# The Gaia alerted fading of the FUor-type star Gaia21elv

Zsófia Nagy,1,2 Sunkyung Park,1,2 Péter Ábrahám,1,2,3 Ágnes Kóspál,1,2,3,4
Fernando Cruz-Sáenz de Miera,1,2 Mária Kun,1,2 Michał Siwak,1,2 Zsófia
Marianna Szabó,1,2,5,6 Máté Szilágyi,1,2,3 Eleonora Fiorellino,7 Teresa
Giannini,8 Jae-Joon Lee,9 Jeong-Eun Lee,10 Gábor Marton,1,2 László
Szabados,1,2 Fabrizio Vitali,8 Jan Andrzejewski,11 Mariusz Gromadzki,12 Simon
Hodgkin,13 Maja Jabłońska,12 Rene A. Mendez,14 Jaroslav Merc,15 Olga
Michniewicz,11 Przemysław J. Mikołajczyk,12,16 Uliana Pylypenko,12 Milena
Ratajczak,12 Łukasz Wyrzykowski,12 Michal Zejmo,11 Paweł Zieliński17
1Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Eötvös
Loránd Research Network (ELKH),
H-1121 Budapest, Konkoly Thege Miklós út 15-17., Hungary
2CSFK, MTA Centre of Excellence, Konkoly-Thege Miklós út 15-17, 1121 Budapest,
Hungary
3ELTE Eötvös Loránd University, Institute of Physics, Pázmány Péter sétány
1/A, H-1117 Budapest, Hungary
4Max Planck Institute for Astronomy, Königstuhl 17, D-69117 Heidelberg,
Germany
5Max Planck Institute for Radio Astronomy, Auf dem Hügel 69, 53121 Bonn,
Germany
6Scottish Universities Physics Alliance (SUPA), School of Physics and
Astronomy, University of St Andrews, North Haugh, St Andrews, KY16 9SS, UK
7INAF-Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131
Napoli, Italy
8INAF-Osservatorio Astronomico di Roma, via di Frascati 33, 00078, Monte
Porzio Catone, Italy
9Korea Astronomy and Space Science Institute 776, Daedeok-daero, Yuseong-gu,
Daejeon, 34055, Republic of Korea
10Department of Physics and Astronomy, Seoul National University, 1 Gwanak-ro,
Gwanak-gu, Seoul 08826, Republic of Korea
11Janusz Gil Institute of Astronomy University of Zielona Gora, Poland
12Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warsaw, Poland
13Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
14Astronomy Department, Universidad de Chile, Casilla 36-D, Santiago, Chile
15Astronomical Institute, Faculty of Mathematics and Physics, Charles
University, V Holešovičkách 2, 180 00 Prague, Czech Republic
16Astronomical Institute, University of Wrocław, ul. M. Kopernika 11, 51-622
Wrocław, Poland
17Institute of Astronomy, Faculty of Physics, Astronomy and Informatics,
Nicolaus Copernicus University in Toruń,
ul. Grudziądzka 5, 87-100 Toruń, Poland
E-mail<EMAIL_ADDRESS>
(Accepted XXX. Received YYY; in original form ZZZ)

###### Abstract

FU Orionis objects (FUors) are eruptive young stars, which exhibit outbursts
that last from decades to a century. Due to the duration of their outbursts,
and to the fact that only about two dozens of such sources are known,
information on the end of their outbursts is limited. Here we analyse follow-
up photometry and spectroscopy of Gaia21elv, a young stellar object, which had
a several decades long outburst. It was reported as a Gaia science alert due
to its recent fading by more than a magnitude. To study the fading of the
source and look for signatures characteristic of FUors, we have obtained
follow-up near infrared (NIR) spectra using Gemini South/IGRINS, and both
optical and NIR spectra using VLT/X-SHOOTER. The spectra at both epochs show
typical FUor signatures, such as a triangular shaped $H$-band continuum,
absorption-line dominated spectrum, and P Cygni profiles. In addition to the
typical FUor signatures, [O i], [Fe ii], and [S ii] were detected, suggesting
the presence of a jet or disk wind. Fitting the spectral energy distributions
with an accretion disc model suggests a decrease of the accretion rate between
the brightest and faintest states. The rapid fading of the source in 2021 was
most likely dominated by an increase of circumstellar extinction. The
spectroscopy presented here confirms that Gaia21elv is a classical FUor, the
third such object discovered among the Gaia science alerts.

###### keywords:

Stars: variables: T Tauri – stars: pre-main sequence

††pubyear: 2023††pagerange: The Gaia alerted fading of the FUor-type star
Gaia21elv–LABEL:lastpage

##  1 Introduction

Studying the accretion in young stellar objects (YSOs) is important to
understand their formation. Most of what we know about accretion in YSOs is
based on the magnetospheric accretion scenario, according to which the
material accretes onto the forming star from the infalling envelope through
the disk, by following the magnetospheric lines (Hartmann et al., 2016). The
accretion rates of YSOs are known to be highly variable, with extreme cases of
eruptive YSOs, which experience outburst events, when their luminosity
increases up to two orders of magnitude. These events are detected as 2-5 mag
brightening in optical and near-infrared (NIR) bands. During the outbursts the
mass accretion rate can increase from $\sim$10-8 $M_{\odot}$ yr-1 in
quiescence to $\sim$10-4 $M_{\odot}$ yr-1 (Audard et al., 2014, Fischer et
al., 2022). Studies with large samples of objects indicate that young stars
experience these events once every $10^{3}-10^{4}$ years (e.g. Fischer et al.,
2019). Episodic accretion is one of the possible explanations for the observed
large luminosity spread of young stellar objects (Fischer et al., 2022). FU
Orionis objects (FUors) are well-studied examples of episodic accretion
(Hartmann & Kenyon, 1996). FUors are low-mass ($<2~{}M_{\odot}$) eruptive YSOs
that exhibit large-amplitude ($>$4 mag) outbursts at optical and infrared
wavelengths. These outbursts are expected to last up to a century, suggesting
that these events will not only increase the final stellar mass by a
significant amount, but also affect the evolution of the circumstellar disc.
The representative characteristics of FUors are brightness increase on a time
scale of 1-10 yr, P Cygni profile of H$\alpha$, Li i 6707 Å absorption, strong
CO absorption features, triangular shape of the $H$-band continuum due to the
strong water absorption bands on both sides of the $H$-band window, typical of
late M-type stars (Hartmann & Kenyon, 1996; Connelley & Reipurth, 2018). So
far the number of confirmed FUors is limited to no more than two dozens
(Audard et al., 2014). One of the important, so far unclear points is the end
of the FUor outbursts, i.e. their return to quiescence. FUor outbursts are
expected to end when the inner disc depletes. However, due to the typically
decades-long duration of the outbursts, no bona fide FUor has returned to
quiescence yet, apart from cases of short, temporary halt in the accretion,
e.g. V899 Mon (Ninan et al., 2015; Park et al., 2021) and V346 Nor (Kraus et
al., 2016, Kóspál et al., 2020). Another example is V1647 Ori, an eruptive YSO
that has shown some FUor characteristics (Aspin et al., 2009), and returned to
quiescence after a ten-years long outburst (Semkov et al., 2018; Giannini et
al., 2018). The spectroscopic deviation of V1647 Ori from well-known FUors,
however, ruled out its FUor classification (Connelley & Reipurth, 2018).

