). We used a MARCS model atmosphere (Gustafsson et al., 2008)
with typical stellar parameters for an RSG (i.e., $T_{eff}=3800$ K, log
$g$=0.0, and metallicity of $\mathrm{[Fe/H]}=-0.21$). We note that several
RSGs in RSGCs are known to have subsolar metallicity (Davies et al., 2008;
Origlia et al., 2013). Atomic and molecular line lists for computing the
synthetic spectra were adopted from Afsar et al. (2016), who combined atomic
and CO molecular lines from Kurucz (2011) and CN and OH molecular lines from
Sneden et al. (2014) and Brooke et al. (2016). We estimated the radial
velocities of the spectral segments and averaged them to obtain the final
radial velocity. Standard deviations of the average radial velocities were
used to represent velocity errors. The heliocentric radial velocities and
velocities in the local standard of rest (LSR) reference system were
calculated from the final radial velocities using the rvcorrect task in IRAF
(Tody, 1986, 1993).

Figure 2: Radial velocity distributions as a function of radial distance from
the cluster center in the sky for six RSGCs. Symbol size represents the
measured radial velocity error of an individual star, with larger symbols
indicating larger errors. Four symbols corresponding to different velocity
error categories ($\sigma_{V_{LSR}}=4.0,1.0,0.6$, and $0.3$ km s-1) are
adopted. The solid line indicates the mean radial velocity ($V_{LSR}=96$ km
s-1) for all RSG candidates in the clusters. The dashed lines represent the
$1\sigma$ (34 km s-1) range from the mean velocity. Histograms of radial
velocity for each cluster are also presented in the right-hand three panels.

The derived radial velocities of the RSG candidates in each cluster are
indicated in Table 1. As predicted in Figure 1, the radial velocity values of
the RSG candidates in the RSGC4 exhibit significant variations, spanning a
wide range from $-64$ to $115$ km s-1. The mean velocity is 18.98 km s-1, with
a standard deviation of 50.26 km s-1. We note that this substantial velocity
difference among the RSG candidates in RSGC4 was not found by Negueruela et
al. (2010). Although they did not estimate the radial velocities of the RSGs,
the Doppler shifts in their spectra appear almost identical (see Figure 4 in
Negueruela et al., 2010). We attribute this discrepancy to the lower
resolution (R$\sim$2500) of their spectra. We note that we observed the same
RSG candidates identified by Negueruela et al. (2010). There is no chance of
observing different stars because they are the most luminous objects in this
field. Chun et al. (2023) reported inconsistent radial velocities for RSG
candidates in RSGC4. Asa’d et al. (2023) also demonstrated a large dispersion
in the radial velocities of RSG candidates in RSGC4, suggesting that RSGC4 may
not be a genuine cluster.

The radial velocities of the RSG candidates in the RSGC4 were compared with
those of the other RSGCs. Previous medium-resolution spectroscopic studies
(Davies et al., 2007, 2008; Negueruela et al., 2011) suggested that all RSGCs
in the Scutum complex were located within $5\sim 6$ kpc from us, potentially
sharing a common radial velocity of 95–123 km s-1. For a proper comparison, we
collected the radial velocities of the RSG candidates that were not included
in our observations from the previous catalogs. These values were added to our
radial velocity measurements. Specifically, we incorporated radial velocity
data from Davies et al. (2008) for RSGC1, Davies et al. (2007) for RSGC2,
Origlia et al. (2016) for RSGC3, and Origlia et al. (2019) for RSGC5 and
Alicante10.

Figure 2 shows the radial velocity distributions in the LSR for the RSG
candidates in the six RSGCs as a function of the radial distance from each
cluster center in the sky. The median values for the sky positions of the RSG
candidates in the clusters were assigned to the cluster center. Symbol size in
the plot corresponds to the velocity error, with larger symbols indicating
larger errors. We adopted four symbols to represent different velocity error
categories: $\sigma_{V_{LSR}}=4.0,1.0,0.6$, and $0.3$ km s-1. Additionally,
histograms of the radial velocity distributions for the candidates in each
cluster are presented in the right-hand three panels.

As previously reported, all RSG candidates in the five clusters (excepting
RSGC4) have common radial velocities within the range of 75–125 km s-1,
suggesting that they are probably members of the Scutum complex. However, the
RSG candidates in RSGC4 show very different radial velocity distributions
compared to the other clusters. Most of the RSG candidates in RSGC4 have low
radial velocities, with a large spread. RSGC4-1 and RSGC4-9 even exhibit
negative $V_{LSR}$ velocities.

##  4 Discussion of the peculiar radial velocities of RSG candidates in RSGC4

The unusual radial velocities observed for the RSG candidates within RSGC4
raise many intriguing questions regarding their membership and the underlying
cause of these anomalous velocities. The most straightforward explanation for
the peculiar radial velocities of the RSG candidates in RSGC4 is that some of
the identified stars may belong to different populations or represent a random
overdensity of field stars rather than true members of the star cluster.
Another consideration is the potential influence of binaries or multiplicity
in star clusters on radial velocity. The third interpretation involves the
presence of unbounded stars in a cluster. A final possibility is that
environmental perturbations might have been responsible for the peculiar
radial velocities. We discuss these four interpretations to better understand
the peculiar radial velocities observed for the RSG candidates in RSGC4,
including the RSG candidates in the other five clusters.

Table 2: The photometry and color information of RSG candidates in six RSGCs

| 2MASS | $Gaia$ | GLIMPSE | DENIS | DENIS-2MASS | $Q1$ | $Q2$
---|---|---|---|---|---|---|---
ID | $J$ | $H$ | $K_{s}$ | $G$ | [3.6] | [4.5] | [5.8] | [8.0] | $J$ | $K_{s}$ |  $\Delta$ $J$ |  $\Delta$ $K_{s}$ |  |
RSGC4-1 | 9.921 | 7.796 | 6.671 | 17.685 | 6.646 | 6.017 | 5.536 | 5.321 | 10.316 | 7.028 | 0.396 | 0.358 | 0.086 | -0.379
RSGC4-2 | 10.86 | 8.452 | 7.324 | 18.828 | 6.691 | 6.731 | 6.367 | 6.308 | 10.765 | 7.374 | 0.095 | 0.054 | 0.376 | 0.818
RSGC4-3 | 9.551 | 7.186 | 5.995 | 17.658 | 5.500 | 5.926 | 4.844 | 4.679 | 9.672 | 6.157 | 0.122 | 0.157 | 0.218 | -0.003
RSGC4-4 | 10.731 | 7.675 | 6.141 | 20.023 | 5.218 | 5.268 | 4.653 | 4.495 | 10.875 | 6.320 | 0.145 | 0.180 | 0.278 | 0.165
RSGC4-5 | 10.037 | 7.527 | 6.264 | 18.433 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 10.209 | 6.467 | 0.169 | 0.207 | 0.224 |  $\cdots$
RSGC4-6 | 10.255 | 7.731 | 6.429 | 19.065 | 6.632 | 6.090 | 5.300 | 5.188 | 10.473 | 6.718 | 0.213 | 0.288 | 0.190 | 0.489
RSGC4-7 | 10.456 | 8.224 | 7.055 | 18.494 | 6.837 | 6.426 | 5.942 | 5.877 | 10.664 | 7.317 | 0.204 | 0.257 | 0.152 | 0.218
RSGC4-8 | 10.416 | 8.030 | 6.623 | 18.019 |  $\cdots$ | 6.513 | 4.754 | 4.625 | 10.527 | 6.894 | 0.107 | 0.274 | -0.041 | -1.460
RSGC4-9 | 9.829 | 7.629 | 6.486 | 17.567 | 5.384 | 5.327 | 4.993 | 4.831 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 0.148 | -1.123
RSGC4-10 | 10.984 | 8.319 | 6.997 | 19.775 | 6.712 | 6.181 | 5.780 | 5.605 | 11.218 | 7.356 | 0.238 | 0.356 | 0.284 | 0.227
RSGC4-11 | 9.610 | 7.469 | 6.342 | 17.452 | 5.688 | 6.135 | 5.349 | 5.118 |  $\cdots$ | 6.642 |  $\cdots$ | 0.302 | 0.106 | -0.017
RSGC4-12 | 9.872 | 7.728 | 6.454 | 18.120 | 5.267 | 4.998 | 4.655 | 4.448 | 9.833 | 6.493 | 0.037 | 0.043 | -0.164 | -1.965
RSGC1-1 | 9.748 | 6.587 | 4.962 | 19.516 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 9.857 | 4.252 | 0.109 | 0.710 | 0.236 |  $\cdots$
RSGC1-2 | 9.904 | 6.695 | 5.029 | 19.549 |  $\cdots$ | 3.585 | 2.736 |  $\cdots$ | 9.933 | 4.330 | 0.029 | 0.699 | 0.210 |  $\cdots$
RSGC1-3 | 9.954 | 6.921 | 5.333 | 19.219 | 4.301 |  $\cdots$ | 3.704 |  $\cdots$ | 9.952 | 4.628 | 0.002 | 0.705 | 0.175 |  $\cdots$
RSGC2-13 | 8.421 | 6.387 | 5.439 | 15.320 | 5.415 | 5.008 | 4.587 | 4.382 | 8.506 | 4.688 | 0.085 | 0.751 | 0.328 | 0.139
RSGC2-16 | 8.235 | 6.444 | 5.597 | 14.447 | 5.561 |  $\cdots$ | 4.870 | 4.762 | 8.218 | 4.578 | 0.017 | 1.019 | 0.266 | 0.392
RSGC2-17 | 8.709 | 6.613 | 5.619 | 15.854 | 7.177 | 6.718 |  $\cdots$ |  $\cdots$ | 8.717 | 4.741 | 0.008 | 0.878 | 0.307 |  $\cdots$
RSGC3-1 | 8.551 | 6.539 | 5.578 | 15.379 |  $\cdots$ | 6.019 | 4.726 | 4.499 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 0.282 | 0.062
RSGC3-6 | 9.062 | 6.968 | 6.036 | 16.130 |  $\cdots$ | 6.246 | 5.243 | 5.199 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 0.416 | 0.758
RSGC3-8 | 9.525 | 7.288 | 6.303 | 16.746 | 6.789 | 6.249 | 5.462 | 5.430 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 0.458 | 0.890
RSGC5-1 | 9.280 | 7.235 | 6.213 |  $\cdots$ | 5.657 | 6.085 | 5.381 | 5.445 | 9.156 | 6.100 | 0.124 | 0.110 | 0.186 | 1.012
RSGC5-4 | 8.063 | 5.910 | 4.851 | 15.182 |  $\cdots$ | 4.308 | 3.843 |  $\cdots$ | 7.981 | 4.142 | 0.079 | 0.708 | 0.242 |  $\cdots$
RSGC5-5 | 8.446 | 6.386 | 5.409 | 15.386 | 4.794 | 5.148 | 4.567 | 4.447 | 8.335 | 5.010 | 0.115 | 0.400 | 0.296 | 0.450
Alicante10-1 | 10.500 | 8.080 | 6.910 | 18.712 | 6.637 |  $\cdots$ | 5.658 | 5.268 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 0.314 | -0.827
Alicante10-2 | 10.457 | 7.943 | 6.707 | 18.672 | 6.651 | 6.342 | 5.644 | 5.576 | 10.334 | 6.687 | 0.116 | 0.013 | 0.278 | 0.726
Alicante10-3 | 9.970 | 7.405 | 6.062 | 18.405 | 5.016 |  $\cdots$ | 4.761 | 4.521 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ | 0.158 | -0.230

