# ATOMS: ALMA Three-millimeter Observations of Massive Star-forming regions -
VIII. A search for hot cores by using C2H5CN, CH3OCHO and CH3OH lines

Sheng-Li Qin,1 Tie Liu,2 Xunchuan Liu,2 Paul F. Goldsmith,3 Di Li,4,5,6 Qizhou
Zhang,7 Hong-Li Liu,1 Yuefang Wu,8 Leonardo Bronfman,9 Mika Juvela,10 Chang
Won Lee,11,12 Guido Garay,9 Yong Zhang,13 Jinhua He,14,15,9 Shih-Ying Hsu,16
Zhi-Qiang Shen,2 Jeong-Eun Lee,17 Ke Wang,18 Ningyu Tang,19 Mengyao Tang,20
Chao Zhang,21 Yinghua Yue,1 Qiaowei Xue,1 Shang-Huo Li,11 Yaping Peng,22
Somnath Dutta,16 Ge Jixing,15 Fengwei Xu,8 Longfei Chen,4 Tapas Baug,23 Lokesh
dewanggan,24 and Anandmayee Tej25
Affiliations are listed at the end of the paper E-mail<EMAIL_ADDRESS><EMAIL_ADDRESS>
(Accepted XXX. Received YYY; in original form ZZZ)

###### Abstract

Hot cores characterized by rich lines of complex organic molecules are
considered as ideal sites for investigating the physical and chemical
environments of massive star formation. We present a search for hot cores by
using typical nitrogen- and oxygen-bearing complex organic molecules (C2H5CN,
CH3OCHO and CH3OH), based on ALMA Three-millimeter Observations of Massive
Star-forming regions (ATOMS). The angular resolutions and line sensitivities
of the ALMA observations are better than 2 arcsec and 10 mJy/beam,
respectively. A total of 60 hot cores are identified with 45 being newly
detected, in which the complex organic molecules have high gas temperatures
($>$ 100 K) and small source sizes ($<$ 0.1 pc). So far this is the largest
sample of hot cores observed with similar angular resolution and spectral
coverage. The observations have also shown nitrogen and oxygen differentiation
in both line emission and gas distribution in 29 hot cores. Column densities
of CH3OH and CH3OCHO increase as rotation temperatures rise. The column
density of CH3OCHO correlates tightly with that of CH3OH. The pathways for
production of different species are discussed. Based on the spatial position
difference between hot cores and UC Hii regions, we conclude that 24 hot cores
are externally heated while the other hot cores are internally heated. The
observations presented here will potentially help establish a hot core
template for studying massive star formation and astrochemistry.

###### keywords:

astrochemistry - ISM:molecules - star:formation

††pubyear: 2022††pagerange: ATOMS: ALMA Three-millimeter Observations of
Massive Star-forming regions - VIII. A search for hot cores by using C2H5CN,
CH3OCHO and CH3OH lines–A.4

##  1 Introduction

Massive stars play an important role in shaping structure and evolution of
galaxies, but also can affect star and planet formation. They also are
dominant sources of heavy elements and UV radiation (see Zinnecker & Yorke
2007 and references therein). How massive stars form is not yet well
understood. During massive star formation processes, the hot core phase is
particularly important since hot cores may trace the physical and chemical
environments where massive stars are born. When clouds collapse to form
massive stars, the material surrounding these objects are heated up, leading
to formation of hot cores with rich chemistry.

Hot cores are characterized by rich line emission from complex organic
molecules (COMs) with higher gas temperature ($>$ 100 K) and smaller source
size ($<$0.1 pc) (e.g, Kurtz et al. 2000; Cesaroni 2005). Of the detected 250
molecular species111https://cdms.astro.uni-koeln.de/classic/molecules (see
also McGuire 2021), most of COMs, as well as simple molecules are frequently
observed in hot cores. Different molecules are used for probing different
physical components at different scales (Tychoniec et al. 2021; van Dishoeck &
Blake 1998; Jøgensen et al. 2021). Especially COMs are thought to play an
important role in prebiotic chemistry which may be linked to the origin of
life (e.g., Herbst & van Dishoeck 2009; Ceccarelli et al. 2017). Therefore
observations toward hot cores are crucial in the study of massive star
formation and astrochemistry.

Hot cores have been observed towards individual cases and large sample of
sources by single dish telescopes (Belloche et al. 2013; Bisschop et al. 2007;
Fontani et al. 2007; Gibb et al. 2000a; Schilke et al. 1997, 2001, 2006;
Halfen et al. 2013; Coletta et al. 2020; Widicus Weaver et al. 2017; Suzuki et
al. 2016, 2018; Crockett et al. 2014; Xie et al. 2021; Hernández-Hernández et
al. 2019; Neill et al. 2014; Möller et al. 2021; Ospina-Zamudio et al. 2018;
Li et al. 2020; Bergin et al. 2010; van der Tak et al. 2000). Due to small
source sizes of hot cores, single dish observations suffer from beam dilution
and sample the emission from both hot cores and surrounding cold envelopes.
The millimeter/submillimeter interferometric arrays (e.g., SMA, NOEMA, and
ALMA) offer larger bandwidth, higher spatial resolution and increased
sensitivity, which have promoted the hot core observations greatly, but mainly
focused on individual sources or small samples (Law et al. 2021; van der Walt
et al. 2021; Beuther et al. 2009; Qin et al. 2008, 2010, 2015; Rong et al.
2016; Rivilla et al. 2017; Bøgelund et al. 2019; Guzmán et al. 2018; Luo et
al. 2009; Csengeri et al. 2019; Sakai et al. 2013, 2018; Zapata et al. 2011;
Mottram et al. 2020; Fuente et al. 2021; Belloche et al. 2016, 2019; Brogan et
al. 2016; Bonfand et al. 2017; Xue et al. 2019; wong & An 2018; Sánchez-Monge
et al. 2010, 2014; Hernández-Hernández et al. 2014; Peng et al. 2019; Liu et
al. 2001, 2002; Remijan et al. 2003, 2004; Palau et al. 2017; Wu et al. 2014;
Taniguchi et al. 2020; Rathborne et al. 2011; Orozco-Aguilera et al. 2017;
Ahmadi et al. 2018; Mottram et al. 2020; Pagani et al. 2017; Tercero et al.
2018). Since these observations were made with different spatial resolution
and spectral setup, it is difficult to make a comparison. More recently, a
large sample including 18 well-known high-mass star-forming regions was
observed by the NOEMA with same spectral setup, suggesting that most molecules
are destroyed in evolved cores having less emission lines (Gieser et al.
2021). Of the detected hot cores, a few show chemical differentiation between
nitrogen- and oxygen-bearing COMs (e.g., Wyrowski et al. 1999; Qin et al.
2010, 2015; Remijan et al. 2004; Kalenskii & Johansson 2010; Suzuki et al.
2018; Lee et al. 2019; Csengeri et al. 2019; Allen et al. 2017; Jiménez-Serra
et al. 2012; Gieser et al. 2019; Mills et al. 2018). Chemical differentiation
and heating mechanism are still longstanding problems due to a lack of large
sample and systematic observations.

ATOMS project has observed 146 massive clumps at 3 mm band with ALMA (Liu et
al. 2020a, 2020b). 90 out of a total of 453 compact dense cores have been
considered as hot core candidates based on number of emission lines (Liu et
al. 2021). Hot cores are rich in COM lines. Especially two typical nitrogen-
and oxygen-bearing molecules C2H5CN and CH3OCHO are frequently detected in hot
cores. Spectral windows (SPWS) 7 and 8 were tuned to observe C2H5CN and
CH3OCHO with more than 3 transitions, which are suitable for identifying hot
cores. Comparing with the submillimeter waveband, the lines at 3 mm have much
less blending problem. Aiming to build up a large sample of hot cores, we
conduct a survey using C2H5CN and CH3OCHO lines based on the ATOMS continuum
and line data at 3 mm band in this work. The two molecules can also be used
for investigating chemical differentiation among these hot cores. The
observations and data reduction are briefly described in Section 2, and the
observational results including hot core identification and parameter
calculation are given in Section 3. We discuss the heating mechanism and
nitrogen and oxygen differentiation in Section 4. The main results and
conclusions are summarized in Section 5.