Therefore, it is not known whether the end of FUor outbursts is an abrupt
event when accretion suddenly stops and the brightness drops back to the
quiescent level in 1-2 years, or it is a slow gradual decrease of the
accretion rate resulting in a slowly decreasing light curve over perhaps
decades. The first scenario would indicate some instability, like the thermal
instability model proposed by Bell & Lin (1994). To understand how FUors end
their outbursts, it is important to increase their sample.

One of the best tools to discover the brightening or fading of eruptive young
star candidates is the Gaia Photometric Science Alerts system, due to its
large sky coverage and typically monthly cadence (Hodgkin et al., 2021).
Several eruptive YSOs have already been discovered based on the Gaia Science
Alerts, including the FUors Gaia17bpi (Hillenbrand et al., 2018) and Gaia18dvy
(Szegedi-Elek et al., 2020), and the EX Lupi-type eruptive YSOs (EXors)
Gaia18dvz (Hodapp et al., 2019), Gaia20eae (Ghosh et al., 2022; Cruz-Sáenz de
Miera et al., 2022) and Gaia19fct (Park et al., 2022). Some additional
eruptive YSOs were found, which cannot be classified as either a FUor or an
EXor, such as Gaia19ajj (Hillenbrand et al., 2019), Gaia19bey (Hodapp et al.,
2020), and Gaia21bty (Siwak et al., submitted). Two Gaia alerted sources with
light curves similar to eruptive YSOs, Gaia20bwa and Gaia20fgx (Nagy et al.,
2022), turned out to be classical T Tauri stars (CTTS), while the brightening
of another Gaia alerted YSO, V555 Ori (Gaia17afn), was confirmed to be caused
by variable circumstellar extinction, rather than a change in its accretion
rate (Nagy et al., 2021). Here we present a study of a previously known YSO,
which triggered the Gaia Science Alerts system due to its fading.

Gaia21elv (ESO H$\alpha$-148 or 2MASS J08410676-4052174, $\alpha_{\rm J2000}$
= 08h 41m 06$\aas@@fstack{s}$75, $\delta_{\rm J2000}$ = -40∘ 52′
17$\aas@@fstack{\prime\prime}$44) had a Gaia alert on 2021 October 6 due to
its quick fading by 1.2 mag over 18 months. Its archival photometry based on
photographic plates of the SuperCOSMOS Sky Survey (SSS) showed a long-term
brightening (Contreras Peña et al., 2019). It is a known young, Class II type
star (Pettersson & Reipurth, 1994, Marton et al., 2019), associated with the
Vela Molecular Ridge (Pettersson & Reipurth, 1994), and in particular, with
the RCW 27 HII region located at a distance of $\sim$1 kpc (Pettersson, 2008).
Its Gaia DR3 (Gaia Collaboration et al., 2022) parallax is 1.0727$\pm$0.0397
mas. The Renormalised Unit Weight Error (RUWE) of 1.291 and the astrometric
excess noise of 0.437 mas suggest that the astrometry is accurate. We derived
a zero-point correction of $-$0.02513 based on Lindegren et al. (2021) for
this parallax. After the zero-point correction, the Gaia DR3 parallax can be
converted to a distance of 910.9$\pm$33.7 pc, which we use in this paper. This
distance is close to the estimate of $905^{+36}_{-26}$ pc by Bailer-Jones et
al. (2021).

In this paper, we provide spectroscopic evidence that Gaia21elv is a FUor, and
discuss the cause of its fading that triggered the Gaia Alerts system. We
describe the photometric and spectroscopic observations in Sect. 2 and present
their results in Sect. 3. We analyse the FUor signatures in the NIR spectra in
Sect. 4, discuss the nature of the fading of the source, and provide a
comparison to other similar sources. We summarize our main findings in Sect.
5.

##  2 Observations

###  2.1 Optical photometry

In 2022 June, we obtained optical photometric observations of Gaia21elv with
the 60-cm Ritchey-Chrétien Rapid Eye Mount (REM) telescope operated by the
Italian National Institute for Astrophysics (INAF) at La Silla (Chile) using
its ROS2 instrument, an optical imager operating at four simultaneous
passbands (Sloan $g^{\prime}r^{\prime}i^{\prime}z^{\prime}$) with a field of
view (FoV) of 9$\aas@@fstack{\prime}$1$\times$9$\aas@@fstack{\prime}$1 and
pixel scale of 0$\aas@@fstack{\prime\prime}$58\. Three images were taken per
filter on four nights, 2022 June 5, 6, 8, and 9. After the usual bias and flat
field correction, and removal of hot pixels, we obtained aperture photometry
for Gaia21elv and about 15 comparison stars in the FoV. We selected the
comparison stars from the APASS9 catalog (Henden et al., 2015) making sure
that they are sufficiently constant in brightness (${\sigma}_{V}<0.08\,$mag).
We calculated the $z$-band brightness of the comparison stars by plotting
their spectral energy distribution (SED) using APASS9
$Bg^{\prime}Vr^{\prime}i^{\prime}$ and 2MASS $JHK_{\rm{s}}$ magnitudes (Cutri
et al., 2003) and interpolating between these points for the effective
wavelength of the $z^{\prime}$ filter, 1.05$\,\mu$m. We used an aperture
radius of 6 pixels (3$\aas@@fstack{\prime\prime}$5) and sky annulus between 20
and 40 pixels (11$\aas@@fstack{\prime\prime}$68 and
23$\aas@@fstack{\prime\prime}$36). Because all comparison stars were much
bluer than Gaia21elv, in order to avoid introducing large uncertainties by
extrapolation, we converted the instrumental magnitudes by averaging the
calibration factors of all comparison stars without fitting a colour term. The
results can be seen in Table 2.

Further observations of the target have been performed with REM between 2022
Oct 26 and 2023 Jan 4, during 12 nights. These observations, taken in Sloan
$g^{\prime}r^{\prime}i^{\prime}$ passbands, were uploaded to the BHTOM
service.111BHTOM - Black Hole TOM: www.bhtom.space 40, 38 and 44 images were
reduced in Sloan $g^{\prime}r^{\prime}i^{\prime}$, respectively.

Photometric observations were obtained with the PROMPT6 telescope located at
Cerro Tololo Inter-American Observatory in Chile. This telescope is a part of
SkyNET robotic network and is supplied with FLI CCD camera with 15.1 $\times$
15.1 arcmin field-of-view (2048 $\times$ 2048 pixels, 0.44 arcsec/pix). All 42
observations (14 frames per band) were taken in Johnson-Cousins $V$, $R$ and
$I$ bands and uploaded to the BHTOM service, where they were reduced and
converted to standard magnitudes (in APASS/V, APASS/r and APASS/i
respectively).