Full Table 2 is published in the machine-readable format.

###  4.1 Contamination with AGB stars

Asymptotic giant branch stars (AGBs) in the field can lead to significant
scattering in the radial velocity distribution. The separation of AGBs and
RSGs based solely on photometry has been a very difficult task. AGBs have
similar red colors to those of RSGs, and their luminosities also overlap with
those of faint RSGs: $M_{\mathrm{bol}}$ from $-5.5$ to $-9.0$ mag for RSGs
(Davies et al., 2007; Clark et al., 2009), and $M_{\mathrm{bol}}$ from $-5.5$
to $-7.0$ mag for AGBs (Alard et al., 2001; Messineo et al., 2014). In
spectroscopic data, strong and broad water absorption features at the edges of
the $H-$band and $K-$band spectra are used to distinguish between AGBs and
RSGs (e.g., Messineo et al., 2014). AGB spectra typically show a highly curved
continuum due to $\mathrm{H_{2}O}$ in their envelopes (e.g., Lançon & Wood,
2000). Unfortunately, our high-resolution IGRINS spectra of the RSG candidates
were not appropriate for detecting continuum absorption by water, as the
continuum shape in high-resolution spectra is unreliable, and the broad water
vapor feature spans multiple orders in the spectra.

Instead, we used the photometric variations of AGBs, especially Mira-AGBs, as
a way to distinguish between AGBs and RSGs. AGBs generally have larger
variable amplitudes than RSGs. The visual amplitude of a Mira-AGB exceeds 2.5
mag, extending up to $\sim 1$ mag in $K_{s}$ (Messineo et al., 2004). In
contrast, the variable amplitude of RSGs is a few tenths of the magnitude in
the $K$ band (Kiss et al., 2006; Groenewegen et al., 2009; Yang & Jiang,
2011). Messineo et al. (2014) conducted tests on the variability of the RSGs
and AGBs using data from the Two Micron All-Sky Survey (2MASS; Cutri et al.,
2003) and the Deep Near-Infrared Survey (DENIS; Epchtein et al., 1999). They
identified variable Mira-AGBs in the Milky Way based on criteria such as
$J(DENIS)>7.5$ mag, and $|J(DENIS)-J(2MASS)|>3\times 0.15$ mag, or
$K_{s}(DENIS)>6.0$ mag, and $|K_{s}(DENIS)-K_{s}(2MASS)|>3\times 0.15$ mag. We
followed these criteria to check the variability of RSG candidates in RSGC4
using the 2MASS and DENIS magnitudes. The photometric magnitudes of 2MASS and
DENIS are listed in Table 2. We found that none of the RSG candidates in RSGC4
were consistent with these criteria, except for RSGC4-9, for which the
criteria could not be applied due to the lack of a DENIS counterpart. This
indicates that the RSGs candidates in RSGC4 do not show the level of
variability observed in the Mira-AGBs.

Figure 3: Left: $(J-H,G-K_{s})$ color-color diagram of stars with $K_{s}<8$ in
the $6^{\circ}\times 2^{\circ}$ Scutum complex area and RSG candidates in the
six RSGCs. The arrow indicates an increasing reddening vector, and the dashed
line shows the linear distribution of stars. Right: $(J-H,Q_{GK_{s}})$ diagram
for the stars in the left panel. The MIST isochrones with [Fe/H]=$-0.25$ and
log$\mathrm{(t_{yr})}=7.0,7.2,7.4,7.8,8.6$ and $9.6$ are indicated by solid
lines of different colors. The core helium burning (CHeB) phases in isochrones
are described by bold lines, while early AGB or thermal pulsing AGB phases are
shown by thin lines. The dash-dotted line indicates the limit $Q_{GK_{s}}$
value of 1.7, which stars in the CHeB cannot attain.

We also studied the variability of RSG candidates in RSGC4 using the unTimely
Catalog (Meisner et al., 2023), which is based on data from the Wide-field
Infrared Survey Explorer (WISE; Wright et al., 2010) and the NEOWISE mission
extension (Mainzer et al., 2011, 2014). We found amplitudes of variation
ranging from 0.2 to 1.0 mag in the W1 and W2 bands of WISE/NEOWISE single
exposure photometry. In the case of unTimely photometry, which is much deeper
and cleaner than WISE/NEOWISE single exposure photometry, the amplitudes of
variation were less than 0.5 mag in both the W1 and W2 bands. It is worth
noting that the expected amplitudes of variation of AGB stars typically
exceeds 2 mag in both the W1 and W2 bands (e.g., Suh, 2021; Groenewegen,
2022). Unfortunately, however, several RSG candidates in RSGC4 were saturated
during certain epochs, potentially resulting in underestimated amplitudes of
variation. Therefore, although we could not infer definite variability similar
to AGB stars for RSG candidates in RSGC4, the detected amplitudes of variation
in WISE/NEOWISE data represent a lower limit of variation. Additional time-
domain survey data in near-infrared, where RSG candidates are not saturated,
are necessary to confirm the contamination of the Mira-AGBs in RSGC4. We note
that the VISTA Variables in the Via Lactea (VVV) survey (Minniti et al., 2010)
did not cover the area of RSGC4, preventing us from investigating their
variability.

We further investigated the contamination with field AGBs in RSGC4 using Gaia
Data Release 3 ($Gaia$ DR3; Gaia Collaboration et al., 2022) and 2MASS data.
The magnitudes in $G$ band of $Gaia$ data are shown in Table 2. The left panel
of Figure 3 shows the $(J-H,G-K_{s})$ color-color diagram (CCD) for stars with
$K_{s}<8$ in the $6^{\circ}\times 2^{\circ}$ area where all six RSGCs are
located. All RSG candidates in the six RSGCs are indicated with the same
symbols as in Figure 2. We find that all the stars closely follow the
reddening line (indicated by an arrow) in the $(J-H,G-K_{s})$ CCD. The RSG
candidates in the six RSGCs are also located based on their distances and
increasing extinctions with distance. The extinction toward RSGC4 appears to
be higher than those toward RSGC2, RSGC3, and RSGC5, but lower than that
toward RSGC1. Therefore the distance to RSGC4 is somewhere between RSGC2 and
RSGC1. This is consistent with previous distance and extinction results for
RSGCs: $A_{K_{s}}=2.6$ and $d=6.6$ kpc for RSGC1 (Davies et al., 2008),
$A_{K_{s}}=1.44$ and $d=5.8$ kpc for RSGC2 (Davies et al., 2007),
$A_{K_{s}}=1.5$ and $d=6.0$ kpc for RSGC3 (Clark et al., 2009),
$A_{K_{s}}=1.9$ and $d=6.6$ kpc for RSGC4 (Negueruela et al., 2010), and
$A_{K_{s}}=1.5$ and $d=6.1$ kpc for RSGC5 (Negueruela et al., 2011). Alicante
10 and RSGC4 appear to be at the same distance and have the same extinction
value, as shown in Figure 3. González-Fernández & Negueruela (2012) reported
6.0 kpc and $E(J-K_{s})=2.6$, corresponding to $A_{K_{s}}=1.93$, for Alicante
10. We note that the distances to and extinctions toward the clusters can
differ by up to 1 kpc and $\Delta A_{K_{s}}=0.2$, respectively, depending on
the methods used for the distance and extinction measurements.

Based on the stellar distributions and the linear fitting line (dashed line)
in the $(J-H,G-K_{s})$ CCD, we defined a new parameter,
$Q_{GK_{s}}=(G-K_{s})-4.24\pm 0.01\times(J-H)$. This parameter is independent
of reddening, similar to the $Q1$111$Q1=(J-H)-1.8\times(H-K_{s})$ parameter of
Messineo et al. (2012). The right panel of Figure 3 shows the $Q_{GK_{s}}$
parameter versus the $(J-H)$ diagram of stars and RSG candidates in the left
panel. The MIST isochrones (Choi et al., 2016) with [Fe/H]=$-0.25$ and several
ages of $\mathrm{log(t_{yr})}$=7.0, 7.2, 7.4, 7.8, 8.6, and 9.6 are also
indicated to describe the stellar evolutionary phase. For clarity, only the
isochrones from core helium burning (CHeB, bold lines) to early AGB or
thermally pulsing AGB (EAGB and TPAGB, thin lines) phases are included in the
plot.