##  2 Observations

The sample selection and basic observational parameters are described in Liu
et al. (2020a). In brief, the ALMA band 3 observations were made toward 146
massive clumps from September to middle November 2019 with both the Atacama
Compact 7-m Array (ACA) and the 12-m array (C43-2 or C43-3 configurations).
The correlator setup was tuned to include eight SPWs with 6 higher spectral
resolutions of $\sim$ 0.2–0.4 km s-1 windows (SPWs 1–6) in the lower sideband
and SPWs 7 and 8 having lower spectral resolutions in the upper side band.
SPWs 7 and 8 have a broad bandwidth of 1875 MHz corresponding to a spectral
resolution of $\sim$1.6 km s-1 which were used for continuum imaging and line
survey purposes. The frequencies of SPWs 7 and 8 range from 97536 to 99442
MHz, and from 99470 to 101390 MHz respectively, covering many of COM lines. In
addition to the COM lines, H40$\alpha$ line at 99023 MHz is included in SPW 7
used for Hii region identification. SiO (2-1) at 86847 MHz are tuned in SPW 4
for tracing shocked gas. Since hot cores have smaller source size and COM
lines suffer from less missing flux problem, we only use the 12 m array data
for identifications of COM lines and hot cores in this work. Data reduction
was done using the CASA software package version 5.6 (McMullin et al. 2007).
The resultant continuum image and line cubes with the 12 m array data for the
146 clumps have angular resolutions of $\sim$ 1.2–1.9 arcsec, and maximum
recoverable angular scales of $\sim$ 14.5–20.3 arcsec. The mean 1 $\sigma$
noise level is better than 10 mJy beam-1 per channel for lines, and 0.4 mJy
beam-1 for continuum. Taking angular resolution of 1.9 arcsec and 3 $\sigma$
level , the position accuracy of line images due to the noise is estimated to
be better than 0.3 arcsec by using formula $\Delta\theta$=0.45
$\frac{\theta_{\rm FWHM}}{\rm{S/N}}$ (Reid et al. 1988).

##  3 Results

###  3.1 Line and hot core identifications

As stated before, 453 compact dense cores were found in 146 massive clumps
(Liu et al. 2021). We have inspected the line emission of the 453 cores one by
one and extracted spectra at the line-rich positions. In total 60 line-rich
cores are considered as hot core candidates. The results are in good agreement
with those by Liu et al. (2021) where 54 cores were found in a statistic
manner to have significant detection of at least 20 COM transitions with line
intensities larger than 3 $\sigma$ level. Then we use the eXtended CASA Line
Analysis Software Suite (XCLASS,222https://xclass.astro.uni-koeln.de/ Möller
et al. 2017) for further line identification and parameter calculation. The
XCLASS accesses the Cologne Database for Molecular Spectroscopy (CDMS; Müller
et al. 2001, 2005) and Jet Propulsion Laboratory molecular databases (JPL;
Pickett et al. 1998). Assuming the molecular gas satisfies local
thermodynamical equilibrium condition (LTE), the XCLASS solves radiative
transfer equation and produces synthetic spectra for specific molecular
transitions by taking source size, beam filling factor, line profile, line
blending, excitation, and opacity into account. In the XCLASS modelling, the
input parameters are source size, beam size, line velocity width, velocity
offset, rotation temperature and column density (Möller et al. 2017). In our
case, we take deconvolved angular sizes of the continuum sources as source
sizes which are listed in Table 1. The velocity offsets are determined when
referred to commonly detected CH3OH line at 100.6389 GHz. Therefore we set
rotation temperatures, column densities and velocity widths as free parameters
to simulate the observed spectra. To obtain optimized rotation temperature and
column density parameters, we employ Modeling and Analysis Generic Interface
for eXternal numerical codes (MAGIX; Möller et al. 2013) for further
calculation. Note that we only consider line-rich species of C2H5CN, CH3OCHO
and CH3OH molecule in this work.

The frequency setup covers many of C2H5CN and CH3OCHO transitions in SPWs 7
and 8. The observed C2H5CN and CH3OCHO transitions span upper level energies
of 30–139 K and 22–58 K, respectively. Therefore we do not expect very hot gas
components to be detected. Only a few CH3OH lines are tuned in our
observations with upper level energies ranging from 17 to 724 K. More than 3
transitions of C2H5CN and CH3OCHO are detected in the 59 line-rich cores. The
line transitions of C2H5CN and CH3OCHO in IRAS 09018-4816 are too weak to
derive rotation temperature, but 3 CH3OH transitions are detected. Rotation
temperatures and column densities can be derived simultaneously. The XCLASS
calculations suggested that all the 59 line-rich cores have gas temperature
derived from C2H5CN and CH3OCHO higher than 100 K. IRAS 09018-4816 has gas
temperature above 100 K based on CH3OH line data. Considering the distances
(Liu et al. 2021a) and source sizes, the 60 cores have source size smaller
than 0.1 pc. We then confirm the 60 cores as hot cores, and give their
coordinates, rotation temperatures and column densities in Table 1\. We have
checked the literature and found that 15 of these hot cores have been reported
before (I18089-1732, Beuther et al. 2004; I18032-2032/G9.62+0.19, Liu et al.
2015; Dall’Olio et al. 2019; I18507+0110/G34.43+0.24, Beltrán et al. 2009; Fu
& Li 2017; Calcutt et al. 2014; I17441-2822/Sgr B2(M), Sánchez-Monge et al.
2017; IRAS18056-1952/G10.47+0.03, Rolffs et al. 2011; I18507+0121/G34.26+0.15
, Sakai et al. 2013; Mookerjea et al. 2007; I16060-5146, I16065-5158,
I12326-6245, I14498-5856, I15254-5621, Araya et al. 2005; Dedes et al. 2011;
I16547-4247, I17233-3606, I18182-1433, Hernández-Hernández et al. 2014;
I17175-3544/NGC6334I, Brogan et al. 2016). Thus, 45 hot cores are newly
detected in our work. This is currently the largest hot core sample observed
with similar angular resolution and spectral coverage. Forty-one cores have
more than 3 CH3OH transitions detected. Note that this is a lower limit since
the frequency setup of C43-2 and C43-3 configurations has a 48 MHz difference
which leads to two CH3OH transitions not covered in C43-3 observations. For
CH3OH with less than 3 transitions detected, column densities are estimated by
assuming gas temperature equal to the rotation temperatures of oxygen-bearing
molecule CH3OCHO, or equal to the rotation temperatures of C2H5CN in case that
CH3OCHO is not detected.

Figure 1: Sample spectra in SPW 7 for four typical hot cores. The observed
spectra are shown as black curves and the XCLASS modelled spectra are coded in
color.

Figure 2: continued

Sample spectra in SPW 8 for four typical hot cores. The observed spectra are
shown as black curves and the XCLASS modelled spectra are coded in color. The
spectra of all the sources are availble in the supplementary material.

Figure 1 presents sample spectra toward a few hot cores with modeled molecular
spectra (C2H5CN, CH3OCHO and CH3OH) overlaid on the observed ones. Clearly
lots of C2H5CN and CH3OCHO lines are observed and large number of lines are
emitted from C2H5CN and CH3OCHO species. The spectra emitted from the hot
cores also show different emission features. From Figure 1 and the figures in
the supplementary material, one can see that line peak intensities of C2H5CN
are larger than those of CH3OCHO in 19 cores while the opposite is seen in 5
cores. It is worth mentioning that C2H5CN line emission is absent in 4 cores
while CH3OCHO is not detected in 9 cores. The differences in line emissions
from these cores may imply differences in physical and chemical environments
among these sources.