We obtained photometric observations with the 1.54m Danish telescope, located
at La Silla, Chile. The telescope is equipped with the CCD camera (E2V231-42)
in the Cassegrain focus, cooled by liquid nitrogen. The FoV is 13.7 $\times$
13.7 arcmin (2048 $\times$ 2048 pixels; pixel scale of 0.4 arcsec/pixel). The
filters used were Johnson-Cousins $BVR_{\rm{c}}I_{\rm{c}}$. In all cases, the
exposure time was 90 seconds.

We collected data using the 50cm CDK telescope equipped with a QHY268M pro
camera. This telescope (ROTUZ) is part of the
DeepSkyChile222https://www.deepskychile.com/en, and belongs to the Janusz Gil
Institute of Astronomy, University of Zielona Gora, Poland. We reduced the
data by applying bias, dark, and flat correction using AstroImageJ software
(Collins et al., 2017). The photometry was done using the BHTOM server. The
photometry done using the BHTOM server is based on the method described in
Zieliński et al. (2019) and Zieliński et al. (2020).

The results are shown in Fig. 1 and are summarized in Tables 2 and 3.

###  2.2 Infrared photometry

In 2022 June, we obtained infrared photometric observations with the REM,
using the infrared imaging camera, REMIR. The reduction of the $JHK$ images,
performed with our own IDL routines, included the construction and subtraction
of a sky image, and flat-fielding. We extracted the instrumental magnitudes
for the target as well as for all good-quality 2MASS stars (i.e. with a 2MASS
photometric quality flag of AAA) in the field in an aperture with a radius of
$\sim 3\aas@@fstack{\prime\prime}7$. No extended nebulosity is visible around
the source on the 2MASS images. The final step was the determination of an
average constant calibration factor between the instrumental and the 2MASS
magnitudes of typically 30–50 stars, and this offset was applied to the target
observations. The results can be found in Table 2.

REMIR was used again between October 2022 and January 2023 for $J$-band
imaging. Each image came from the five single images jittered along a circle
thanks to a dithering wedge from which a median sky was derived. Every image
was then sky-subtracted with the median sky. Subsequently, the five images
were re-aligned and averaged into a single $J$ band exposure. Calibrated
images were then uploaded to the BHTOM service, reduced and matched to 2MASS
$J$ band as described above for the optical data.

We used mid-infrared photometry from the Wide-field Infrared Survey Explorer
(WISE) and NEOWISE surveys from the NASA/IPAC Infrared Science Archive.
NEOWISE observes the full sky on average twice per year with multiple
exposures per epoch. For a comparison with the photometry from other
instruments, we computed the average of multiple exposures of a single epoch.
NEOWISE $W1$ and $W2$ photometry is known to display a photometric bias for
saturated sources. We corrected for this bias using the correction curves
given in the Explanatory Supplement to the NEOWISE Data Release Products. We
derived the average of the uncertainties of the single exposures (err1). We
also calculated the standard deviation of the points we averaged per season
(err2). For the error of the data points averaged per epoch we used the
maximum of err1 and err2.

Figure 1: Light curve of Gaia21elv in Gaia G (black), WISE $W1$ (blue) and
$W2$ (grey) bands, and in $g$ band from the ASAS-SN (green). The solid
vertical line shows the epoch of the Gemini/IGRINS spectrum, the dashed
vertical line shows the epoch of the VLT/X-SHOOTER, and the arrow shows the
epoch of the Gaia alert. The ranges covered by the colour-magnitude diagrams
in Figures 4 and 5 during and after the fading phase, respectively, are also
indicated.

###  2.3 Spectroscopy

We obtained high-resolution (R$\sim$45,000) NIR spectra of Gaia21elv on 2020
November 14 (Program ID: GS-2020B-Q-218, PI: S. Park) using the Immersion
GRating INfrared Spectrograph (IGRINS; Yuk et al., 2010; Park et al., 2014;
Mace et al., 2016) of Gemini South, in the $H$ and $K$ bands. The spectrum was
obtained with a slit size of 0.34″ $\times$ 5″. Gaia21elv was observed with
two sets of ABBA nodding observations to subtract the sky background better.
The total exposure time of Gaia21elv was 192 sec with 24 sec exposure of each
frame. The data were reduced using the IGRINS pipeline (Lee & Gullikson, 2017)
for flat-fielding, sky subtraction, correcting the distortion of the
dispersion direction, wavelength calibration, and combining the spectra. In
order to correct for telluric absorption features, a nearby A0 telluric
standard star (HIP 21514) was observed right before the target. Then, the
telluric correction and flux calibration were applied as done in Park et al.
(2018). Finally, barycentric velocity correction using barycorrpy (Kanodia &
Wright, 2018) was applied ($V_{\rm{bary}}$ = 16.715 km s-1).

A spectrum using the X-SHOOTER instrument of the Very Large Telescope (VLT) at
ESO’s Paranal Observatory in Chile (Vernet et al., 2011) was taken on 2021
December 12 (Program ID: 108.23M6, PI: Z. Nagy). X-SHOOTER simultaneously
covers a wavelength range from 300 nm to 2480 nm, and the spectra are divided
into three arms, the ultraviolet (UVB, 300 – 550 nm), the visible (VIS, 500 –
1020 nm), and the near-infrared (NIR, 1000 – 2480 nm). The observations were
performed with the narrow slits of 1′′, 0.9′′, and 0.4′′ in the UVB, VIS, and
NIR respectively, leading to spectral resolution of R $\sim$ 5400, 8900, and
11600, respectively. The exposure time was 1800 s in each of the three arms.
We obtained additional exposures with the 5′′ slits, which resulted in data
without slit losses, which we used for the correct flux calibration of the
spectra obtained with the narrower slits. The ABBAAB nodding pattern was used.
The observations were processed with the official ESO pipeline. Telluric
correction was performed using ESO’s Molecfit program (Kausch et al., 2015;
Smette et al., 2015) running in the same EsoReflex environment (Freudling et
al., 2013).

Figure 2: Colour-magnitude diagram based on WISE $W1$ and $W2$ data.

##  3 Results

###  3.1 Light and colour variations

Figure 1 shows the optical and infrared light curves of Gaia21elv, including
archival data from 1977 (Contreras Peña et al., 2019 and references therein),
the All-Sky Automated Survey for Supernovae (ASAS-SN, Shappee et al., 2014,
Kochanek et al., 2017), and the Asteroid Terrestrial-impact Last Alert System
(ATLAS, Tonry et al., 2018, Smith et al., 2020, Heinze et al., 2018) survey
downloaded from the ATLAS Forced Photometry web service (Shingles et al.,
2021). Based on these data, the eruption occurred around between 1991 and
1996. The amplitude of the brightening was 4-4.5 mag from a quiescent 16.5-17
mag to around 12 mag in the $R$-band. A slow fading of the source is already
seen after 2010 based on data points from Contreras Peña et al. (2019)
(collected from the AAVSO Photometric All Sky Survey (APASS) DR9 (Henden et
al., 2015), the VST Photometric Halpha Survey (VPHAS$+$) DR2 (Drew et al.,
2014), the Bochum Galactic disc survey (Hackstein et al., 2015)), and the Gaia
$G$-band light curve.