The isochrones indicate that the stars in the CHeB phase do not surpass a
specific $Q_{GK_{s}}$ value (i.e., 1.7 in this study). The majority of RSG
candidates in the five RSGCs, except RSGC4, have $Q_{GK_{s}}$ values below
1.7, which is consistent with the isochrone trend. We note that only six stars
in three clusters (three in RSGC2, two in RSGC5, and one in Alicante 10) have
$Q_{GK_{s}}>1.7$. Three stars in RSGC2 are spectroscopically confirmed members
(ID-2, ID-8, and ID-52; Davies et al., 2007). The two stars in RSGC5 are the
main RSG and faint star (ID-A7 and ID-A12; Negueruela et al., 2011),
respectively. The star in Alicante 10 has been spectroscopically confirmed as
an RSG (ID-Near07, González-Fernández & Negueruela, 2012). Unfortunately, data
for these six stars were not obtained during the IGRINS observation; thus, we
did not investigate their spectroscopic properties in this study.

Conversely, more than half RSG candidates in RSGC4 have $Q_{GK_{s}}$ values
larger than $1.7$. The isochrones indicate that the stars in the EAGB with
$\mathrm{log(t_{yr})}=7.8$ or the TPAGB phase with $\mathrm{log(t_{yr})}=8.6$
and $9.6$ can have large $Q_{GK_{s}}$ values. If we adopted TPAGBs with
$\mathrm{log(t_{yr})}=8.6$ and $9.6$ for the stars with $Q_{GK_{s}}>1.7$, the
distance to and foreground extinction toward RSGC4 should be equal to or less
than those of RSGC2. However, as clearly shown in the $(J-H,G-K_{s})$ CCD, the
distance and extinction of RSGC4 are greater than those of RSGC2. We find that
the EAGBs with $\mathrm{log(t_{yr})}=7.8$ can partially explain the red colors
and bright magnitudes of stars in RSGC4 by adjusting the distance and
extinction values ($d$=6.28 kpc and $A_{K_{s}}=1.69$), with only slight
changes. We note that the isochrones from the updated PARSEC code (Bressan et
al., 2012) also yield the same results for the $Q_{GK_{s}}$ values of the RSG
candidates in RSGC4. This indicates that the stars with $Q_{GK_{s}}>1.7$ in
RSGC4 may not be CHeB stars (except RSG with high mass-loss and thick
circumstellar dust), but instead be foreground EAGB stars with initial masses
of at least 6.48 $\mathrm{M_{\odot}}$ at almost the same distance as RSGC4.

Figure 4: The stellar number density maps for the Scutum complex region
constructed using different selection criteria. The selection criteria are
indicated in each panel. The positions of the six RSGCs are indicated by red
circles with labels. The color bars in the each panel show the number density
smoothed by the Gaussian filter.

Assuming that stars with $Q_{GK_{s}}>1.7$ were not RSGs and cluster members,
we investigated the overdensities associated with six clusters in the sky.
Figure 4 shows the stellar density maps for the $\sim 6^{\circ}\times
2^{\circ}$ field of view around six clusters. The density map in the upper
panel was constructed by selecting the stars that met conditions similar to
those (i.e., $K_{s}<8$, $J-K_{s}>2$, and $0.1<Q_{1}<0.5$) established by
Negueruela et al. (2010). Our additional criterion ($Q_{GK_{s}}<1.7$) was
incorporated into the conditions of Negueruela et al. (2010), and the
resulting density map is presented in the bottom panel. We counted the stars
in bins of $1^{\prime}.2\times 1^{\prime}.2$ widths, and then the number of
stars in bins were smoothed by Gaussian filter with a width equivalent to 10
bins. The densities from the background to 6$\sigma$ above the level are
depicted in both panels.

In the upper panel of Figure 4, we found significant overdensities in the
RSGC1, RSGC2, and RSGC3 regions, along with some overdensities in the RSGC5
and Alicante 10 areas. However, a dominant density clump was not found in the
RSGC4 area, which contradicts the finding of a clear overdensity by Negueruela
et al. (2010). In the bottom panel, the clusters, except RSGC4, stand out more
clearly from the surrounding field when the stars with $Q_{GK_{s}}>1.7$ are
discarded. However the clumpy structure of RSGC4 appears to be a random
fluctuation similar to the background field. Therefore, we cautiously suggest
that RSGC4 could be a random coincidence of foreground AGBs and RSGs along the
line of sight.

###  4.2 Binary systems in cluster

Massive stars with initial masses of $\sim 8-30M_{\sun}$ are frequently found
in binary systems (e.g., Kobulnicky & Fryer, 2007; Sana et al., 2013; Dunstall
et al., 2015; Almeida et al., 2017). Interaction with their companion stars
during their lifetimes significantly alters their structures and final fates
(Sana et al., 2012; Kobulnicky et al., 2014). The orbital motion of a binary
system in a cluster can introduce a large scattering in the radial velocity
distribution. Therefore, detecting periodic radial velocity variations in our
RSG candidates would help constrain the binary system in RSGS4. However, this
is a long-term endeavor requiring long-time observations over a few thousands
of days.

Most detected RSG binary systems are composed of RSG+B-type companions (e.g.,
108 Galactic RSG+B binary systems; Pantaleoni González et al., 2020), as
predicted by stellar evolution considerations. The unique spectral features of
B-type companions, such as Balmer absorption lines with additional flux in the
UV spectra of B stars or the H$\alpha$ emissions of Be stars, could be
detected, as already shown in the RSG+B binary spectra obtained by Neugent et
al. (2019). However, detecting the spectral features of the B-type companion
stars in our near-infrared spectra is impossible. Instead, the effect of the
strong interactions between companions can be observed in the spectra. Mass
transfer from previously evolved stars (e.g., RSG+WR, although rare) could
have altered the atmospheric chemical compositions of the RSGs (e.g., Smith &
Lambert, 1988). Significant mass loss toward the companions may have
eliminated the outer envelopes of the RSGs. In these processes, the feature of
chemical pollution that cannot be explained by a single normal RSG evolution
can be detected in some RSG binary systems. Therefore, we searched for any
peculiar spectral features in our high-resolution RSG spectra and compared
them with those for typical RSGs.

Figure 5: Comparison of the $H$-band spectra of RSGC4-8, RSGC4-9 (peculiar
RSGs), and RSGC4-5 (normal RSG). The spectra of V1016 Cyg (D-type) and AG Peg
(S-type) symbiotic stars are also described as orange solid lines to show
similarities in spectral absorption features. Several atomic (Si, Ca, Ti, and
Fe) and molecule (OH, CN, and CO) absorption lines are indicated by vertical
lines of different colors.  Figure 6: Comparison of the $K$-band spectra of
RSGC4-8, RSGC4-9, and RSGC4-5.

Figure 1 shows that the spectra of RSGC4-1, 8, 9, and 12 (hereafter referred
to as peculiar RSG candidates) display different spectral features from those
of the other RSG candidates in RSGC4. We note that the latter RSG candidate
spectra show spectral shapes and absorption features similar to those of the
RSG candidates in the other five clusters. Thus, we consider these RSG
candidates to be representative of normal RSG candidates in the Scutum complex
region. The spectra of RSGC4-1, 8, 9, and 12 have a relatively low CO bandhead
absorptions compared to those of normal RSG candidates in RSGC4, whereas the
atomic absorptions of RSGC4-8 and RSGC4-9 are much deeper than those of the
other normal RSG candidates (see Figures 1, 5 and 6). Although there are
slight differences in the atomic absorption strengths, the spectra of RSGC4-8
and RSGC4-9 closely resemble each other. The spectra of RSGC4-1 and 12
simultaneously show spectral absorption features of both the normal RSG
candidates and RSGC4-8 and 9.

We compared the spectra of peculiar RSG candidates with single normal RSG
model spectra. The several atomic abundances in model spectra were adjusted to
mimic the observed spectra. However, we could not simulate any theoretical
spectra that closely resembled the observed absorptions of the peculiar RSG
candidates. Notably, the spectra of normal RSG candidates were well
represented by single RSG models, although the atomic and molecular linelists
(Afsar et al., 2016) could not perfectly describe the RSGs. Thus, there seem
to be some unknown physical properties of peculiar RSG candidates that cannot
be explained by single RSG models.

We searched the several spectral libraries and the relevant literature to find
spectra similar to those of peculiar RSG candidates. Eventually, we found a
hint that their spectra were similar to the spectra of D-type symbiotic stars
(e.g., V1016 Cyg). Symbiotic stars are interacting binary systems consisting
of an evolved cool giant and a compact companion, typically an AGB star and a
white dwarf. Fortunately, several symbiotic stars were also observed using
IGRINS and are available in the Raw & Reduced IGRINS Spectral Archive
(RRISA)222 https://igrinscontact.github.io/. Thus, we could directly compare
our spectra with those of symbiotic stars.

Figures 5 and 6 show the spectra comparisons of RSGC4-8 and 9 as peculiar RSG
candidates and 5 as a normal RSG candidate in the $H$\- and $K$-bands,
respectively. The spectra of the AG Peg and V1016 Cyg symbiotic stars are also
included as orange solid lines. AG Peg is a normal S-type symbiotic star, and
its spectrum is characterized by a stellar continuum and the absorptions of
normal late-type giants. Therefore, the spectrum of AG Peg do not differ
significantly from the spectrum of RSGC4-5 (i.e., normal RSG candidate), as
shown in Figures 5 and 6.