###  3.2 Line images

Line images of various molecules can provide valuable information on their
spatial distributions and possible chemical routes. We choose three molecular
line transitions of C2H5CN, CH3OCHO and CH3OH at 98523, 98792 and 100639 MHz
for the images, respectively. The sample images are shown in Figure 2, and
other images are presented in the supplementary material. Overall, most of the
C2H5CN, CH3OCHO and CH3OH emissions are associated with the continuum images,
though there are position offsets between peaks of the line and continuum
images. All the line images show compact source structure and the three line
emissions mainly distribute over small regions, indicating hot core
properties. It can be seen that the emission peaks of C2H5CN are coincident
with those of CH3OCHO in 28 cores. Considering position accuracy of 0.3 arcsec
in line images, 29 cores clearly show nitrogen and oxygen separation, i.e.,
the emissions of C2H5CN and CH3OCHO peak at different positions. So far only a
few hot cores exhibit nitrogen and oxygen differentiation in space. Our
observations provide the largest hot core sample with positional separation
between nitrogen- and oxygen-bearing molecules.

Figure 3: Sample images of the continuum and organic molecular lines. In the
left panels, the background shows the three-color image composed by
H40$\alpha$ (red), SiO (green) and Spitzer 4.5 $\micron$ (blue), and the white
contours represent the 3 mm continuum; the green rectangles mark the imaging
regions of the right panels. In the right panel, the background shows the 3 mm
continuum. The red, cygn and yellow contours represent the integrated
intensities of C2H5CN, CH3OH and CH3OCHO, respectively. The contour levels are
10 to 90 percent (stepped by 20 percent) of the peak values. The innermost
contour has a level of 95 percent of the peak value. The images of all the
sources are availble in the supplementary material.

###  3.3 Rotation temperatures, column densities and abundances relative to
CH3OH

From Table 1, one can see that the rotation temperatures of C2H5CN, CH3OCHO
and CH3OH range from 100–285 K, 100–230 K and 100–290 K respectively. Most of
the cores have gas temperatures of 100–200 K for the three species. When
compared with C2H5CN and CH3OCHO, CH3OH has the largest column densities
ranging from 8.6$\times$1016 to 1.5 $\times$1019 cm-2 which is 2–3 orders of
magnitude higher than those of C2H5CN and 1–2 orders of magnitude higher than
those of CH3OCHO. Figure 3 shows the relationships between rotation
temperatures and column densities for the three molecules in our sample.
Column densities of CH3OH increase as their rotation temperatures rise. A
linear fitting to the data gives log(N(CH3OH))=0.022Trot+14 with correlation
coefficient of 0.86. A similar trend is seen for CH3OCHO with the relation
log(N(CH3OCHO))=0.017Trot+14 and correlation coefficient of 0.68. The linear
fitting to C2H5CN data gives a correlation coefficent of 0.29, suggesting that
column densities of C2H5CN are not sensitive to its rotation temperatures,
though a weak increasing trend is seen. The results appear to indicate that
the two oxygen-bearing molecules are chemically-related species or the two
species reside in the same astrophysical environments.

Figure 4: Relationships between column densities and rotation temperatures for
CH3OH, CH3OCHO and C2H5CN. The open squares indicate the derived column
densities and rotation temperatures for the hot cores. The bars indicate the
1$\sigma$ errors. The data points without error bars in the upper panel are
for the sources without three CH3OH line detected. The linear least-squares
fit is shown as red solid line.

Figure 4 presents the relationships of column densities among the three
species. Overall, column densities of both C2H5CN and CH3OCHO have positive
correlation with that of CH3OH. The linear fittings give
log(N(CH3OCHO))=1.1log(N(CH3OH))–2.5 and log(N(C2H5CN))=1.2log(N(CH3OH))–5.1
with correlation coefficients of 0.95 and 0.82 for CH3OCHO and C2H5CN,
respectively. The models suggested that oxygen-bearing molecules CH3OCHO and
CH3OH and nitrogen-bearing molecule C2H5CN have different forming pathways
(Charnley et al. 1992; Rodgers & Charnley 2001). Probably the column densities
of the three molecules depend on the initial cloud environments. As shown in
Figure 3, the column densities of the oxygen-bearing molecules of CH3OCHO and
CH3OH correlate well with the rotation temperatures while the nitrogen-bearing
molecule C2H5CN does not follow the same trend. A possible explanation is that
excitation of CH3OCHO and CH3OH depends on both temperature and density while
excitation of C2H5CN is mainly related to its column density.

Among the detected COMs so far, CH3OH has the highest gas phase abundance
relative to H2. The infrared observations have shown that CH3OH is the most
abundant molecule relevant to water ice (e.g., Dartois et al. 1999; Gibb et
al. 2000b; Ehrenfreund & Charnley 2000). Grain surface chemical models also
suggested that formation of many of other COMs (especially the oxygen-bearing
molecules) is related to CH3OH. From the abundance criteria, the CH3OH can be
taken as one of the reference molecules to compare with other sources. The
relative abundances defined by f = N(x)/N(CH3OH) are listed in Table 1, where
x is the specific molecule CH3OCHO or C2H5CN. The relative abundances range
from 8.4$\times$10-4 to 4$\times$10-2 for C2H5CN and from 2.2$\times$10-2 to
2.8 $\times$10-1 for CH3OCHO, respectively, which are in agreement with
previous results in hot cores (Bøelund et al. 2019; Mookerjea et al. 2007;
Allen et al. 2017; Guzmán et al. 2018; Law et al. 2021; Feng et al. 2016;
Bonfand et al. 2017, 2019; Molet et al. 2019; Qin et al. 2010).

Figure 5: Correlations of column densities of CH3OH, CH3OCHO and C2H5CN. The
open squares indicate the column densities and the bars indicate the 1$\sigma$
errors. The linear least-squares fit is shown as red solid line.

##  4 Discussion

###  4.1 Heating mechanism

Observationally the evolutionary sequence of high-mass star formation starts
from massive pre-stellar cores, to high-mass protostellar objects (HMPOs), hot
cores, HC Hii and UC Hii regions where massive stars are already formed (e.g.
Beuther et al. 2007; Menten et al. 2005). During the processes of massive star
formation, the gravitational energy is converted into thermal energy to form
the HMPOs. The radiation from HMPOs, HC Hii and UC Hii regions can heat up
their immediate environments to form hot cores. Therefore hot cores are
observed to be associated with dense and hot regions near the young high-mass
protostellar objects and the UC Hii regions.

In this section, we want to differentiate internal and external heating
mechanisms by comparing the relative position of line images of COMs and UC
Hii regions. Radio continuum at centimeter wavelength and radio recombination
lines are characteristics of the UC Hii regions associated with newly formed
high-mass young stars. ATOMS covers the H40$\alpha$ line transition which can
be used for identifying UC Hii regions. Figure 2 presents sample images of the
continuum, organic molecular, H40$\alpha$ and SiO lines. From the left panels
of Figure 2 and the figures in the supplementary material, 28 UC Hii regions
are identified based on H40$\alpha$ line images. When comparing the continuum
and H40$\alpha$ images with the CH3OH line images, we find that the
morphologies of CH3OH line images are similar to those of continuum images,
and that the CH3OH emissions peak at the peak positions of the continuum, for
most sources without associated UC Hii regions. We note that although IRAS
08303–4303, IRAS 13484–6100, IRAS 16484–4603, IRAS 17158–3901, IRAS
17233-3606, and IRAS 18182–1433 have no UC Hii region counterparts, their
CH3OH emission peaks are located offset from those of continuum emissions.
Probably shocks traced by the 4.5 $\micron$ and SiO push the molecular
materials away. For the sources having H40$\alpha$ line emission, peaks of
CH3OH are offset from the continuum images except for IRAS 18056–1952.
Previous molecular line images were observed to be not always consistent with
the continuum emission in cases where continuum emission includes
contributions from dust and free-free emission of UC Hii regions. Also no
internal energy sources were identified, suggesting that these hot cores are
externally heated (e.g., Wyrowski et al. 1999; De Buizer et al. 2003;
Mookerjea et al. 2007). Then we simply classify the observed hot cores in our
work into two categories: externally heated hot cores and internally heated
hot cores. Twenty four hot cores are thought to be heated by adjacent UC Hii
regions and the other sources are classified as internally heated hot cores,
suggesting that some hot cores are not simply linked to HMPOs and precursors
to UC Hii regions in evolutionary sequence during massive star formation (Law
et al. 2021), but they only trace physical and chemical environments of hot,
dense and line-rich regions. Note that we consider IRAS 18056-1952 and
I18032-2032c1 as internally heated since both line and continuum images of the
two sources peak at UC Hii region positions.