In 2021, the source started a more rapid fading, and had a Gaia alert in 2021
October due to its 1.2 mag fading in 18 months. After the Gaia alert, a
temporary brightening by about 0.2 mag was seen in early 2022, and after that,
the source stayed at the same brightness for several months, around 14.25 mag
in Gaia $G$-band. Between 2022 July and November, the source brightened again,
by about 0.3 mag as is seen in the lower panel of Fig. 1. A slow long-term
fading is also seen in the WISE data points.

Figure 2 shows a colour-magnitude diagram based on the WISE $W1$ and $W2$
bands. As the changes are mostly grey, extinction can be excluded as the
physical mechanism between the flux changes observed at the WISE wavelengths.

Figure 3 shows the $J-H$ vs $H-K_{s}$ diagram for the bright state (2MASS data
point from 1999 February) and for the faint state (REM data point from 2022
June). The difference between the two data points in this diagram ($\Delta
J\sim 0.61$ mag, $\Delta(J-H)\sim 0.16$ mag, $\Delta(H-K_{s})\sim 0.13$ mag)
may be consistent with the reddening of the source between 1999 and 2022. In
this case, the colour change implies a visual extinction increase by
$A_{V}\sim 2$ mag. However, the colour change in the $J-H$ vs $H-K_{s}$
diagram can also be caused by accretion. Eruptive young stars in the $J-H$ vs
$H-K_{s}$ plot usually move toward or away from the main sequence (e.g.
Szegedi-Elek et al., 2020).

Figure 4 shows a colour-magnitude diagram during the fading, as shown in Fig.
1 based on the $o$ and $c$ band magnitudes from the ATLAS survey. There is an
indication of a long-term increasing trend of the extinction. Since the period
of the quick fading in 2021 is not sampled well by these data points (as seen
in Fig. 1), it is not clear based on them, whether the increasing extinction
also applies for this period.

Figure 5 shows colour-magnitude diagrams after the fading of the source, based
on the $o$ and $c$ band magnitudes from the ATLAS survey, $g-r$ versus $g$ and
$r-i$ versus $r$ colour-magnitude diagrams based on our follow-up observations
between 2022 June and 2023 January. The periods covered by these figures are
also indicated in Fig. 1. These colour-magnitude diagrams show extinction-
related variations between 2022 June and 2023 January. The colour-magnitude
diagram based on the ATLAS $o$ and $c$ band also includes data points from a
period between 2021 October and 2022 May. These data points do not show an
extinction-related trend, indicating, that mechanisms other than the
extinction may also play a role in this post-fading phase.

Based on the colour variations alone, it is not possible to make a conclusion
on the origin of the brightness variations of Gaia21elv. The $o$ and $c$ band
data from the ATLAS survey as well as the $g-r$ versus $g$ and $r-i$ versus
$r$ colour-magnitude diagrams suggest extinction-related brightness variations
both during the fading and the brightening. Such extinction-related variations
are not seen in the WISE colour-magnitude diagrams, whereas the $J-H$ vs
$H-K_{s}$ diagram can be interpreted both as a result of extinction and
accretion. Therefore, we do not make a conclusion on the origin of the
brightness variations based on the colour variations, and will further
investigate it in Sect. 3.3.

Figure 3: ($J-H$) versus ($H-K_{S}$) colour–colour diagram for the bright
state (2MASS data point from 1999 February) and during the fading (REM data
point from 2022 June). The solid curve shows the colours of the zero-age main-
sequence, and the dotted line represents the giant branch (Bessell & Brett,
1988). The long-dashed lines delimit the area occupied by the reddened normal
stars (Cardelli et al., 1989). The dash–dotted line is the locus of unreddened
CTTS (Meyer et al., 1997) and the grey shaded band borders the area of the
reddened $K_{S}$-excess stars. Figure 4: Colour-magnitude diagram based on $o$
and $c$ magnitudes from the ATLAS survey during the fading of Gaia21elv. The
typical error bar is shown in the lower left corner.

Figure 5:  Left panel: Colour-magnitude diagram based on $o$ and $c$
magnitudes from the ATLAS survey after the fading of Gaia21elv. The typical
error bar is shown in the lower left corner. Middle and right panels: Colour-
magnitude diagrams based on follow-up photometry shown in Tables 2 and 3.

###  3.2 Reddening and spectral features

Figure 6: Optical and NIR spectra of Gaia21elv taken with VLT/X-SHOOTER and
Gemini South/IGRINS in comparison with those of FU Ori (taken also with
VLT/X-SHOOTER, ESO archival data from program 094.C-0233). Arbitrary scaling
factors were applied for a better comparison of the spectra. The Gemini
South/IGRINS spectrum was smoothed for a better comparison.

Figure 6 shows the spectra taken at the two epochs in optical and NIR using
Gemini South/IGRINS and VLT/X-SHOOTER and their comparison to the
VLT/X-SHOOTER spectrum of FU Ori.

Following the method of Connelley & Reipurth (2018), we used the X-SHOOTER
spectrum to estimate the visual extinction toward the source by comparing it
to the spectrum of FU Ori, which has a low and well known extinction
($A_{V}=$1.7$\pm$0.1 mag; e.g. Siwak et al., 2018, Lykou et al., 2022). We
dereddened the spectrum of Gaia21elv with increasing $A_{V}$ until it matched
the scaled, flux calibrated spectrum of FU Ori. The resulting $\Delta A_{V}$
is $\sim$4 mag, which suggests $A_{V}\sim 5.7$ mag for Gaia21elv in its faint
state.

Table 1 lists the lines we identified in the VLT/X-SHOOTER spectrum of
Gaia21elv. Most detected lines are seen in absorption, such as Ba ii, Li i, Na
D, K i, Al i, He i, Pa$\beta$, and Mg i (Fig. 7). Some of these absorption
lines show two (or more) components, such as the Ba ii, He i, and Pa$\beta$
lines. Some lines show a P Cygni profile, such as H$\alpha$ and H$\beta$ (Fig.
8) and the Ca ii triplet (Fig. 9). Forbidden lines of [O i], [Fe ii], and [S
ii] were detected in emission (Fig. 10). These lines may indicate the presence
of a jet associated with Gaia21elv, similarly to what was seen for the
classical FUor V1057 Cyg (e.g. Szabó et al., 2021). Forbidden emission lines
in young stars were also suggested to trace disk winds (Paatz & Camenzind,
1996, Iguchi & Itoh, 2016, Ballabio et al., 2020). The $H$ and $K$-band
spectra were observed at two different epochs: in 2020 November, just before
the rapid fading of the source (Gemini South/IGRINS) and in 2021 December,
soon after the Gaia alert reporting the fading (VLT/X-SHOOTER). These spectra
display very similar features (Fig. 6), including a triangular shaped $H$-band
continuum and the CO-bandhead features in absorption, both typical FUor
signatures. Fig. 11 shows lines detected at both epochs, such as Mg i,
Br$\gamma$, Na i, and Ca i. The line profiles did not change significantly
between the two epochs.