In contrast, V1016 Cyg is a famous D-type symbiotic star comprising a Mira-AGB
and a white dwarf. This star shows an infrared (IR) excess from the dust
produced by the evolved Mira-AGB variable. Interestingly, in Figures 5 and 6,
the spectrum of V1016 Cyg not only shows entirely different absorption
features from those of AG Peg and RSGC4-5 but also closely traces very well
the peculiar absorption features of RSGC4-8 and 9. The only difference between
V1016 Cyg and the peculiar RSG candidates is the presence of strong emission
lines in V1016 Cyg. It is not clear why our peculiar RSG candidates show no
emission lines, but if there was no shell-burning in the accretion region
around the hot component, weak emission or no emission would be possible
(e.g., SU Lyncis; Mukai et al., 2016). Considering this point, our peculiar
RSG candidates in RSGC4 are not precisely D-type symbiotic stars but seem to
exhibit similar or corresponding properties to D-type symbiotic stars.

Figure 7: The distributions of RSG candidates in six RSGCs in $(Q1,Q2)$,
$(Q2,[3.6]-[4.5])$, and $(K_{s}-[8.0],Q1)$ CCDs. The dashed lines indicate the
criteria of RSG established by Messineo et al. (2012), and the shaded region
in each panel is where most Galactic RSGs are distributed. The peculiar RSG
candidates with ID numbers in RSGC4 are located in the region where IR
excesses are expected.

One of the most prominent properties of D-type symbiotic stars is the IR
excess resulting from dust produced by an evolved Mira-AGB variable. Messineo
et al. (2012) conducted a photometric classification of Galactic evolved
stars, analyzing their distributions in near- and mid-infrared color-color
diagrams using the 2MASS and Galactic Legacy Infrared Midplane Extraordinaire
(GLIMPSE) catalogs. Using the reddening-free $Q1$ and
$Q2$333$Q2=(J-K_{s})-2.69\times(K_{s}-[8.0])$ parameters, GLIMPSE
$[3.6]-[4.5]$, and $K_{s}-[8.0]$ colors, they assigned specific color regions
for objects with IR excess and RSGs. We also examined the same CCDs for the
RSG candidates in our RSGCs to assess their IR excess properties. The
magnitudes in the mid-infrared region of GLIMPSE are provided in Table 2.

Figure 7 shows the distributions of RSG candidates in the six RSGCs in the
(Q1, Q2), (Q2, $[3.6]-[4.5]$), and ($K_{s}-[8.0]$, Q1) CCDs. As determined by
Messineo et al. (2012), the shaded region in each CCD indicates the area where
most Galactic RSGs are distributed; $0.1<Q1<0.5$, $-1.1<Q2<1.5$, and
$Q1>-0.2\times(K_{s}-[8.0])+0.34$. Apparently, most of RSG candidates in the
six RSGCs are situated inside the shaded area of normal RSGs, while three or
four peculiar RSG candidates in RSGC4 are located in the region of IR excess:
$Q1<0.1$, $Q2<-1.1$ (Messineo et al., 2012) and $K_{s}-[8.0]>0.5$ (van Loon et
al., 2003). Therefore, these peculiar RSG candidates are likely to be the
stars with circumstellar materials due to their high mass loss.

The mass-loss rates of RSGs range from $\sim 10^{-7}$ to $\sim
10^{-4}M_{\sun}$yr-1 (e.g., Josselin et al., 2000; Verhoelst et al., 2009),
which are comparable to Mira-AGBs (from $\sim 10^{-7}$ to $\sim
10^{-4}M_{\sun}$yr-1; Habing & Olofsson, 2003) but not maser sources (from
$\sim 10^{-5}$ to $\sim 10^{-3}M_{\sun}$yr-1; Ortiz et al., 2002). We note
that Mira-AGBs and SiO or OH/IR maser stars also have an IR excess and can be
located in the IR-excess areas on the CCDs. However, as mentioned earlier, we
did not detect Mira-AGB variability. No maser sources in this field were
reported by Negueruela et al. (2010). In this respect, the peculiar RSG
candidates in RSGC4 do not appear to be Mira-AGBs or maser sources. They seem
to be the binary systems containing AGB or RSG stars surrounded by dust,
resulting from the significant mass loss by their companions.

We obtained additional IGRINS spectra for some RSG candidates in RSGC4 in 2021
to examine radial velocity changes. The measured velocities are indicated in
Table 1. The radial velocities of seven RSG candidates (RSGC4-1, 2, 3, 4, 5,
6, and 7) in RSGC4, observed in 2019 by Asa’d et al. (2023), are also included
in Table 1. Most of the RSG candidates observed in 2021 and by Asa’d et al.
(2023) show slight velocity changes of 2–3 km $\mathrm{s^{-1}}$, which is
acceptable considering the vigorous atmospheric activity of RSGs.

Interestingly, however, two peculiar RSG candidates with D-type absorption
features in our study (i.e., RSGC4-8 and 9) show different velocity changes.
RSGC4-9 show a large velocity variation of approximately $20$ km
$\mathrm{s^{-1}}$. Typical radial velocity variations due to binary motions
are between 20 km s-1 and 30 km s-1 (e.g., Wright, 1977; Eaton & Shaw, 2007;
Patrick et al., 2019). On the other hand, the velocity variation of RSGC4-8 is
about 5 km s-1 and changes rapidly within a month. This phenomenon contradicts
the long-term variation expected of an RSG binary system. Another peculiar RSG
candidate, RSGC4-1, shows a velocity change of about 5–8 km s-1. Thus, if
these three peculiar RSG candidates are real binary systems, it is highly
likely that these systems contain high-mass AGBs, consistent with the
foreground AGB contamination results described in the previous section. It is
also interesting that RSGC4-3 exhibits the second largest velocity change
(15–17 km s-1) despite its spectrum and photometric colors appearing to be
those of a normal RSG candidate. To conclude on the binarity of this star,
additional radial velocity monitoring is necessary.

Regardless of whether our peculiar RSG candidates are indeed AGBs or genuine
RSGs, we have concluded that only five stars (RSGC4-1, 3, 8, 9, and 12) among
the 12 RSG candidates in RSGC4 have the potential features of binary systems
in terms of spectra, photometric colors, and variations of radial velocity.
Among these, three (RSGC4-3, 8, and 9) are confirmed binary systems. When we
rejected these five RSG candidates in the cluster, there still remained a
large scattering of 46 km s-1 in the velocity distribution. Although
additional multi-epoch observations for other RSG candidates are needed, it is
apparent that the effect of binary motion alone cannot fully explain the large
scattering in the radial velocity distribution and the offset from the mean
velocity of the RSG candidates in the RSGCs.

###  4.3 Unbounded stars in the cluster

Figure 8: Left: Relative spatial distribution of RSG candidates in RSGC4 from
the cluster center. The relative PM vectors are also indicated by arrows. The
dotted circle indicates the region within $3^{\prime}$ of the cluster center.
The ID numbers of candidates are indicated, and the approximate motions of
candidates based on $\Phi_{\mathrm{rel}}$ are indicated in parentheses (see
text in Section 4.3). Right: Distribution of $\Phi_{\mathrm{rel}}$ as a
function of $\mu_{\mathrm{rel}}$ for RSG candidates in RSGC4. The potential
binary stars and the stars with RUWE $<$ 1.4 are indicated by open squares and
open circles, respectively. The red dashed lines indicate the angle criteria
($\pm 70\arcdeg$ and $\pm 110\arcdeg$) used to determine the approximate
motions of RSG candidates. The KDE plot of $\Phi_{\mathrm{rel}}$ is shown in
the right-hand panel.

A large offset velocity of RSG candidates in RSGC4 of more than 30 km s-1
compared to the mean velocity of RSGs in the Scutum complex reminds us of
potential runaway or walkaway stars (Blaauw, 1956, 1961; Gies & Bolton, 1986;
Dray et al., 2005; Eldridge et al., 2011; Boubert & Evans, 2018). Given that a
significant fraction (10%–50%) of OB main-sequence stars are runaway stars
(Gies & Bolton, 1986), and most of them undergo the RSG evolutionary phases,
we expect that relatively numerous RSGs would be found as runaway or walkaway
stars. However, very few runaway RSGs with peculiar velocities have been found
so far in the Milky Way: Betelgeuse at 56 km s-1 (Noriega-Crespo et al.,
1997), $\mu$ Cep at 22 km s-1 (Cox et al., 2012), and IRC-10414 at 70 km s-1
(Gvaramadze et al., 2014).

The RSG candidates in RSGC4 are probably runaway stars with low velocities
(Gvaramadze et al., 2012), or simply escaping stars from the cluster. Since
one-dimensional radial velocity alone cannot precisely describe the motion of
stars, we examined the proper motion (PM) of RSG candidates in RSGC4 from
$Gaia$ DR3 data. We identified 12 RSG candidates in RSGC4 from $Gaia$ DR3
data, and the proper motions are presented in Table 1. If the several RSG
candidates in RSGC4 are real runaway or walkaway stars, they may be placed in
the outer region of the cluster, and the moving direction of the stars could
show outward movement from the center of the cluster.

The relative spatial distributions of the RSG candidates in RSGC4 from the
center of the cluster are plotted in the left panel of Figure 8. The relative
PM vectors of stars to the median PM ($\mu_{\alpha,\mathrm{med}}$cos
$\delta_{\mathrm{med}}=-1.334$ mas yr-1 and $\mu_{\delta,\mathrm{med}}=-5.417$
mas yr-1) of RSG candidates are shown as red arrows in the left panel of
Figure 8. We have indicated the direction of the relative PM vectors, and the
length of the arrows does not represent the magnitude of the velocity. The
center of the cluster was determined by the median value of the sky positions
of the RSG candidates.