Figure 5 shows the cumulative distributions of the rotational temperatures and
column densities of C2H5CN and CH3OCHO of the two groups of hot cores, the
internally heated ones and the externally heated ones, respectively. No
statistical difference of the rotational temperatures of C2H5CN between the
two groups can be found. The rotational temperatures of CH3OCHO tend to be
higher in the externally heated hot cores than in the internally heated hot
cores, although this difference is not very much significant with a p-value of
0.14 given by the Kolmogorov-Smirnov test. The externally and internally
heated cores show no obvious difference (with a p-value of 0.25) in the
cumulative distributions of the column densities of CH3OCHO. While significant
difference (with a p-value of 0.027) can be seen between the cumulative
distributions of the column densities of C2H5CN ($N_{\rm C_{2}H_{5}CN}$). The
values of $N_{\rm C_{2}H_{5}CN}$ in the externally heated hot cores are a
factor of two higher than those in the internally heated hot cores. Of course,
one may expect that higher temperature component from highly excited lines are
shifting towards the UC Hii regions for the externally heated hot cores while
higher temperature components will be located at the dusty continuum peaks for
internally heated hot cores. Future high angular resolution and high frequency
observations covering high energy line transitions can verify the occurrence
and characteristics of externally and internally heated mechanisms in detail.

Figure 6: The cumulative distributions of the rotational temperatures (upper)
and column densities (lower) of C2H5CN (left) and CH3OCHO (right). The
parameters of the internally heated ones and the externally heated ones are
shown in blue and orange colors, respectively. The vertical dashed lines
represent the corresponding median values. The p-value of the KS-test between
the cumulative distributions of the two groups of hot cores is shown in the
upper-left corner of each panel.

###  4.2 Nitrogen and oxygen differentiation

Nitrogen and oxygen differentiation has been suggested in some hot cores
(Blake et al. 1987; Wyrowski et al. 1999; Fayolle et al. 2015; Qin et al.
2015, 2010), showing different rotation temperatures and abundances, and
nitrogen- and oxygen-bearing COMs peaking at different spatial positions.
Observations have shown that many hot cores have excitation temperatures of
nitrogen-bearing COMs higher than those of oxygen-bearing COMs (van’t Hoff et
al. 2020, and references therein). Derived abundances of oxygen-bearing and
nitrogen-bearing COMs have positive correlations with the other oxygen-bearing
and nitrogen-bearing COMs respectively (e.g., Bisschop et al. 2007; Bergner et
al. 2017; Suzuki et al. 2018). The observations have also shown that nitrogen-
bearing COMs peak at protostar position while oxygen-bearing COMs are peaked
offset from the central source (Qin et al., 2010; Fayolle et al. 2015).

From Table 1, rotation temperatures of C2H5CN in most hot cores are generally
lower than those of CH3OH, which is inconsistent with previous observations.
As stated in Section 3.1, a possible explanation is that our 3 mm observations
of C2H5CN lines have low upper level energies of 30–139 K, and then the hot
components of the cores are not sampled. The LTE calculations find CH3OCHO and
CH3OH molecules to have larger column density than that of the nitrogen-
bearing molecule C2H5CN. Column density of CH3OH correlates well with that of
CH3OCHO, but also has a positive correlation with C2H5CN which also conflicts
with previous results as stated above. Probably our targeted lines of the
C2H5CN and CH3OCHO have low upper level energies which only sample the freshly
evaporated species, and then can not probe the innermost and hottest structure
of the hot cores. Nitrogen and oxygen separation is clearly seen in 29 hot
cores in which C2H5CN and oxygen-bearing molecules peak at different spatial
positions (see Figure 2 and the figures in the supplementary material).

###  4.3 Chemistry

COMs are suggested to be particularly important in both astrophysics and
astrochemistry, but also to be linked to origin of life. However no firm
conclusions about their formation are reached, even for commonly detected
CH3OCHO and C2H5CN. Various chemical models are proposed to interpret their
origination (Charnley et al. 1992, 1995; Caselli et al. 1993; Millar et al.
1991; Garrod et al. 2006, 2008; Taquet et al. 2012, 2015, 2016; Rodgers &
Charnley 2001, 2003; Pols et al. 2018; Choudhury et al. 2015). In summary,
main chemical routes are gas-phase and grain-surface chemical reactions as
well as interaction between gas-phase and grain-surface molecules. Large
sample observations of their relative abundances and spatial distributions
will be able to provide clues in investigating their forming pathways.

Higher gas-phase abundances of CH3OH are derived from (sub)millimeter
wavelength observations toward massive star formation regions, which is
attributed to icy CH3OH evaporated from grain mantles leading to large
abundance by various chemical models. From Figure 3, column densities of CH3OH
increase as rotation temperatures rise, providing additional evidence that
CH3OH originates from grain surface and is then released into gas phase when
gas temperature is higher than 100 K. Our large sample observations give
positive correlation between CH3OH and CH3OCHO which appears to indicate that
CH3OCHO is formed in gas phase and is related to the evaporated CH3OH as
proposed by Charnley et al. (1992, 1995). However, gas phase mechanisms by
molecule-ion reactions have very low efficiency in producing CH3OCHO (Horn et
al. 2004; Geppert et al. 2006). The relationship between the observed column
densities and rotation temperatures of CH3OCHO show the same trend as of CH3OH
that has also been repeorted by previous observations (Law et al. 2021), which
may hint that CH3OCHO also originates from grain surface and then is
evaporated into phase as temperature increases. The warm up model developed by
Garrod et al. (2006) suggested that CH3OCHO is mainly synthesized on grain
surface in lower temperatures and gas phase reactions play a dominant role in
forming CH3OCHO when the gas temperature is above 200 K. In this scheme,
formation of CH3OCHO is attributed to both grain surface and gas phase
reactions. In our case, the observations of CH3OCHO transitions at 3 mm band
have low upper level energies and should sample freshly evaporated species.
The rotation temperatures of CH3OCHO in most cores are lower than 200 K. We
then favor CH3OCHO observed by us at lower frequency band is synthesized on
grain surface. Future higher angular resolution observations of high energy
level lines can detect inner and even hotter components of the hot cores, and
can test if the gas phase reactions play an important role in forming CH3OCHO.