Figure 7: Examples of absorption lines detected toward Gaia21elv. Figure 8: Hydrogen Balmer lines in the VLT/X-SHOOTER spectrum of Gaia21elv. Figure 9: Ca II triplet lines observed for Gaia21elv using VLT/X-SHOOTER, compared to those observed using VLT/X-SHOOTER for FU Ori. Figure 10: Forbidden lines in emission detected toward Gaia21elv. Figure 11: Comparison of lines detected at both epochs using Gemini South/IGRINS (red) and VLT/X-SHOOTER (blue). Arbitrary scaling factors were applied for a better comparison of the spectra. Table 1: Lines detected in the X-SHOOTER spectrum of Gaia21elv. The FWHM values were derived using a Gaussian fitting, and are not provided for line profiles, which cannot be fitted by a Gaussian. For lines with multiple components, we provide the parameters of the one with the highest intensity. Species | Lab. $\lambda$ | Obs. $\lambda$ | EW | FWHM | Note
---|---|---|---|---|---
| [nm] | [nm] | [nm] | [nm] |
$[$S ii$]$ | 406.860 | 406.702 |  $-$1.01$\pm$0.05 | 0.20$\pm$0.03 | emission
H$\delta$ | 410.171 | 410.038 | 0.05$\pm$0.01 | 0.10$\pm$0.01 | absorption
H$\gamma$ | 434.047 | 433.850 | 0.13$\pm$0.03 | 0.32$\pm$0.01 | absorption
H$\beta$ | 486.129 | 485.997 | 0.13$\pm$0.01 | … | P Cygni absorption
H$\beta$ | 486.129 | 486.251 |  $-$0.04$\pm$0.01 | 0.16$\pm$0.02 | P Cygni emission
Na D | 588.995 | 588.898 | 0.30$\pm$0.01 | 0.27$\pm$0.01 | absorption
Na D | 589.592 | 589.508 | 0.25$\pm$0.01 | 0.24$\pm$0.02 | absorption
$[$O i$]$ | 630.030 | 629.801 |  $-$0.28$\pm$0.02 | 0.29$\pm$0.01 | emission
Ba ii | 649.690 | 649.515 | 0.08$\pm$0.01 | 0.38$\pm$0.01 | absorption
H$\alpha$ | 656.282 | 656.155 | 0.42$\pm$0.01 | … | P Cygni absorption
H$\alpha$ | 656.282 | 656.377 |  $-$0.09$\pm$0.01 | 0.18$\pm$0.02 | P Cygni emission
Li i | 670.776 | 670.785 | 0.03$\pm$0.01 | 0.13$\pm$0.01 | absorption
$[$S ii$]$ | 673.082 | 672.960 |  $-$0.05$\pm$0.01 | 0.28$\pm$0.01 | emission
$[$Fe ii$]$ | 715.517 | 715.364 |  $-$0.08$\pm$0.01 | 0.24$\pm$0.01 | emission
K i | 766.490 | 766.457 | 0.15$\pm$0.01 | 0.20$\pm$0.02 | absorption
K i | 769.896 | 769.851 | 0.10$\pm$0.01 | 0.20$\pm$0.02 | absorption
Ca ii | 849.802 | 849.647 | 0.05$\pm$0.01 | 0.19$\pm$0.02 | P Cygni absorption
Ca ii | 849.802 | 849.854 |  $-$0.04$\pm$0.01 | 0.12$\pm$0.01 | P Cygni emission
Ca ii | 854.209 | 854.028 | 0.10$\pm$0.01 | 0.28$\pm$0.01 | P Cygni absorption
Ca ii | 854.209 | 854.276 |  $-$0.06$\pm$0.01 | 0.16$\pm$0.01 | P Cygni emission
Ca ii | 866.214 | 866.043 | 0.09$\pm$0.01 | 0.23$\pm$0.01 | P Cygni absorption
Ca ii | 866.214 | 866.283 |  $-$0.04$\pm$0.01 | 0.14$\pm$0.01 | P Cygni emission
Pa8 | 954.620 | 954.607 | 0.04$\pm$0.01 | 0.21$\pm$0.02 | absorption
He i | 1083.025 | 1081.46 | 0.60$\pm$0.10 | 1.20$\pm$0.20 | absorption, two components
Pa$\beta$ | 1281.807 | 1281.819 | … | … | absorption, two components
Al i | 1312.342 | 1312.382 | 0.06$\pm$0.01 | 0.18$\pm$0.01 | absorption
Al i | 1315.075 | 1315.107 | 0.07$\pm$0.01 | 0.22$\pm$0.01 | absorption

To interpret the CO bandhead features observed at the two epochs, we used an
isothermal slab model to find a best-fitting CO column density and excitation
temperature of the absorbing material, similarly to Kóspál et al. (2011) and
Park et al. (2021). We found the best-fitting CO column density to be
$\sim$1022 cm-2, and a best-fitting excitation temperature of $2800\pm 100$ K
at the first epoch (Gemini South/IGRINS) and $2300\pm 100$ K at the later
epoch (VLT/X-SHOOTER). The results are shown in Figure 12. In Sect. 4.1 we
analyse the spectra in more detail and compare the observed features to those
seen in FUors.

Figure 12: CO overtone features of Gaia21elv shown in black observed using
Gemini South/IGRINS (top panel) and VLT/X-SHOOTER (bottom panel). The best fit
models are overplotted in red.

###  3.3 Spectral Energy Distribution modeling

In the following, we analyse the Spectral Energy Distribution (SED) of
Gaia21elv at three different epochs. To create a SED for the state of the
maximum brightness, we used archival data from APASS9 (Henden et al., 2015),
DENIS (Epchtein et al., 1994), 2MASS (Cutri et al., 2003), and the ALLWISE
(Cutri et al., 2013) catalogues. A comparison of the DENIS $I$-band flux from
1996 December with the APASS9 $i^{\prime}$-band flux from 2010 December shows
that the brightness of the star did not change significantly between these
dates, thus the fact that the used archival data correspond to different
epochs is not expected to affect the modeling of the SED in the bright state.
In addition to the epoch of the bright state, we compiled an SED for 2020
Oct–Nov that is very close to the epoch of the Gemini/IGRINS spectrum, and as
such, it is just before the fast fading phase of the source. We used the
available ASAS-SN $g$, Gaia $G$ and WISE $W1$ data, as well as photometry in
the cyan and orange bands of the ATLAS survey for this epoch. The third epoch
we considered is the epoch of the VLT/X-SHOOTER spectrum in 2021 December, as
it represents the faint state at the end of the fast fading of the source. We
obtained synthetic photometry in the APASS9 and 2MASS bands from the X-SHOOTER
spectrum, and also used the NEOWISE $W1$ data point closest to this epoch. The
three SEDs are shown in Fig. 13.