In Figure 8, five RSG candidates (RSGC4-6, 9, 10, 11, and 12) appear to be
located more than $3^{\prime}$ from the cluster center, while seven RSG
candidates are closely grouped together within $3^{\prime}$ of the center,
forming a clump. We note that the potential binary stars in RSGC4 (open
squares) have large Renormalised Unit Weight Error (RUWE) values ($>1.4$) of
$Gaia$ DR3 data. The relative PM vectors seem to show inward motions,
contradicting the expected outward motion of runaway or walkaway stars.

To investigate inward motion in detail, we calculated the orientation angles
of relative PM vectors, ($\Phi_{\mathrm{rel}}$), between the radial vector of
a given star from the center of the cluster and its relative PM vector. We
described this angle as a function of the amplitude of the relative PM vectors
in the right panel of Figure 8. The amplitude of the relative PM vectors was
calculated using the following equation:

$\mu_{\mathrm{rel}}=\sqrt{(\mu_{\alpha}\cos\delta-\mu_{\alpha,\mathrm{med}}\cos\delta_{\mathrm{med}})^{2}+(\mu_{\delta}-\mu_{\delta,\mathrm{med}})^{2}}.$
(1)

The value of $\Phi_{\mathrm{rel}}$ describes the overall expansion or
contraction motion of stars in the cluster, as demonstrated by Lim et al.
(2019); stars with $\Phi_{\mathrm{rel}}=0\arcdeg$ have expansion motion, while
those with $\pm 180\arcdeg$ have contraction motion.

In the right panel of Figure 8, it is difficult to decide whether the stars
have expansion or contraction motions due to the large errors. However, the
RSG candidates in RSGC4 seem to be distributed around $-150\arcdeg$ and
80$\arcdeg$ of $\Phi_{\mathrm{rel}}$ in Kernel Density Estimate (KDE) plot.
The bimodal distribution in $\Phi_{\mathrm{rel}}$ suggests marginal inward
motions of RSG candidates in RSGC4. Based on $\Phi_{\mathrm{rel}}$, we
determined approximate motions of RSG candidates as following criteria:

  * •

$|\Phi_{\mathrm{rel}}|>110\arcdeg$ : inward motion (‘I’)

  * •

$70\arcdeg<|\Phi_{\mathrm{rel}}|<110\arcdeg$ : tangential motion (‘T’)

  * •

$|\Phi_{\mathrm{rel}}|<70\arcdeg$ : outward motion (‘O’)

  * •

Undecided due to large error : (‘?’)

Determined motions are indicated in the left panel of Figure 8.

According to $\Phi_{\mathrm{rel}}$, there are 4 stars exhibiting inward
motions, 4 stars showing tangential motions, and 2 stars displaying outward
motions. The bimodal distribution of $\Phi_{\mathrm{rel}}$ in the KDE plot
represents the stars with tangential and inward motions. We do not find any
spatial dependence among stars with tangential and inward motions. The stars
concentrated within $3^{\prime}$ of the center, as well as those outside of
it, exhibit various motion patterns. Among the four RSG candidates with
peculiar absorption lines, only one star (RSGC4-12) exhibits inward motion,
while RSGC4-1, 8, and 9 show tangential motions to the radial vector from the
center of the cluster. We find that only two stars (RSGC4-7 and 11) among 12
RSG candidates in RSGC4 are likely to show outward motion. We also
investigated the $\Phi_{\mathrm{rel}}$ distributions of RSG candidates in
other clusters; notably, we found no distinct expansion or contraction motions
of RSG candidates in the other clusters.

The marginal inward and tangential motions of the stars indicates that the RSG
candidates in RSGC4 are neither the runaway nor walkaway stars from the
cluster. Considering the young age of $\sim$20 Myr for this cluster
(Negueruela et al., 2010), the inward motions and concentrations of some RSG
candidates could indicate mass segregation. The absence of main-sequence stars
(e.g. Froebrich & Scholz, 2013) and the more spatially extended feature of
this cluster compared to others may support the idea of mass segregation in
this cluster. However, in this case, the reason for the large spread in the
radial velocity of the bounded RSG candidates resulting from mass segregation
remains unclear.

###  4.4 Environmental effects

Figure 9: Left: Action space map of normalized
($J_{\mathrm{hor}},J_{\mathrm{ver}}$) for the RSG candidates in the six RSGCs.
The RSG candidates in RSGC4 with low $J_{\mathrm{hor}}$ and $J_{\mathrm{ver}}$
are indicated in the subplot of the figure. The stars with RUWE $<$ 1.4 are
indicated by open circles. The potential binary stars in RSGC4 are indicated
by open squares. The red dashed line shows the limits of the action space map.
Right: Map of inclination against eccentricity for the RSG candidates in the
six RSGCs. The RSG candidates in RSGC4 with high eccentricity are shown in the
subplot.

The close proximity of RSGCs to the junction where the spiral arms meet the
Galactic bar naturally leads us to consider the environmental effects of the
unusual radial velocities of the RSG candidates in RSGC4. The spiral arms or
bars of the galaxy are places where inflowing gaseous material and stars from
the outer disk are easily accumulated. Thus, cloud-cloud collisions are
expected in these regions. These collisions inevitably lead to strong star
formation (Loren, 1976; Tan, 2000; Sheth et al., 2005; Inoue & Fukui, 2013)
and massive star formation (Anathpindika, 2010; Motte et al., 2014).

Particularly at the Lindblad resonance radii, where the bar intersects with
the spiral arm, stars and gas experience shocks and loss angular momentum
(Combes & Gerin, 1985). Consequently, the material migrates toward the central
regions, yielding a ring-like structure (e.g., Schwarz, 1981; Shlosman et al.,
1990; Athanassoula, 1992) characterized by intense star formation and nuclear
starburst (e.g., Knapen et al., 1995; Wong & Blitz, 2000; Mazzuca et al.,
2008). Indeed, the presence of numerous massive stars (López-Corredoira et
al., 1999), RSGC clusters (Figer et al., 2006; Davies et al., 2007; Clark et
al., 2009), and stellar and H II regions with ring-like structures (Bertelli
et al., 1995; Davies et al., 2009) at the outer Lindblad resonance of the
Galactic bar indicates recent intense starburst activity in this region.

In this respect, the tip of the bar is a dynamically complex structure, and
the young stars and massive star clusters formed in this region are
significantly influenced by their surroundings. For example, the tidal force
from the bar potential or shocks resulting from encounters with dense giant
molecular clouds can have a strong impact on the morphology and dynamic
evolution of these clusters, sometimes leading to their complete dissolution
(Spitzer, 1958; Gnedin & Ostriker, 1999; Gieles et al., 2006).

Recently, Whitmore et al. (2023) provided new insights into the formation of
young massive star clusters in the bar region using JWST observations of NGC
1365. They reported that the collision of material streamers with gas in the
bar and overshooting stars from the opposite bar lane play crucial roles in
star cluster formation. On the other side, it is also known that the formation
of spiral arms and bars in the Milky Way is influenced by the merging and
accretion of gas-rich dwarf galaxies (Purcell et al., 2011). Several
theoretical results (Schweizer, 1987; Ashman & Zepf, 1992) and observational
evidence (van den Bergh, 1971; Arp & Sandage, 1985; Larsen, 2006; Roche et
al., 2015) suggest that such mergers lead to the formation of massive star
clusters. Therefore, the peculiar radial velocity properties of the RSG
candidates in RSGC4 could be the result of secular evolution influenced by
environmental factors.

Based on the radial velocities, distances to the clusters from literature (see
Section 4.1), and proper motions of $Gaia$ DR3 data, we derived the actions
and orbital properties of RSG candidates in the six RSGCs using Agama galaxy
modeling architecture (Vasiliev, 2019) to investigate the dynamical properties
affected by their surroundings. We utilized the potential of McMillan (2017)
and the Stäckel fudge approach (Binney, 2012). The map of normalized
horizontal action $J_{\mathrm{hor}}=J_{\mathrm{\phi}}/J_{\mathrm{tot}}$ and
vertical action
$J_{\mathrm{ver}}=(J_{\mathrm{z}}-J_{\mathrm{R}})/J_{\mathrm{tot}}$, where
$J_{\mathrm{tot}}$ is the sum of absolute values of
$J_{\mathrm{R}},J_{\mathrm{\phi}}$ and $J_{\mathrm{z}}$ (see e.g., Myeong et
al., 2019), is shown in the left panel of Figure 9.

The left panel of Figure 9 shows that most of the RSG candidates, except in
RSGC1 and RSGC4, have disk-like motions with a horizontal action of
$J_{\mathrm{hor}}\sim 1.0$ and a vertical action of $J_{\mathrm{ver}}\sim
0.0$, consistent with the expected behavior of stars in the disk. In contrast,
the RSG candidates in RSGC4 show a large scattering in both the
$J_{\mathrm{hor}}$ and $J_{\mathrm{ver}}$ distributions. Interestingly, they
have relatively low values for both $J_{\mathrm{hor}}$ and $J_{\mathrm{ver}}$
compared to the other RSG candidates. Other interesting features are
unexpected scattering and the relatively large vertical actions of the RSG
candidates in RSGC1. These dynamical properties are also clearly shown in the
eccentricity and inclination in the right panel of Figure 9. Half of RSG
candidates in RSGC4 show eccentricities greater than 0.6, and the majority
have inclinations of more than $5^{\circ}$. Most RSG candidates in RSGC1 have
a large inclination of more than $10^{\circ}$ compared to the other RSG
candidates. Although several RSG candidates have a RUWE $>$1.4 and more
accurate astrometric solutions are necessary, it is important to note that the
current action values, eccentricities, and inclinations of the RSG candidates
possess a certain level of reliability. For example, as expected, a large
number of RSG candidates with RUWE$>$1.4 in other clusters still show disk-
like motions, and most RSG candidates in RSGC1 have RUWE values less than 1.4.
We confirmed that the dynamical properties of each cluster, indicated by the
action values, remained relatively stable even when the distance to each
cluster or proper motion was changed, with offset values applied.