C2H5CN can be formed by ion-molecule reactions in gas phase, but the processes
are quite slow. Grain surface reactions through hydrogenation of HC3N is a
more efficient mechanism (Blake et al. 1987; Caselli et al. 1993; Charnley et
al. 2004). Contrary to the two complex oxygen-bearing molecules, the column
densities of C2H5CN did not show positive correlation with its rotation
temperatures. A possible explanation is that C2H5CN tends to have higher gas
temperature than the oxygen-bearing COMs while our lower energy level lines
can only trace lower excitation temperatures in the outer envelopes of the hot
cores (Öberg et al. 2013). Large amount of C2H5CN are not fully evaporated
from grain surface. Surprisingly the column densities of C2H5CN correlate well
with those of CH3OH which is inconsistent with previous observations. Previous
observations have shown that nitrogen-bearing molecules have strong
correlation with other nitrogen-bearing molecules and no correlation with
oxygen-bearing COMs (Bisschop et al. 2007; Bergner et al. 2017; Suzuki et al.
2018). As argued before, our observations may only sample freshly evaporated
species which have not involved in subsequent gas phase reactions yet. The
observed correlation between C2H5CN and CH3OH may reflect initial chemical
environments or physical differences (Van’t Hoff et al. 2020).

The spatial separation between C2H5CN and the two oxygen-bearing molecules can
be seen from Figure 2 and the figures in the supplementary material. But no
obvious temperature differentiation is observed as in Orion KL, W3(OH) complex
and G34.26+0.15 (Blake et al. 1987; Crockett et al. 2014; Wyrowski et al.
1999; Qin et al. 2015; Mookerjea et al. 2007), due to our targeted lines
having low upper level energies. Another reason is that in most cases our beam
may cover part of both nitrogen and nitrogen cores. Nitrogen and oxygen
differentiation is a long-standing problem. Different chemical models give
different explanations (Caselli et al. 1993; Garrod et al. 2008; Rodgers &
Charnley 2003; Van’t Hoff et al. 2020). Higher resolution observations of
highly excited lines from various COMS toward a large sample of hot cores are
needed to interpret the observed differentiation.

##  5 Conclusions

We have performed a systematic hot core survey employing C2H5CN, CH3OCHO and
CH3OH commonly observed in hot cores, based on the data obtained by the ATOMS
project. We summarize the main results in the following.

1\. The ATOMS 3 mm band were set up to cover multiple C2H5CN, CH3OCHO and
CH3OH transitions, which were then used for deriving their rotation
temperatures and column densities. We identify a dense core as a hot core if
more than 3 transitions of the COMs are identified and have rotation
temperature above 100 K. In total, 60 hot cores are identified, out of which
45 are newly detected, thus representing the largest hot core sample with
similar angular resolution and spectral coverage.

2\. The observations have shown that line intensities of C2H5CN are stronger
than those of CH3OCHO lines in 19 cores while the opposite is observed in 5
cores. There are total absence of C2H5CN line emission in 4 cores while
CH3OCHO lines are not detected in 9 cores. Probably the different spectral
features reflect the differences in physical and chemical environments.

3\. Line images of the three molecules show compact source structure
concentrated on small regions for the hot cores. There are 28 hot cores
associated with UC Hii regions. Based on relative positions between UC Hii
regions and CH3OH emission peaks, we classify the detected cores into two
categories: externally heated and internally heated hot cores. 24 hot cores
are externally heated by radiation from adjacent UC Hii regions while the
others are heated internally.

4\. Our large sample shows that column densities of CH3OCHO and CH3OH
correlate well with their rotation temperatures, and column densities of the
two molecules have a positive correlation. The results support that CH3OCHO
and CH3OH originate from the grain surface chemistry.

5\. Nitrogen and oxygen differentiation is observed in 29 hot cores with
nitrogen-bearing and oxygen-bearing molecules peaking at different positions,
but no clear differences in temperature and column density are seen from our
data.

In summary, our ALMA 12 m array observations provide the largest robust hot
core sample with similar angular resolution and spectral coverage. 24 hot
cores are considered to be heated by external energy sources, and therefore
hot cores may be not considered at evolutionary sequence of massive star
formation. The targeted lines of ATOMS have upper level energies of C2H5CN and
CH3OCHO less than 139 K, hence the hottest components are not observed. The
heating mechanisms as well as nitrogen and oxygen differentiation in rotation
temperature are needed to be verified by future observations of highly excited
lines of various COMs. The future observations can also test whether gas phase
chemical reactions play an important role in higher temperatures, as suggested
by chemical models.

## Acknowledgements

This work has been supported by the National Key R&D Program of China (No.
2017YFA0402701), by the National Natural Science Foundation of China (grant
Nos. 12033005, 12073061, 12122307, 12103045, 11973013, and 11873086), and by
the international partnership program of Chinese Academy of Sciences through
grant No.114231KYSB20200009, Shanghai Pujiang Program 20PJ1415500, and the
science research grants from the China Manned Space Project with No. CMS-
CSST-2021-B06. This research was carried out in part at the Jet Propulsion
Laboratory, which is operated by the California Institute of Technology under
contract with NASA. J.-E. Lee was supported by the National Research
Foundation of Korea (NRF) grant funded by the Korea government (MSIT; grant
No. 2021R1A2C1011718). C.W.L. is supported by the Basic Science Research
Program through the National Research Foundation of Korea (NRF) funded by the
Ministry of Education, Science and Technology (NRF-2019R1A2C1010851). This
work is sponsored in part by the Chinese Academy of Sciences (CAS), through a
grant to the CAS South America Center for Astronomy (CASSACA) in Santiago,
Chile. G.G. and L.B. acknowledge support by the ANID BASAL project FB210003.
This paper makes use of the following ALMA data: ADS/JAO.ALMA#2019.1.00685.S.
ALMA is a partnership of ESO (representing its member states), NSF (USA), and
NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI
(Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA
Observatory is operated by ESO, AUI/NRAO, and NAOJ.

## Data Availability

The data underlying this article are available in the ALMA archive.