As we will discuss in Sec. 4, the properties of Gaia21elv resemble those of FU
Orionis-type stars. In these objects the circumstellar matter is expected to
form an accretion disc (Hartmann & Kenyon, 1996). To estimate the properties
of the accretion disc in Gaia21elv at the three epochs, we modelled the SEDs
using a steady, optically thick and geometrically thin viscous accretion disc,
whose mass accretion rate is constant in the radial direction. This method was
successfully applied to estimate the accretion rate in several eruptive YSOs
including HBC 722 (Kóspál et al., 2016), V582 Aur (Ábrahám et al., 2018),
2MASS 22352345 + 7517076 (Kun et al., 2019), Gaia18dvy (Szegedi-Elek et al.,
2020), V1057 Cyg (Szabó et al., 2021), and V1515 Cyg (Szabó et al., 2022). In
this model, the temperature profile of the disc is defined based on Hartmann &
Kenyon (1996) as:

$T(r)=\left[\frac{3GM_{\star}\dot{M}}{8\pi
R^{3}_{\star}\sigma}\left(1-\sqrt{\frac{R_{\star}}{r}}\right)\right]^{1/4},$
(1)

where $r$ is the distance from the star, $R_{\star}$ is the stellar radius,
$M_{\star}$ is the stellar mass, $\dot{M}$ is the accretion rate, and
$G,\sigma$ are the gravitational and Stefan-Boltzmann constants, respectively.
The model SED was calculated by integrating black-body emission in concentric
annuli between the inner disc radius and the outer disc radius. The resulting
SED was then reddened by different AV values.

One of the input parameters of the model is the inclination, and as it is
unknown for Gaia21elv, we used an intermediate value of 45∘. We assumed a
distance of $910.9$ pc, as derived above from the Gaia DR3 parallax and its
zero-point correction. There is a known degeneracy in the model between the
inner disc radius and AV. To break this degeneracy we adopted the AV value of
$\sim$5.7 mag obtained from the X-SHOOTER spectrum in Sect. 3. This choice
fixed the inner disc radius to $R_{\rm{in}}=2R_{\odot}$, a reasonable value,
as it is the same as determined for FU Ori by Zhu et al. (2007).

The remaining free parameters of the disc model are $M_{\star}\dot{M}$, AV,
and $R_{\rm{out}}$. Finding the best $M_{\star}\dot{M}$ and AV combinations
was performed with $\chi^{2}$ minimization over a large grid in both the
accretion rate and the extinction, by taking into account all flux values
between 0.4 and 4.0 $\mu$m. The formal uncertainties of the data points were
set to a homogeneous 5% of the measured flux values. We ran several models
assuming different outer disc radii in the range between 0.2 and 2 au, and
found that the WISE data points are reasonably well fitted with
$R_{\rm{out}}=1$ au, though this value is less constrained than the other two
parameters. The best-fitting visual extinctions and products of the stellar
mass and the accretion rate are plotted in Fig. 14. Since the outcome of the
model is the product $M_{\star}\dot{M}$, the true accretion rate depends on
the stellar mass. However, FUors are typically low-mass objects (Hartmann &
Kenyon, 1996), thus our obtained values provide a good approximation to the
accretion rate.

Considering the results for all three epochs, the three data points suggest
that the accretion rate followed a monotonic decay in the last 15 years. Our
models suggest a slight increase of the extinction toward the source from 3.6
mag to 4.4 mag between the maximum brightness and the Gemini epoch in 2020
November. Remarkably, the quick fading in 2021, corresponding to the Gaia
alert, was mostly caused by an increase in extinction. The accretion
luminosity of the source also dropped in parallel to the accretion rate
between the first and last epoch, from 106 L⊙ to 68 L⊙, although the absolute
values depend on the unknown inclination angle, too.

Figure 13: The SED of Gaia21elv at the three modelled epochs. The SED at the
brightest state based on archival data is shown with circles. The SED close to
the epoch of the Gemini/IGRINS spectrum is shown with asterisks. The SED at
the epoch of the VLT/X-SHOOTER spectrum representing the faint state is shown
with triangles. Solid curves show the results of the accretion disc models for
the individual epochs. Figure 14: Visual extinctions and the product of the
stellar mass and accretion rate for the three epochs based on the accretion
disc models. The long-term light curve using the Gaia $G$ magnitudes and the
data from Contreras Peña et al. (2019) are shown as a comparison.

##  4 Discussion

###  4.1 Classification of Gaia21elv as a FUor

To investigate, whether Gaia21elv is indeed a FUor, we used the criteria from
Connelley & Reipurth (2018), which they list in their Table 3. In the
following, we list these defining characteristics and check if Gaia21elv
fulfills them.

  * -

The eruption is observed for each bona fide FUor, unlike for FUor-like and
peculiar objects. This criterion is fulfilled for Gaia21elv. The date of the
eruption can be constrained based on the light curve shown in Contreras Peña
et al. (2019) in their figure B2, which includes data points from the
literature starting from 1977 (Fig. 1). The outburst of Gaia21elv based on the
long term light curve occurred between 1991 and 1996.

  * -

Bona fide FUors have well defined CO absorption features. Strong CO absorption
was also observed for Gaia21elv (Fig. 12) at both of our observing epochs.

  * -

Water vapor bands can be identified in the NIR spectra of bona fide FUors,
including the feature at 1.33 $\mu$m and the triangular shaped $H$-band
continuum, which is due to water vapor bands on each end of the $H$-band (Fig.
6). Gaia21elv shows these features at both epochs.

  * -

Bona fide FUors show other molecular bands in their $J$-band spectra, such as
those from vanadium oxide (at 1.05 $\mu$m and 1.19 $\mu$m) and titanium oxide
(0.88, 0.92, and 1.11 $\mu$m). The X-SHOOTER spectrum of Gaia21elv shows all
these molecular bands as wide absorption features (Fig. 6).

  * -

Another characteristic of FUors is their hydrogen lines, especially the
Pa$\alpha$, $\beta$, $\gamma$, and $\delta$ lines, are in absorption,
Br$\gamma$ line is very weak, with the rest of the Brackett series not
observed. For Gaia21elv the Pa$\beta$ and Pa$\delta$ lines are indeed seen in
absorption, however, the other two Paschen lines are not detected. It was not
possible to detect the Pa$\alpha$ line due to the poor atmospheric
transmission at its wavelength (1.87 $\mu$m). The Br$\gamma$ line shows a weak
absorption, while the rest of the Brackett series is not detected, similarly
to what was expected for FUors.

  * -

FUors show very few, if any, emission lines, and even those are typically the
emission components of P Cygni profiles. Gaia21elv shows a few P Cygni
profiles in H$\alpha$, H$\beta$, and the Ca II triplet, and in addition to
those, there are forbidden lines of [O i], [Fe ii], and [S ii] in emission.
The absorption lines and P Cygni profiles typically detected in the spectra of
FUors are related to the disc, while the forbidden emission lines trace a jet
or disk wind. Forbidden emission lines are not always detected in the spectra
of known FUors, but were identified for a few examples, including the
classical FUors V2494 Cyg (Connelley & Reipurth, 2018 and references therein)
and V1057 Cyg (Szabó et al., 2021), therefore, their detection does not rule
out a classification as a bona fide FUor.