The vertical motion of RSGC1 and the high eccentricity of RSGC4 might indicate
dynamical evolution resulting from the environmental effects that the clusters
experienced during their formation in single-star forming event. Encounters
with dense and giant molecular clouds in the disk could influence the
dynamical evolution of clusters, increasing their vertical motion. Tidal
forces from bar potential or local gaseous substructures (e.g., Elmegreen &
Hunter, 2010) in the Scutum complex region can contribute to the disruption of
clusters.

Park et al. (2020), in their $\sim 2$ Myr timespan simulation, have shown that
a strong tidal field can dissolve a single star-forming cloud, resulting in
several bound star clusters that contain an overabundance of massive stars of
$20~{}\mathrm{M}_{\sun}$. Notably, the distribution of star clusters in their
simulation (see the rightmost panel of their Figure 5) closely resembles the
distribution of RSGCs in the sky shown in Figure 4. The small, sparse group of
massive stars labeled ‘2b’ in their Figure 5 could be a candidate for RSGC4.
However, it remains unclear how these environmental factors could selectively
affect only two out of the six clusters at almost the same distance, even
leading to different dynamic evolutionary properties. To get a better
understanding, properties of stars in clusters, such as the velocities of
massive stars in clusters, are necessary from additional simulation spanning
20 Myr. Without a detailed comparison with simulation results, it is more
likely that for the RSG candidates in RSGC4, stars moving toward the bar tip
along the bar lane overshoot the bar tip, and then overlap with the stars
traveling along the arms or gas streamers. In this scenario, the stars moving
along the bar lane appear to be moving toward us, while the stars traveling
along the arm seem to be moving away from us. This interpretation is supported
by the discovery of massive stars with ages of 10–20 Myr in several overshoot
regions and the NE bridge feature of NGC 1365 using JWST (Whitmore et al.,
2023).

Additionally, the distribution of a significant number of RSGCs in the narrow
Scutum complex region raises the possibility that RSGC4 might be a binary
cluster or an interacting cluster. Previous studies by Rozhavskii et al.
(1976) and Subramaniam et al. (1995) suggested that 8–20% of open clusters
born in star complex regions could be binary clusters. Resonant trapping in
the Scutum complex region can potentially lead to the formation of interacting
or merging star clusters (de La Fuente Marcos & de La Fuente Marcos, 2009).
Therefore, the distinct radial velocities and action values observed for the
RSG candidates in RSGC4 could be attributed to such interactions.

In binary cluster scenario, bimodalities or clumpy structures (Kuhn et al.,
2014; Darma et al., 2019) in the spatial positions of cluster member stars can
be expected. Although cluster pairs are likely to share common kinematics,
detecting slightly different motions among member stars belonging to different
clusters is plausible, considering their young ages. Despite these
expectations, we could not find apparent bimodalities in the spatial positions
and motions, including radial velocities, proper motions, and even action
values, of the RSG candidates in RSGC4. To confirm the binary cluster
scenarios for RSGC4, conducting additional statistical tests on the spatial
and motion properties of main-sequence members within the cluster would be
helpful.

##  5 Summary and conclusion

In this study, we obtained high-resolution ($R\sim 45,000$) near-infrared
spectra for 60 RSG candidates in six RSGCs in the Scutum complex region, where
the Scutum-Crux arm intersects the tip of the Galactic bar, using IGRINS at
the Gemini South telescope. From their spectra, we find that the RSG
candidates in RSGC4 share no common radial velocities, unlike the consistent
radial velocities found in the other cluster member stars. The RSG candidates
in RSGC4 show significant scattering in the radial velocity distribution and
an offset in the mean velocity compared with the mean radial velocity of the
RSG candidates in the other RSGCs. We have discussed four explanations (AGB
contamination, binary systems, unbounded stars, and environmental effects) to
interpret the unusual radial velocities observed in the RGS candidates of the
RSGC4.

AGB contamination was checked by investigating the photometric variations and
the new reddening-free parameter, $Q_{GK_{s}}$. We find no significant
differences between the magnitudes of 2MASS and DENIS. Definite variability
similar to that of AGB stars was not detected in WISE/NEOWISE data due to the
saturation of RSG candidates. However, a lower limit of the amplitudes of
variation, approximately 1.0 mag in both W1 and W2, was just detected.
Interestingly, we find that most RSG candidates in RSGC4 have $Q_{GK_{s}}$
values greater than 1.7, which is the limit that stars in the CHeB
evolutionary phase cannot reach. Stars in the early AGB or thermal pulsing AGB
phases can exceed this limit and have large values of $Q_{GK_{s}}$. Indeed, we
find that most RSG candidates in the other RSGCs have $Q_{GK_{s}}$ values
below this limit. Therefore, it is possible that several RSG candidates in
RSGC4 could be massive AGB stars, although we cannot rule out the presence of
RSGs surrounded by dust.

We find that the spectra of RSGC4-1, 8, 9, and 12 have different absorption
features from those of the other stars in the clusters. The peculiar
absorptions in their spectra cannot be explained by single normal RSG model
spectra, suggesting the possibility that these stars are binary systems.
Indeed, the spectra of RSGC4-8 and 9 are very similar to those of D-type
symbiotic stars (AGB + white dwarf), except for the absence of emission lines.
Furthermore, these two stars exhibit large or fast radial velocity variations,
providing further evidence of their binary nature. In several color-color
diagrams, we also find that four RSG candidates with peculiar absorption
features have IR excesses similar to D-type symbiotic stars. We also discover
radial velocity changes in RSGC4-1 and 3 through multi-epoch observations.
However, even if we excluded these five binary systems from the 12 RSG
candidates in RSGC4, significant scattering remained in the velocity
distribution. Therefore, the binary systems cannot fully explain the peculiar
radial velocities found in the RSGC4.

We investigated the relative spatial distributions, relative proper motions,
and their orientation angles ($\Phi_{\mathrm{rel}}$) of the RSG candidates in
RSGC4 to determine whether there were runaway or walkway stars in the cluster.
Despite the limited number of RSG candidates, 7 of 12 stars are located within
$3^{\prime}$ of the center of the cluster. Slight bimodal distributions in the
orientation angles of the relative proper motions indicate marginal inward and
tangential motions of the RSG candidates in RSGC4. We did not observe any
spatial dependence among stars with tangential and inward motions. Therefore,
the RSG candidates in RSGC4 are neither the runaway nor walkway stars
influenced by supernovae events.

The environmental effects on the clusters were investigated by calculating the
$J_{hor}$ and $J_{ver}$ action values as well as orbit parameters, such as
eccentricity and inclination. We find that most of the RSG candidates in the
clusters, except in RSGC1 and RSGC4, show circular disk-like motions and low
inclination, as anticipated. In contrast, RSGC4 exhibits relatively low
$J_{hor}$ and $J_{ver}$ values and high eccentricity. RSGC1 has vertical
motion with relatively high $J_{ver}$ values and high inclination compared
with the other RSGCs.

Encounters with giant molecular clouds in the disk or the tidal force of bar
potentials can induce increased vertical motion of clusters or disruption of
clusters. However, it remains unclear how the various differences in dynamical
evolutionary properties among the clusters can result from the same
environmental effects. For RSG candidates in RSGC4, it is more likely that the
stars with radial motions toward the bar tip along the bar lane overshoot and
then overlap with the stars traveling along the arms or gas streamers. We also
considered binary or interacting clusters as the origin of the peculiar radial
velocities and action values of RSGC4. However, no apparent bimodalities were
identified in the spatial positions and motions of the RSG candidates in
RSGC4.

Overall, based on our analysis (the possibility of massive AGB stars, some
binary systems of AGB-like stars, no runaway/walkaway stars, and dynamical
evolutionary properties), we tentatively conclude that RSGC4 may not be a
genuine star cluster dominated by RSGs. Instead, it could be a complex
assembly of stars, including RSGs and AGBs distributed along the line of sight
at approximately the same distance. These stars might originate from diverse
environments, such as spiral arms and bar lanes. The vertical motions of the
RSG candidates in RSGC1 are also noteworthy. Although RSGC4 might not be a
star cluster, its dynamic properties, along with those of RSGC1 and other
RSGCs with disk-like motions, suggest a complicated and hierarchical secular
evolution of star clusters in the Scutum complex region. In addition, our
results caution against relying solely on density crowding to identify
clusters in the disk and bulge regions. Further investigation of the chemical
abundances of RSGs and field massive stars, and detailed comparisons based on
simulation, will provide keys to constraining whether RSGC4 is a true cluster
and understanding the secular evolution in this region.

We are grateful to the anonymous referee for detailed comments and suggestions
that greatly improved this paper. S.H.C. acknowledges support from the
National Research Foundation of Korea (NRF) grant funded by the Korea
government (MSIT) (NRF-2021R1C1C2003511) and the Korea Astronomy and Space
Science Institute under R&D program (Project No. 2023-1-830-00) supervised by
the Ministry of Science and ICT. This work was supported by K-GMT Science
Program (PID: GS-2018A-Q-418 and GS-2021A-Q-417) of Korea Astronomy and Space
Science Institute (KASI). This work used the Immersion Grating Infrared
Spectrometer (IGRINS) that was developed under a collaboration between the
University of Texas at Austin and the KASI with the financial support of the
Mt. Cuba Astronomical Foundation, of the US National Science Foundation under
grants AST-1229522 and AST-1702267, of the McDonald Observatory of the
University of Texas at Austin, of the Korean GMT Project of KASI, and Gemini
Observatory. Based on observations obtained at the international Gemini
Observatory, a program of NSF’s NOIRLab, which is managed by the Association
of Universities for Research in Astronomy (AURA) under a cooperative agreement
with the National Science Foundation on behalf of the Gemini Observatory
partnership: the National Science Foundation (United States), National
Research Council (Canada), Agencia Nacional de Investigación y Desarrollo
(Chile), Ministerio de Ciencia, Tecnología e Innovación (Argentina),
Ministério da Ciência, Tecnologia, Inovações e Comunicações (Brazil), and
Korea Astronomy and Space Science Institute (Republic of Korea).