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Table 1: Physical parameters of the hot cores Source | RA | DEC | $\theta_{\rm source}$ |  | C2H5CN |  |  |  | CH3OCHO |  |  |  CH3OH |
---|---|---|---|---|---|---|---|---|---|---|---|---|---
| h m s |  ${\circ}$ $\prime$ $\prime\prime$ | $\prime\prime$ | Trot (K) | N (cm-2) | f |  | Trot (K) | N (cm-2) | f |  | Trot (K) | N (cm-2)
I08303–4303S | 08:32:08.68 | –43:13:45.78 | 2.2 | 104$\pm$16 | (7.1$\pm$1.0)$\times$1014 | (2.8$\pm$0.4)$\times$10-3 |  | 101$\pm$9 | (1.5$\pm$0.2)$\times$1016 | (6.1$\pm$0.1)$\times$10-2 |  | 101 | 2.5$\times$1017
I08470–4243S | 08:48:47.79 | –42:54:27.90 | 1.8 | 125$\pm$18 | (1.3$\pm$0.2)$\times$1015 | (1.5$\pm$0.3)$\times$10-3 |  | 200$\pm$30 | (2.1$\pm$0.2)$\times$1017 | (2.5$\pm$0.1)$\times$10-1 |  | 202$\pm$32 | (8.5$\pm$1.1)$\times$1017
I09018–4816 | 09:03:33.46 | –48:28:01.69 | 3.4 |  |  |  |  |  |  |  |  | 160$\pm$8 | (1.9$\pm$0.1)$\times$1017
I11298–6155E | 11:32:05.59 | –62:12:25.62 | 3.1 |  |  |  |  | 124$\pm$6 | (1.3$\pm$0.3)$\times$1016 | (8.3$\pm$5.3)$\times$10-2 |  | 124 | 1.6$\times$1017
I12326–6245E,S | 12:35:35.09 | –63:02:31.91 | 1.3 | 130$\pm$32 | (3.1$\pm$0.2)$\times$1015 | (8.4$\pm$0.5)$\times$10-3 |  | 137$\pm$33 | (2.3$\pm$0.2)$\times$1016 | (6.2$\pm$0.6)$\times$10-2 |  | 137 | 3.7$\times$1017
I13079–6218 | 13:11:13.75 | –62:34:41.55 | 2.4 | 100$\pm$13 | (2.1$\pm$0.2)$\times$1016 | (1.2$\pm$0.2)$\times$10-2 |  | 110$\pm$9 | (1.1$\pm$0.3)$\times$1017 | (8.5$\pm$2)$\times$10-2 |  | 110 | 1.3$\times$1018
I13134–6242 | 13:16:43.2 | –62:58:32.3 | 2.2 | 139$\pm$4 | (8.2$\pm$0.5)$\times$1015 | (7.5$\pm$0.5)$\times$10-3 |  | 160$\pm$5 | (2.1$\pm$0.1)$\times$1017 | (1.9$\pm$0.1)$\times$10-1 |  | 160 | 1.1$\times$1018
I13140–6226S | 13:17:15.49 | –62:42:24.42 | 5.5 | 139$\pm$4 | (5.9$\pm$0.1)$\times$1014 | (7.4$\pm$0.2)$\times$10-3 |  | 120$\pm$17 | (6.7$\pm$0.1)$\times$1015 | (8.4$\pm$0.1)$\times$10-2 |  | 120 | 8.1$\times$1016
I13471–6120E,S | 13:50:41.81 | –61:35:10.67 | 1 | 134$\pm$8 | (2.2$\pm$0.5)$\times$1015 | (1.7$\pm$0.4)$\times$10-3 |  | 126$\pm$10 | (1.1$\pm$0.2)$\times$1017 | (8.5$\pm$1.3)$\times$10-2 |  | 126 | 1.3$\times$1018
I13484–6100 | 13:51:58.31 | –61:15:41.5 | 2.5 | 131$\pm$18 | (1.6$\pm$0.6)$\times$1015 | (1.5$\pm$0.5)$\times$10-2 |  |  |  |  |  | 126 | 1.1$\times$1017
I14498–5856S | 14:53:42.68 | –59:08:52.89 | 2.8 | 144$\pm$13 | (8.6$\pm$0.1)$\times$1014 | (1.9$\pm$0.1)$\times$10-3 |  | 106$\pm$3 | (1.5$\pm$0.1)$\times$1016 | (3.3$\pm$0.3)$\times$10-2 |  | 135$\pm$8 | (4.5$\pm$0.4)$\times$1017
I15254–5621E | 15:29:19.39 | –56:31:22.34 | 0.8 | 148$\pm$3 | (4.2$\pm$0.1)$\times$1015 | (2.3$\pm$0.2)$\times$10-3 |  | 146$\pm$14 | (9.2$\pm$0.2)$\times$1016 | (5.2$\pm$0.4)$\times$10-2 |  | 165$\pm$13 | (1.8$\pm$0.1)$\times$1018
I15437–5343 | 15:47:32.73 | –53:52:38.8 | 2.3 | 128$\pm$20 | (1.3$\pm$0.1)$\times$1015 | (1.7$\pm$0.1)$\times$10-3 |  | 106$\pm$11 | (2.1$\pm$0.1)$\times$1016 | (2.8$\pm$0.2)$\times$10-2 |  | 170$\pm$28 | (7.5$\pm$0.4)$\times$1017
I15520–5234E,S | 15:55:48.47 | –52:43:06.75 | 2.9 | 118$\pm$13 | (1.6$\pm$0.1)$\times$1015 | (3.6$\pm$0.3)$\times$10-3 |  | 160$\pm$30 | (3.7$\pm$0.1)$\times$1016 | (8.4$\pm$0.4)$\times$10-2 |  | 168$\pm$28 | (4.4$\pm$0.1)$\times$1017
I16060–5146E,S | 16:09:52.64 | –51:54:54.49 | 1.9 | 110$\pm$5 | (3$\pm$0.2)$\times$1015 | (2$\pm$0.1)$\times$10-2 |  | 110$\pm$22 | (2.5$\pm$0.2)$\times$1016 | (1.7$\pm$0.2)$\times$10-1 |  | 110 | 1.5$\times$1017
I16065–5158E,S | 16:10:19.99 | –52:06:07.25 | 3.1 | 125$\pm$15 | (2$\pm$0.1)$\times$1016 | (1.5$\pm$0.2)$\times$10-2 |  | 150$\pm$43 | (8.9$\pm$0.1)$\times$1016 | (6.8$\pm$0.1)$\times$10-2 |  | 174$\pm$14 | (1.3$\pm$0.1)$\times$1018
I16071–5142E | 16:10:59.59 | –51:50:23.37 | 4.2 | 161$\pm$6 | (1.1$\pm$0.1)$\times$1016 | (1.1$\pm$0.1)$\times$10-2 |  | 150$\pm$20 | (1.1$\pm$0.1)$\times$1017 | (1.1$\pm$0.1)$\times$10-1 |  | 190$\pm$16 | (9.8$\pm$0.2)$\times$1017
I16076–5134E,S | 16:11:26.59 | –51:41:57.84 | 4.3 | 110$\pm$18 | (3.4$\pm$0.2)$\times$1015 | (2$\pm$0.2)$\times$10-2 |  | 121$\pm$32 | (2.5$\pm$0.3)$\times$1016 | (1.5$\pm$0.2)$\times$10-1 |  | 130$\pm$14 | (1.7$\pm$0.1)$\times$1017
I16164–5046E | 16:20:11.08 | –50:53:14.75 | 2.8 | 133$\pm$20 | (7.4$\pm$0.1)$\times$1015 | (8.1$\pm$0.2)$\times$10-3 |  | 157$\pm$19 | (1.2$\pm$0.2)$\times$1017 | (1.3$\pm$0.2)$\times$10-1 |  | 187$\pm$74 | (9.1$\pm$0.1)$\times$1017
I16172–5028E,S | 16:21:02.97 | –50:35:12.6 | 1.8 | 110$\pm$32 | (1.9$\pm$0.3)$\times$1015 | (3.3$\pm$0.5)$\times$10-3 |  |  |  |  |  | 204$\pm$18 | (5.7$\pm$0.1)$\times$1017
I16272–4837c1 | 16:30:58.77 | –48:43:53.57 | 2.2 | 220$\pm$8 | (4.2$\pm$0.1)$\times$1016 | (1.3$\pm$0.1)$\times$10-2 |  | 230$\pm$37 | (8.9$\pm$0.2)$\times$1017 | (2.8$\pm$0.1)$\times$10-1 |  | 240$\pm$21 | (3.2$\pm$0.1)$\times$1018
I16272–4837c2 | 16:30:58.68 | –48:43:51.32 | 2 | 120$\pm$3 | (3.1$\pm$0.1)$\times$1015 | (6.9$\pm$0.1)$\times$10-3 |  | 150$\pm$9 | (3.9$\pm$0.1)$\times$1016 | (8.7$\pm$0.1)$\times$10-2 |  | 180$\pm$6 | (4.5$\pm$0.1)$\times$1017
I16272–4837c3 | 16:30:57.29 | –48:43:39.87 | 2.1 | 103$\pm$11 | (2.1$\pm$0.1)$\times$1015 | (6.9$\pm$0.1)$\times$10-3 |  | 123$\pm$12 | (3.8$\pm$0.1)$\times$1015 | (8.7$\pm$0.1)$\times$10-2 |  | 123$\pm$6 | (2.1$\pm$0.1)$\times$1017