  * -

FUors show weak absorption lines of Na i (2.208 $\mu$m) and Ca i (2.256
$\mu$m) (Connelley & Reipurth, 2018). As shown in Fig. 11, these lines are
detected in the spectra of Gaia21elv at both epochs.

  * -

Another spectroscopic signature of FUors is the He i line at 1.083 $\mu$m,
which is also present in the spectrum of Gaia21elv (Fig. 6). The He i line
detected toward Gaia21elv is double-peaked, where the higher intensity
component is largely blueshifted, detected at a velocity of around $-$400 km
s-1, and the lower intensity component is seen at a velocity of around $+$25
km s-1 (Fig. 7). Most bona fide FUors show blueshifted absorption lines, with
a mean velocity of $-$350 km s-1 (see Fig. 4. in Connelley & Reipurth, 2018).

Another characteristics of FUors is that their spectral type is wavelength-
dependent (Hartmann & Kenyon, 1996). To check whether this applies to
Gaia21elv, we used the VLT/X-SHOOTER spectrum, and compared it to the
synthetic stellar spectra calculated by Coelho et al. (2005) in the 300 nm to
1.8 $\mu$m wavelength range. These stellar templates are given for effective
temperatures in the range between 3500 K and 6000 K in steps of 250 K. We
compared the VLT/X-SHOOTER spectrum to these stellar templates at optical and
at NIR wavelengths, separately. At optical wavelengths, the best match was
found with the stellar template corresponding to an effective temperature of
5500$\pm$250 K, while at NIR wavelengths, the best fit corresponds to an
effective temperature of 3750$\pm$250 K. This is consistent with the
expectation for FUors, that the stellar type is wavelength-dependent.

Based on the above criteria from Connelley & Reipurth (2018) as well as its
wavelength-dependent spectral type, we conclude that Gaia21elv can be
classified as a bona fide FUor. This classification is consistent with the
high accretion luminosity of the source implied by our accretion disc
modelling.

###  4.2 On the recent fading of Gaia21elv

Until now, no bona fide FUor is known to have completely ended its outburst.
This is why it is important to monitor their brightness variations, and study
their fading episodes. A temporary fading of V346 Nor was reported by Kraus et
al. (2016) and Kóspál et al. (2020), which was due to a decrease in the
accretion rate, however, after the fading, the star brightened again to nearly
reach its outburst brightness. Another eruptive young star, V899 Mon, which
shows properties of both FUors and EXors, faded to quiescence for a little
less than a year (Ninan et al., 2015; Park et al., 2021). However, this
quiescent phase was followed by another outburst. In addition to their fading
being temporary, neither V346 Nor, nor V899 Mon is a bona fide FUor. The long-
term fading of a classical FUor, V1515 Cyg was recently reported by Szabó et
al. (2022): its fading started around 2006 and is approximately consistent
with an exponential decay with an e-folding time of 12 years. Another
classical FUor, V733 Cep also shows long-term fading (Park et al., in prep.),
which was found to be the result of a decrease in the accretion rate.

Brightness variations of young stars are only partly related to changes in the
accretion rate (Fischer et al., 2022). The other main process is variable
circumstellar extinction. To probe whether the fading of Gaia21elv was the
result of a decrease of the accretion rate, we estimated the accretion rate by
fitting the SEDs with an accretion disc model in Sec. 3. The accretion rates
derived for Gaia21elv are typical of FUors (Fischer et al., 2022 and
references therein). The accretion rate between the brightest and faintest
states decreased by $\sim$36%. However, according to the accretion disc models
fitted to the SEDs, the decreasing accretion rate was combined with increasing
circumstellar extinction, especially between 2020 and 2022. It is most likely,
that the increased circumstellar extinction dominated the rapid fading of the
source that triggered the Gaia Alerts system in 2021. After the Gaia alert,
the brightness of the source also started a slow increase, though it is still
almost a magnitude fainter than in early 2020, before the start of this fading
episode. The decrease found between the accretion rates at the brightest and
faintest states indicates an e-folding time of about 25 years. Based on our
results, the fading of Gaia21elv found by the Gaia alert is likely a temporary
event. Future photometric and spectroscopic monitoring of the source is
important to provide more information on the evolution of its outburst.

##  5 Summary

We analysed the photometry and spectroscopy of a young star exhibiting a long-
term outburst and a recent fading alerted by the Gaia Science Alerts system.

Optical and NIR spectra confirm that Gaia21elv is a bona fide FUor. This is
the third FUor which was found based on the Gaia alerts. In addition to the
classical FUor signatures, forbidden emission lines were detected, which are
typically tracing a jet or disk winds.

Fitting the SEDs at the maximum brightness and and its faint state using an
accretion disc model suggests a decrease in the accretion rate. However,
fitting the SED at an epoch close to the onset of the quick fading in late
2020-2021 indicates that this episode was mostly caused by an increase of
circumstellar extinction.

In the future, a photometric and spectroscopic monitoring of Gaia21elv is
important to characterize its behavior after its fading episode.

## Acknowledgements

We thank the referee for comments which helped to improve our paper.

This project has received funding from the European Research Council (ERC)
under the European Union’s Horizon 2020 research and innovation programme
under grant agreement No 716155 (SACCRED).

We acknowledge support from the ESA PRODEX contract nr. 4000132054.

G.M. and Z.N. were supported by the János Bolyai Research Scholarship of the
Hungarian Academy of Sciences.

G.M. has received funding from the European Union’s Horizon 2020 research and
innovation programme under grant agreement No. 101004141.

Zs.M.Sz. acknowledges funding from a St Leonards scholarship from the
University of St Andrews. For the purpose of open access, the author has
applied a Creative Commons Attribution (CC BY) licence to any Author Accepted
Manuscript version arising.

E.F. and T.G. acknowledge financial support from the project PRIN-INAF 2019
"Spectroscopically Tracing the Disk Dispersal Evolution (STRADE)".

We acknowledge ESA Gaia, DPAC and the Photometric Science Alerts Team
(http://gsaweb.ast.cam.ac.uk/alerts).

This work used the Immersion Grating Infrared Spectrometer (IGRINS) that was
developed under a collaboration between the University of Texas at Austin and
the Korea Astronomy and Space Science Institute (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.

This work was supported by K-GMT Science Program (PID: GS-2020B-Q-218) of
Korea Astronomy and Space Science Institute (KASI).

Based on observations collected at the European Southern Observatory under ESO
programme 108.23M6.

This work has made use of data from the Asteroid Terrestrial-impact Last Alert
System (ATLAS) project. The Asteroid Terrestrial-impact Last Alert System
(ATLAS) project is primarily funded to search for near earth asteroids through
NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575; byproducts of the NEO
search include images and catalogs from the survey area. This work was
partially funded by Kepler/K2 grant J1944/80NSSC19K0112 and HST GO-15889, and
STFC grants ST/T000198/1 and ST/S006109/1. The ATLAS science products have
been made possible through the contributions of the University of Hawaii
Institute for Astronomy, the Queen’s University Belfast, the Space Telescope
Science Institute, the South African Astronomical Observatory, and The
Millennium Institute of Astrophysics (MAS), Chile.