## References

  * Afsar et al. (2016) Afsar, M., Sneden, C., Frebel, A., et al. 2016, ApJ, 819, 103
  * Alard et al. (2001) Alard, C., Blommaert, J. A. D. L., Cesarsky, C., et al. 2001, ApJ, 552, 289
  * Almeida et al. (2017) Almeida, L. A., Sana, H., Taylor, W., et al. 2017, A&A, 598, A84
  * Anathpindika (2010) Anathpindika, S. V. 2010, MNRAS, 405, 1431
  * Arp & Sandage (1985) Arp, H., & Sandage, A. 1985, AJ, 90, 1163
  * Asa’d et al. (2023) Asa’d, R., Ivanov, V. D., Negueruela, I., et al. 2023, AJ, 165, 212
  * Ashman & Zepf (1992) Ashman, K. M., & Zepf, S. E. 1992, ApJ, 384, 50
  * Athanassoula (1992) Athanassoula, E. 1992, MNRAS, 259, 345
  * Banerjee et al. (2012) Banerjee, S., Kroupa, P., & Oh, S. 2012, ApJ, 746, 15
  * Bastian et al. (2005) Bastian, N., Gieles, M., Efremov, Y. N., & Lamers, H. J. G. L. M. 2005, A&A, 443, 79
  * Baumgardt et al. (2008) Baumgardt, H., Kroupa, P., & Parmentier, G. 2008, MNRAS, 384, 1231
  * Bertelli et al. (1995) Bertelli, G., Bressan, A., Chiosi, C., Ng, Y. K., & Ortolani, S. 1995, A&A, 301, 381
  * Binney (2012) Binney, J. 2012, MNRAS, 426, 1324
  * Blaauw (1956) Blaauw, A. 1956, PASP, 68, 495
  * Blaauw (1961) —. 1961, Bull. Astron. Inst. Netherlands, 15, 265
  * Bono et al. (2013) Bono, G., Matsunaga, N., Inno, L., Lagioia, E. P., & Genovali, K. 2013, in Cosmic Rays in Star-Forming Environments, ed. D. F. Torres & O. Reimer, Vol. 34, 115
  * Boubert & Evans (2018) Boubert, D., & Evans, N. W. 2018, MNRAS, 477, 5261
  * Bressan et al. (2012) Bressan, A., Marigo, P., Girardi, L., et al. 2012, MNRAS, 427, 127
  * Brooke et al. (2016) Brooke, J. S. A., Bernath, P. F., Western, C. M., et al. 2016, J. Quant. Spec. Radiat. Transf., 168, 142
  * Camargo et al. (2016) Camargo, D., Bica, E., & Bonatto, C. 2016, MNRAS, 455, 3126
  * Choi et al. (2016) Choi, J., Dotter, A., Conroy, C., et al. 2016, ApJ, 823, 102
  * Chun (2020) Chun, S.-H. 2020, in Star Clusters: From the Milky Way to the Early Universe, ed. A. Bragaglia, M. Davies, A. Sills, & E. Vesperini, Vol. 351, 189–191
  * Chun et al. (2023) Chun, S.-H., Oh, H., Myeong, G., & Lee, J.-J. 2023, in American Astronomical Society Meeting Abstracts, Vol. 55, American Astronomical Society Meeting Abstracts, 402.01
  * Clark et al. (2009) Clark, J. S., Negueruela, I., Davies, B., et al. 2009, A&A, 498, 109
  * Combes & Gerin (1985) Combes, F., & Gerin, M. 1985, A&A, 150, 327
  * Cox et al. (2012) Cox, N. L. J., Kerschbaum, F., van Marle, A. J., et al. 2012, A&A, 537, A35
  * Cutri et al. (2003) Cutri, R. M., Skrutskie, M. F., van Dyk, S., et al. 2003, 2MASS All Sky Catalog of point sources.
  * Darma et al. (2019) Darma, R., Arifyanto, M. I., & Kouwenhoven, M. B. N. 2019, in Journal of Physics Conference Series, Vol. 1231, Journal of Physics Conference Series, 012028
  * Davies et al. (2007) Davies, B., Figer, D. F., Kudritzki, R.-P., et al. 2007, ApJ, 671, 781
  * Davies et al. (2008) Davies, B., Figer, D. F., Law, C. J., et al. 2008, ApJ, 676, 1016
  * Davies et al. (2009) Davies, B., Origlia, L., Kudritzki, R.-P., et al. 2009, ApJ, 696, 2014
  * de La Fuente Marcos & de La Fuente Marcos (2009) de La Fuente Marcos, R., & de La Fuente Marcos, C. 2009, A&A, 500, L13
  * Dray et al. (2005) Dray, L. M., Dale, J. E., Beer, M. E., Napiwotzki, R., & King, A. R. 2005, MNRAS, 364, 59
  * Dunstall et al. (2015) Dunstall, P. R., Dufton, P. L., Sana, H., et al. 2015, A&A, 580, A93
  * Eaton & Shaw (2007) Eaton, J. A., & Shaw, F. G. 2007, AJ, 133, 2669
  * Eldridge et al. (2011) Eldridge, J. J., Langer, N., & Tout, C. A. 2011, MNRAS, 414, 3501
  * Elmegreen & Hunter (2010) Elmegreen, B. G., & Hunter, D. A. 2010, ApJ, 712, 604
  * Epchtein et al. (1999) Epchtein, N., Deul, E., Derriere, S., et al. 1999, VizieR Online Data Catalog, II/240
  * Figer et al. (2006) Figer, D. F., MacKenty, J. W., Robberto, M., et al. 2006, ApJ, 643, 1166
  * Froebrich & Scholz (2013) Froebrich, D., & Scholz, A. 2013, MNRAS, 436, 1116
  * Fujii & Portegies Zwart (2011) Fujii, M. S., & Portegies Zwart, S. 2011, Science, 334, 1380
  * Gaia Collaboration et al. (2022) Gaia Collaboration, Vallenari, A., Brown, A. G. A., et al. 2022, arXiv e-prints, arXiv:2208.00211
  * Garzón et al. (1997) Garzón, F., López-Corredoira, M., Hammersley, P., et al. 1997, ApJ, 491, L31
  * Gieles et al. (2006) Gieles, M., Portegies Zwart, S. F., Baumgardt, H., et al. 2006, MNRAS, 371, 793
  * Gies & Bolton (1986) Gies, D. R., & Bolton, C. T. 1986, ApJS, 61, 419
  * Gnedin & Ostriker (1999) Gnedin, O. Y., & Ostriker, J. P. 1999, ApJ, 513, 626
  * González-Fernández & Negueruela (2012) González-Fernández, C., & Negueruela, I. 2012, A&A, 539, A100
  * Groenewegen (2022) Groenewegen, M. A. T. 2022, A&A, 659, A145
  * Groenewegen et al. (2009) Groenewegen, M. A. T., Sloan, G. C., Soszyński, I., & Petersen, E. A. 2009, A&A, 506, 1277
  * Gustafsson et al. (2008) Gustafsson, B., Edvardsson, B., Eriksson, K., et al. 2008, A&A, 486, 951
  * Gvaramadze et al. (2014) Gvaramadze, V. V., Menten, K. M., Kniazev, A. Y., et al. 2014, MNRAS, 437, 843
  * Gvaramadze et al. (2012) Gvaramadze, V. V., Weidner, C., Kroupa, P., & Pflamm-Altenburg, J. 2012, MNRAS, 424, 3037
  * Habing & Olofsson (2003) Habing, H. J., & Olofsson, H. 2003, Asymptot. giant branch Stars
  * Inoue & Fukui (2013) Inoue, T., & Fukui, Y. 2013, ApJ, 774, L31
  * Ivanov et al. (2002) Ivanov, V. D., Borissova, J., Pessev, P., Ivanov, G. R., & Kurtev, R. 2002, A&A, 394, L1
  * Josselin et al. (2000) Josselin, E., Blommaert, J. A. D. L., Groenewegen, M. A. T., Omont, A., & Li, F. L. 2000, A&A, 357, 225
  * Kiss et al. (2006) Kiss, L. L., Szabó, G. M., & Bedding, T. R. 2006, MNRAS, 372, 1721
  * Knapen et al. (1995) Knapen, J. H., Beckman, J. E., Shlosman, I., et al. 1995, ApJ, 443, L73
  * Kobulnicky & Fryer (2007) Kobulnicky, H. A., & Fryer, C. L. 2007, ApJ, 670, 747
  * Kobulnicky et al. (2014) Kobulnicky, H. A., Kiminki, D. C., Lundquist, M. J., et al. 2014, ApJS, 213, 34
  * Kroupa et al. (2001) Kroupa, P., Aarseth, S., & Hurley, J. 2001, MNRAS, 321, 699
  * Kuhn et al. (2014) Kuhn, M. A., Feigelson, E. D., Getman, K. V., et al. 2014, ApJ, 787, 107
  * Kurucz (2011) Kurucz, R. L. 2011, Canadian Journal of Physics, 89, 417
  * Lada & Lada (2003) Lada, C. J., & Lada, E. A. 2003, ARA&A, 41, 57
  * Lançon & Wood (2000) Lançon, A., & Wood, P. R. 2000, A&AS, 146, 217
  * Larsen (2006) Larsen, S. S. 2006, in Planets to Cosmology: Essential Science in the Final Years of the Hubble Space Telescope, ed. M. Livio & S. Casertano, Vol. 18, 35