I16318–4724E | 16:35:33.96 | –47:31:11.59 | 2.2 | 130$\pm$5 | (2.7$\pm$0.1)$\times$1016 | (9.6$\pm$0.8)$\times$10-3 |  | 148$\pm$15 | (1.9$\pm$0.5)$\times$1017 | (6.8$\pm$0.2)$\times$10-2 |  | 192$\pm$18 | (2.8$\pm$0.2)$\times$1018
I16344–4658E | 16:38:09.49 | –47:04:59.73 | 1.7 | 150$\pm$4 | (5.5$\pm$0.1)$\times$1015 | (1.1$\pm$0.1)$\times$10-2 |  | 160$\pm$15 | (5.2$\pm$0.1)$\times$1016 | (1.1$\pm$0.1)$\times$10-1 |  | 160$\pm$12 | (4.8$\pm$0.1)$\times$1017
I16348–4654E,S | 16:38:29.65 | –47:00:35.67 | 0.8 | 270$\pm$13 | (2$\pm$0.1)$\times$1017 | (1.3$\pm$0.1)$\times$10-2 |  | 213$\pm$12 | (2$\pm$0.1)$\times$1018 | (1.3$\pm$0.1)$\times$10-1 |  | 220$\pm$12 | (1.5$\pm$0.1)$\times$1019
I16351–4722E,S | 16:38:50.50 | –47:28:00.68 | 1.5 | 121$\pm$10 | (5.7$\pm$0.1)$\times$1015 | (7.6$\pm$0.1)$\times$10-3 |  | 150$\pm$31 | (6$\pm$0.3)$\times$1016 | (8$\pm$0.4)$\times$10-2 |  | 150 | 7.5$\times$1017
I16458–4512S | 16:49:30.04 | –45:17:44.58 | 1.1 |  |  |  |  | 120$\pm$4 | (2.1$\pm$0.1)$\times$1016 | (1.4$\pm$0.1)$\times$10-1 |  | 120 | 1.5$\times$1017
I16484–4603 | 16:52:04.66 | –46:08:33.85 | 1.3 | 120$\pm$12 | (1.7$\pm$0.3)$\times$1015 | (1$\pm$0.3)$\times$10-3 |  | 127$\pm$5 | (6.1$\pm$0.3)$\times$1016 | (3.6$\pm$0.3)$\times$10-2 |  | 151$\pm$4 | (1.7$\pm$0.1)$\times$1018
I16547–4247S | 16:58:17.18 | –42:52:07.57 | 2.5 | 114$\pm$8 | (3.1$\pm$0.1)$\times$1015 | (7.8$\pm$0.3)$\times$10-3 |  | 150$\pm$20 | (2.2$\pm$0.2)$\times$1016 | (5.5$\pm$0.4)$\times$10-2 |  | 170$\pm$5 | (4$\pm$0.1)$\times$1017
I17008–4040 | 17:04:22.91 | –40:44:22.91 | 1.7 | 161$\pm$7 | (1.6$\pm$0.1)$\times$1016 | (5$\pm$0.1)$\times$10-3 |  | 150$\pm$14 | (2.8$\pm$0.1)$\times$1017 | (8.8$\pm$0.1)$\times$10-2 |  | 180$\pm$12 | (3.2$\pm$0.1)$\times$1018
I17016–4124c1E | 17:05:10.97 | –41:29:06.95 | 0.9 | 153$\pm$6 | (3.6$\pm$0.1)$\times$1016 | (7.8$\pm$0.1)$\times$10-3 |  | 143$\pm$14 | (4.9$\pm$0.1)$\times$1017 | (1.1$\pm$0.1)$\times$10-1 |  | 193$\pm$14 | (4.6$\pm$0.1)$\times$1018
I17016–4124c2 | 17:05:11.20 | –41:29:07.05 | 1.8 | 101$\pm$12 | (5$\pm$0.2)$\times$1015 | (1.7$\pm$0.1)$\times$10-3 |  | 165$\pm$32 | (2$\pm$0.3)$\times$1017 | (6.6$\pm$0.1)$\times$10-2 |  | 180$\pm$11 | (3.1$\pm$0.1)$\times$1018
I17158–3901c1 | 17:19:20.43 | –39:03:51.58 | 2.9 | 152$\pm$9 | (1$\pm$0.1)$\times$1015 | (3.2$\pm$0.4)$\times$10-3 |  | 151$\pm$12 | (2.2$\pm$0.2)$\times$1016 | (7.1$\pm$0.5)$\times$10-2 |  | 150$\pm$2 | (3.1$\pm$0.2)$\times$1017
I17158–3901c2S | 17:19:20.47 | –39:03:49.20 | 2.8 | 152$\pm$3 | (1.5$\pm$0.1)$\times$1015 | (1.4$\pm$0.1)$\times$10-3 |  |  |  |  |  | 150 | 1.1$\times$1017
I17175–3544E | 17:20:53.42 | –35:46:57.72 | 3.4 | 174$\pm$9 | (5.4$\pm$0.2)$\times$1016 | (8.6$\pm$0.4)$\times$10-3 |  | 183$\pm$8 | (1.4$\pm$0.1)$\times$1018 | (2.2$\pm$0.1)$\times$10-1 |  | 182$\pm$6 | (6.3$\pm$0.1)$\times$1018
I17220–3609E,S | 17:25:25.22 | –36:12:45.34 | 1.6 | 183$\pm$14 | (1.6$\pm$0.1)$\times$1016 | (5.7$\pm$0.3)$\times$10-3 |  | 173$\pm$22 | (3.8$\pm$0.1)$\times$1017 | (1.4$\pm$0.1)$\times$10-1 |  | 176$\pm$6 | (2.8$\pm$0.1)$\times$1018
I17233–3606E,S | 17:26:42.46 | –36:09:17.85 | 3.9 | 160$\pm$4 | (3.9$\pm$0.1)$\times$1016 | (4.1$\pm$0.2)$\times$10-2 |  | 100$\pm$14 | (8.9$\pm$0.5)$\times$1016 | (9.3$\pm$0.6)$\times$10-2 |  | 165$\pm$8 | (9.6$\pm$0.4)$\times$1017
I17441–2822E | 17:47:20.17 | –28:23:04.74 | 1.3 | 160$\pm$18 | (2.5$\pm$0.1)$\times$1016 | (1.8$\pm$0.1)$\times$10-2 |  | 144$\pm$7 | (9.6$\pm$0.2)$\times$1016 | (6.9$\pm$0.2)$\times$10-2 |  | 207$\pm$8 | (1.4$\pm$0.1)$\times$1018
I18032–2032c1 | 18:06:14.92 | –20:31:43.22 | 0.9 |  |  |  |  | 204$\pm$17 | (1.2$\pm$0.1)$\times$1017 | (1.1$\pm$0.1)$\times$10-1 |  | 204 | 1.1$\times$1018
I18032–2032c2 | 18:06:14.88 | –20:31:39.59 | 2.4 | 127$\pm$5 | (2$\pm$0.1)$\times$1016 | (4$\pm$0.3)$\times$10-2 |  | 130$\pm$12 | (7$\pm$0.2)$\times$1016 | (1.5$\pm$0.4)$\times$10-1 |  | 146$\pm$5 | (4.7$\pm$0.2)$\times$1017
I18032–2032c3 | 18:06:14.80 | –20:31:37.26 | 1.1 | 110$\pm$3 | (2.4$\pm$0.1)$\times$1015 | (4$\pm$0.3)$\times$10-2 |  |  |  |  |  | 110 | 1.8$\times$1017
I18032–2032c4 | 18:06:14.66 | –20:31:31.57 | 2.1 | 130$\pm$16 | (8$\pm$0.1)$\times$1015 | (3.8$\pm$0.1)$\times$10-3 |  | 140$\pm$13 | (1.7$\pm$0.2)$\times$1017 | (8.1$\pm$0.3)$\times$10-2 |  | 157$\pm$8 | (2.1$\pm$0.1)$\times$1018
I18056–1952S | 18:08:38.23 | –19:51:50.31 | 1.1 | 285$\pm$9 | (1.9$\pm$0.1)$\times$1018 | (6.3$\pm$0.1)$\times$10-2 |  | 133$\pm$5 | (2.1$\pm$0.1)$\times$1018 | (7$\pm$0.3)$\times$10-2 |  | 226$\pm$9 | (3$\pm$0.1)$\times$1019
I18089–1732 | 18:11:51.45 | –17:31:28.96 | 1.7 | 277$\pm$9 | (4.1$\pm$0.1)$\times$1016 | (1.6$\pm$0.1)$\times$10-2 |  | 128$\pm$14 | (2.3$\pm$0.5)$\times$1017 | (8.8$\pm$0.2)$\times$10-2 |  | 200$\pm$7 | (2.6$\pm$0.1)$\times$1018
I18117–1753S | 18:14:39.51 | –17:52:00.08 | 1.6 | 180$\pm$28 | (9.5$\pm$0.1)$\times$1015 | (5.9$\pm$0.2)$\times$10-3 |  | 160$\pm$25 | (1.3$\pm$0.2)$\times$1017 | (8.1$\pm$1.1)$\times$10-2 |  | 163$\pm$5 | (1.6$\pm$0.1)$\times$1018
I18159–1648c1 | 18:18:54.66 | –16:47:50.28 | 1.7 | 157$\pm$26 | (7.6$\pm$0.2)$\times$1015 | (7.6$\pm$0.1)$\times$10-3 |  | 161$\pm$18 | (2.4$\pm$0.1)$\times$1017 | (2.4$\pm$0.4)$\times$10-1 |  | 228$\pm$12 | (1.1$\pm$0.2)$\times$1018
I18159–1648c2 | 18:18:54.34 | –16:47:49.97 | 1.9 | 104$\pm$20 | (1.1$\pm$0.2)$\times$1015 | (1.8$\pm$0.3)$\times$10-3 |  | 110$\pm$22 | (2.9$\pm$0.3)$\times$1016 | (4.8$\pm$0.4)$\times$10-2 |  | 165$\pm$11 | (6$\pm$0.1)$\times$1017
Table 2: continued