This project used data obtained via BHTOM (https://bhtom.space), which has
received funding from the European Union’s Horizon 2020 research and
innovation program under grant agreements No. 101004719.

## Data availability

The data underlying this article will be shared on reasonable request to the
corresponding author.

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##  Appendix A Photometry

Table 2: REM photometry of Gaia21elv. JD$\,{-}\,$2 450 000 | $g^{\prime}$ | $r^{\prime}$ | $i^{\prime}$ | $z^{\prime}$ | $J$ | $H$ | $K_{s}$
---|---|---|---|---|---|---|---
9736.50 | 16.91$\pm$0.10 | 14.81$\pm$0.03 | 13.51$\pm$0.03 | 12.63$\pm$0.05 | … | … | …
9738.50 | 17.02$\pm$0.24 | 14.83$\pm$0.03 | 13.48$\pm$0.03 | 12.61$\pm$0.04 | 9.90$\pm$0.04 | 8.69$\pm$0.05 | 7.79$\pm$0.04
9739.47 | 16.84$\pm$0.08 | 14.89$\pm$0.02 | 13.57$\pm$0.02 | 12.67$\pm$0.04 | 9.91$\pm$0.06 | 8.71$\pm$0.06 | 7.79$\pm$0.09
9740.49 | 16.93$\pm$0.11 | 14.87$\pm$0.02 | 13.53$\pm$0.03 | 12.71$\pm$0.03 | 9.85$\pm$0.07 | 8.68$\pm$0.06 | 7.82$\pm$0.02
9878.86 | … | … | … | … | 9.81$\pm$0.18 | … | …
9883.86 | 16.87$\pm$0.09 | 14.71$\pm$0.08 | 13.31$\pm$0.13 | … | 10.18$\pm$0.43 | … | …
9889.84 | 16.79$\pm$0.21 | 14.66$\pm$0.05 | 13.38$\pm$0.08 | … | … | … | …
9896.75 | 16.73$\pm$0.17 | 14.59$\pm$0.07 | 13.27$\pm$0.09 | … | … | … | …
9901.86 | 16.92$\pm$0.15 | 14.67$\pm$0.09 | 13.35$\pm$0.15 | … | … | … | …
9906.86 | 16.88$\pm$0.17 | 14.66$\pm$0.08 | 13.30$\pm$0.09 | … | 9.70$\pm$0.49 | … | …
9926.72 | 16.79$\pm$0.21 | 14.61$\pm$0.12 | 13.28$\pm$0.09 | … | … | … | …
9933.72 | 16.72$\pm$0.27 | 14.68$\pm$0.13 | 13.33$\pm$0.13 | … | … | … | …
9938.72 | 16.69$\pm$0.20 | 14.63$\pm$0.11 | 13.31$\pm$0.10 | … | … | … | …
9943.75 | 16.82$\pm$0.09 | … | 13.37$\pm$0.02 | … | … | … | …
Table 3: Photometry from other telescopes obtained for Gaia21elv. JD$\,{-}\,$2 450 000 | $B$ | $V$ | $g^{\prime}$ | $r^{\prime}$ | $i^{\prime}$ | Telescope
---|---|---|---|---|---|---
9867.86 | 18.23$\pm$0.11 | 15.64$\pm$0.04 | … | 14.20$\pm$0.14 | 13.04$\pm$0.08 | Danish 1.54-m
9875.86 | 18.28$\pm$0.03 | 15.67$\pm$0.11 | … | 14.25$\pm$0.10 | 13.07$\pm$0.10 | Danish 1.54-m
9904.69 | … | 15.71$\pm$0.05 | … | 14.43$\pm$0.06 | … | PROMPT6
9908.70 | 17.95$\pm$0.11 | 15.87$\pm$0.08 | … | 14.42$\pm$0.06 | … | UZPW 50cm
9909.68 | … | 15.61$\pm$0.04 | … | … | 13.01$\pm$0.05 | PROMPT6
9911.68 | … | … | 16.86$\pm$0.03 | 14.61$\pm$0.04 | 13.26$\pm$0.04 | UZPW 50cm
9913.82 | … | … | … | 14.67$\pm$0.05 | 13.35$\pm$0.06 | UZPW 50cm
9916.84 | … | … | … | 14.67$\pm$0.05 | 13.26$\pm$0.04 | UZPW 50cm
9917.84 | … | … | 16.91$\pm$0.04 | 14.53$\pm$0.07 | 13.28$\pm$0.05 | UZPW 50cm
9920.69 | … | 15.65$\pm$0.06 | … | 14.39$\pm$0.07 | 13.08$\pm$0.05 | PROMPT6
9925.84 | … | … | 16.91$\pm$0.03 | 14.69$\pm$0.04 | 13.33$\pm$0.04 | UZPW 50cm
9932.65 | … | … | 16.94$\pm$0.03 | 14.70$\pm$0.04 | 13.31$\pm$0.04 | UZPW 50cm
9934.59 | … | … | 16.81$\pm$0.09 | 14.66$\pm$0.04 | 13.31$\pm$0.04 | UZPW 50cm
9943.58 | … | … | 16.87$\pm$0.04 | 14.66$\pm$0.05 | 13.28$\pm$0.05 | UZPW 50cm
9946.56 | … | … | 16.86$\pm$0.04 | 14.71$\pm$0.03 | 13.33$\pm$0.03 | UZPW 50cm
9947.57 | … | … | 16.91$\pm$0.03 | 14.69$\pm$0.03 | 13.28$\pm$0.03 | UZPW 50cm
9949.73 | … | … | 16.96$\pm$0.03 | 14.72$\pm$0.05 | 13.31$\pm$0.04 | UZPW 50cm
9950.84 | … | … | 16.93$\pm$0.04 | 14.70$\pm$0.04 | 13.31$\pm$0.03 | UZPW 50cm
9952.71 | … | … | 16.91$\pm$0.04 | 14.74$\pm$0.04 | 13.34$\pm$0.04 | UZPW 50cm
9954.61 | … | 15.76$\pm$0.05 | … | 14.44$\pm$0.07 | 13.16$\pm$0.06 | PROMPT6
9955.86 | … | … | 16.95$\pm$0.03 | 14.71$\pm$0.03 | 13.34$\pm$0.05 | UZPW 50cm
9959.66 | … | 15.70$\pm$0.06 | … | 14.39$\pm$0.07 | 13.11$\pm$0.05 | PROMPT6
9962.64 | … | … | 16.91$\pm$0.03 | 14.67$\pm$0.04 | 13.30$\pm$0.04 | UZPW 50cm
9963.73 | … | … | 16.89$\pm$0.02 | 14.65$\pm$0.03 | 13.28$\pm$0.04 | UZPW 50cm
9964.63 | … | 15.65$\pm$0.06 | … | 14.35$\pm$0.06 | 13.08$\pm$0.06 | PROMPT6
9966.87 | … | … | 16.88$\pm$0.03 | … | … | UZPW 50cm
9969.62 | … | 15.66$\pm$0.05 | … | 14.36$\pm$0.07 | 13.09$\pm$0.05 | PROMPT6