  * Lee et al. (2017) Lee, J.-J., Gullikson, K., & Kaplan, K. 2017, igrins/plp 2.2.0, Zenodo, doi:10.5281/zenodo.845059
  * Lim et al. (2019) Lim, B., Nazé, Y., Gosset, E., & Rauw, G. 2019, MNRAS, 490, 440
  * López-Corredoira et al. (1999) López-Corredoira, M., Garzón, F., Beckman, J. E., et al. 1999, AJ, 118, 381
  * Loren (1976) Loren, R. B. 1976, ApJ, 209, 466
  * Mace et al. (2018) Mace, G., Sokal, K., Lee, J.-J., et al. 2018, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 10702, Ground-based and Airborne Instrumentation for Astronomy VII, ed. C. J. Evans, L. Simard, & H. Takami, 107020Q
  * Magrini et al. (2009) Magrini, L., Sestito, P., Randich, S., & Galli, D. 2009, A&A, 494, 95
  * Mainzer et al. (2011) Mainzer, A., Bauer, J., Grav, T., et al. 2011, ApJ, 731, 53
  * Mainzer et al. (2014) Mainzer, A., Bauer, J., Cutri, R. M., et al. 2014, ApJ, 792, 30
  * Mazzuca et al. (2008) Mazzuca, L. M., Knapen, J. H., Veilleux, S., & Regan, M. W. 2008, ApJS, 174, 337
  * McMillan (2017) McMillan, P. J. 2017, MNRAS, 465, 76
  * Meisner et al. (2023) Meisner, A. M., Caselden, D., Schlafly, E. F., & Kiwy, F. 2023, AJ, 165, 36
  * Messineo et al. (2004) Messineo, M., Habing, H. J., Menten, K. M., Omont, A., & Sjouwerman, L. O. 2004, A&A, 418, 103
  * Messineo et al. (2012) Messineo, M., Menten, K. M., Churchwell, E., & Habing, H. 2012, A&A, 537, A10
  * Messineo et al. (2014) Messineo, M., Zhu, Q., Ivanov, V. D., et al. 2014, A&A, 571, A43
  * Minniti et al. (2010) Minniti, D., Lucas, P. W., Emerson, J. P., et al. 2010, New A, 15, 433
  * Moe & Di Stefano (2017) Moe, M., & Di Stefano, R. 2017, ApJS, 230, 15
  * Motte et al. (2014) Motte, F., Nguyên Luong, Q., Schneider, N., et al. 2014, A&A, 571, A32
  * Mukai et al. (2016) Mukai, K., Luna, G. J. M., Cusumano, G., et al. 2016, MNRAS, 461, L1
  * Myeong et al. (2019) Myeong, G. C., Vasiliev, E., Iorio, G., Evans, N. W., & Belokurov, V. 2019, MNRAS, 488, 1235
  * Negueruela et al. (2011) Negueruela, I., González-Fernández, C., Marco, A., & Clark, J. S. 2011, A&A, 528, A59
  * Negueruela et al. (2010) Negueruela, I., González-Fernández, C., Marco, A., Clark, J. S., & Martínez-Núñez, S. 2010, A&A, 513, A74
  * Neugent et al. (2019) Neugent, K. F., Levesque, E. M., Massey, P., & Morrell, N. I. 2019, ApJ, 875, 124
  * Noriega-Crespo et al. (1997) Noriega-Crespo, A., van Buren, D., Cao, Y., & Dgani, R. 1997, AJ, 114, 837
  * Oh & Kroupa (2016) Oh, S., & Kroupa, P. 2016, A&A, 590, A107
  * Oh et al. (2015) Oh, S., Kroupa, P., & Pflamm-Altenburg, J. 2015, ApJ, 805, 92
  * Origlia et al. (2013) Origlia, L., Oliva, E., Maiolino, R., et al. 2013, A&A, 560, A46
  * Origlia et al. (2016) Origlia, L., Oliva, E., Sanna, N., et al. 2016, A&A, 585, A14
  * Origlia et al. (2019) Origlia, L., Dalessandro, E., Sanna, N., et al. 2019, A&A, 629, A117
  * Ortiz et al. (2002) Ortiz, R., Blommaert, J. A. D. L., Copet, E., et al. 2002, A&A, 388, 279
  * Pantaleoni González et al. (2020) Pantaleoni González, M., Maíz Apellániz, J., Barbá, R. H., & Negueruela, I. 2020, Research Notes of the American Astronomical Society, 4, 12
  * Park et al. (2014) Park, C., Jaffe, D. T., Yuk, I.-S., et al. 2014, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 9147, Ground-based and Airborne Instrumentation for Astronomy V, 91471D
  * Park et al. (2020) Park, S.-M., Goodwin, S. P., & Kim, S. S. 2020, MNRAS, 494, 325
  * Patrick et al. (2019) Patrick, L. R., Lennon, D. J., Britavskiy, N., et al. 2019, A&A, 624, A129
  * Perets & Šubr (2012) Perets, H. B., & Šubr, L. 2012, ApJ, 751, 133
  * Poveda et al. (1967) Poveda, A., Ruiz, J., & Allen, C. 1967, Boletin de los Observatorios Tonantzintla y Tacubaya, 4, 86
  * Purcell et al. (2011) Purcell, C. W., Bullock, J. S., Tollerud, E. J., Rocha, M., & Chakrabarti, S. 2011, Nature, 477, 301
  * Renzo et al. (2019) Renzo, M., Zapartas, E., de Mink, S. E., et al. 2019, A&A, 624, A66
  * Roche et al. (2015) Roche, N., Humphrey, A., Gomes, J. M., et al. 2015, MNRAS, 453, 2349
  * Rozhavskii et al. (1976) Rozhavskii, F. G., Kuz’mina, V. A., & Vasilevskii, A. E. 1976, Astrophysics, 12, 204
  * Ryu & Lee (2018) Ryu, J., & Lee, M. G. 2018, ApJ, 856, 152
  * Sana et al. (2012) Sana, H., de Mink, S. E., de Koter, A., et al. 2012, Science, 337, 444
  * Sana et al. (2013) Sana, H., de Koter, A., de Mink, S. E., et al. 2013, A&A, 550, A107
  * Schwarz (1981) Schwarz, M. P. 1981, ApJ, 247, 77
  * Schweizer (1987) Schweizer, F. 1987, in Nearly Normal Galaxies. From the Planck Time to the Present, ed. S. M. Faber, 18
  * Sheth et al. (2005) Sheth, K., Vogel, S. N., Regan, M. W., Thornley, M. D., & Teuben, P. J. 2005, ApJ, 632, 217
  * Shlosman et al. (1990) Shlosman, I., Begelman, M. C., & Frank, J. 1990, Nature, 345, 679
  * Smith & Lambert (1988) Smith, V. V., & Lambert, D. L. 1988, ApJ, 333, 219
  * Sneden (1973) Sneden, C. 1973, ApJ, 184, 839
  * Sneden et al. (2014) Sneden, C., Lucatello, S., Ram, R. S., Brooke, J. S. A., & Bernath, P. 2014, ApJS, 214, 26
  * Spitzer (1958) Spitzer, Lyman, J. 1958, ApJ, 127, 17
  * Subramaniam et al. (1995) Subramaniam, A., Gorti, U., Sagar, R., & Bhatt, H. C. 1995, A&A, 302, 86
  * Suh (2021) Suh, K.-W. 2021, ApJS, 256, 43
  * Tan (2000) Tan, J. C. 2000, ApJ, 536, 173
  * Tody (1986) Tody, D. 1986, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 627, Instrumentation in astronomy VI, ed. D. L. Crawford, 733
  * Tody (1993) Tody, D. 1993, in Astronomical Society of the Pacific Conference Series, Vol. 52, Astronomical Data Analysis Software and Systems II, ed. R. J. Hanisch, R. J. V. Brissenden, & J. Barnes, 173
  * Tutukov (1978) Tutukov, A. V. 1978, A&A, 70, 57
  * van den Bergh (1971) van den Bergh, S. 1971, A&A, 12, 474
  * van Loon et al. (2003) van Loon, J. T., Gilmore, G. F., Omont, A., et al. 2003, MNRAS, 338, 857
  * Vasiliev (2019) Vasiliev, E. 2019, MNRAS, 482, 1525
  * Verhoelst et al. (2009) Verhoelst, T., van der Zypen, N., Hony, S., et al. 2009, A&A, 498, 127
  * Whitmore et al. (2023) Whitmore, B. C., Chandar, R., Rodríguez, M. J., et al. 2023, ApJ, 944, L14
  * Wong & Blitz (2000) Wong, T., & Blitz, L. 2000, ApJ, 540, 771
  * Wright et al. (2010) Wright, E. L., Eisenhardt, P. R. M., Mainzer, A. K., et al. 2010, AJ, 140, 1868
  * Wright (1977) Wright, K. O. 1977, JRASC, 71, 152
  * Yang & Jiang (2011) Yang, M., & Jiang, B. W. 2011, ApJ, 727, 53
  * Yuk et al. (2010) Yuk, I.-S., Jaffe, D. T., Barnes, S., et al. 2010, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 7735, Ground-based and Airborne Instrumentation for Astronomy III, 77351M
  * Zinnecker & Yorke (2007) Zinnecker, H., & Yorke, H. W. 2007, ARA&A, 45, 481