Source | RA | DEC | $\theta_{\rm source}$ |  | C2H5CN |  |  |  | CH3OCHO |  |  |  CH3OH |
---|---|---|---|---|---|---|---|---|---|---|---|---|---
| h m s |  ${\circ}$ $\prime$ $\prime\prime$ | $\prime\prime$ | Trot (K) | N (cm-2) | f |  | Trot (K) | N (cm-2) | f |  | Trot (K) | N (cm-2)
I18182–1433S | 18:21:09.05 | –14:31:47.88 | 2.1 | 120$\pm$10 | (1.1$\pm$0.1)$\times$1015 | (5.2$\pm$0.5)$\times$10-3 |  | 122$\pm$4 | (2.4$\pm$0.2)$\times$1016 | (1.1$\pm$0.1)$\times$10-1 |  | 120 | 2.1$\times$1017
I18236–1205S | 18:26:25.79 | –12:03:53.08 | 1.4 | 110$\pm$8 | (1.8$\pm$0.1)$\times$1015 | (1.1$\pm$0.1)$\times$10-2 |  |  |  |  |  | 138$\pm$20 | (1.7$\pm$0.1)$\times$1017
I18290–0924S | 18:31:44.13 | –09:22:12.25 | 1.4 | 120$\pm$4 | (3.3$\pm$0.9)$\times$1015 | (5.8$\pm$1.6)$\times$10-3 |  |  |  |  |  | 120 | 5.7$\times$1017
I18316–0602 | 18:34:20.91 | –05:59:42.0 | 2.9 | 186$\pm$35 | (2.8$\pm$0.1)$\times$1015 | (3.2$\pm$0.1)$\times$10-3 |  | 108$\pm$204 | (2.1$\pm$0.15)$\times$1016 | (2.4$\pm$0.2)$\times$10-2 |  | 167$\pm$7 | (8.7$\pm$0.3)$\times$1017
I18411–0338S | 18:43:46.23 | –03:35:29.77 | 1 | 134$\pm$4 | (9.6$\pm$0.5)$\times$1015 | (1.1$\pm$0.1)$\times$10-2 |  | 160$\pm$15 | (7$\pm$0.1)$\times$1016 | (7.7$\pm$0.5)$\times$10-2 |  | 160$\pm$14 | (9.1$\pm$0.6)$\times$1017
I18469–0132E,S | 18:49:33.05 | –01:29:03.34 | 0.9 | 124$\pm$8 | (5.1$\pm$0.1)$\times$1015 | (3.6$\pm$0.2)$\times$10-2 |  | 136$\pm$15 | (9.4$\pm$0.2)$\times$1016 | (6.7$\pm$0.4)$\times$10-2 |  | 145$\pm$9 | (1.4$\pm$0.1)$\times$1018
I18507+0110E,S | 18:53:18.56 | +01:14:58.23 | 1.3 | 172$\pm$6 | (9$\pm$0.2)$\times$1016 | (8.2$\pm$0.2)$\times$10-3 |  | 200$\pm$11 | (1.6$\pm$0.2)$\times$1018 | (1.5$\pm$0.2)$\times$10-1 |  | 240$\pm$16 | (1.1$\pm$0.1)$\times$1019
I18507+0121 | 18:53:18.01 | +01:25:25.56 | 1.7 | 160$\pm$4 | (3.7$\pm$0.1)$\times$1016 | (8.8$\pm$0.3)$\times$10-3 |  | 200$\pm$12 | (6.1$\pm$0.1)$\times$1017 | (1.5$\pm$0.1)$\times$10-1 |  | 220$\pm$8 | (4.2$\pm$0.1)$\times$1018
I18517+0437S | 18:54:14.24 | +04:41:40.65 | 2.3 | 190$\pm$30 | (8$\pm$1)$\times$1014 | (8.4$\pm$1.1)$\times$10-4 |  | 140$\pm$23 | (2.8$\pm$0.3)$\times$1016 | (2.9$\pm$0.4)$\times$10-2 |  | 164$\pm$8 | (9.5$\pm$0.3)$\times$1017
I19078+0901c1E,S | 19:10:13.16 | +09:06:12.49 | 1.7 | 140$\pm$22 | (2.5$\pm$0.2)$\times$1015 | (1.5$\pm$0.1)$\times$10-2 |  |  |  |  |  | 140 | 1.7$\times$1017
I19078+0901c2 | 19:10:14.13 | +09:06:24.67 | 1.8 | 120$\pm$12 | (3.2$\pm$0.2)$\times$1015 | (1.5$\pm$0.1)$\times$10-2 |  | 115$\pm$6 | (4.6$\pm$0.3)$\times$1016 |  |  | 170$\pm$8 | (8.7$\pm$0.3)$\times$1017
I19095+0930E,S | 19:11:53.99 | +09:35:50.27 | 0.5 | 160$\pm$11 | (7.5$\pm$0.2)$\times$1015 | (1.8$\pm$0.1)$\times$10-3 |  | 121$\pm$3 | (1.3$\pm$0.1)$\times$1017 | (3.1$\pm$0.1)$\times$10-2 |  | 156$\pm$15 | (4.2$\pm$0.1)$\times$1018

Notes: The superscript E in column 1 indicates that the hot core is externally
heated, while S denotes that there is spatial separation between nitrogen- and
oxygen-bearing molecules. $\theta_{\rm source}$ is the source size derived
from the deconvolved continuum size on assuming $\pi\theta^{2}$=$\pi$ab, where
a and b are major and minor axes of the continuum source from Gaussian
fitting. f denotes the relative abundance of the molecular species (C2H5CN or
CH3OCHO) with respect to CH3OH.

## Author affiliations

1Department of Astronomy, Yunnan University, Kunming 650091, People’s Republic
of China
2Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan
Road, Shanghai 200030, People’s Republic of China
3Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove
Drive, Pasadena, CA 91109, USA
4National Astronomical Observatories, Chinese Academy of Sciences, Beijing
100101, People’s Republic of China
5University of Chinese Academy of Sciences, Beijing 100049, People’s Republic
of China
6NAOC-UKZN Computational Astrophysics Centre, University of KwaZulu-
Natal,Durban 4000, South Africa
7Center for Astrophysics, Harvard & Smithsonian, 60 Garden Street,Cambridge,
MA 02138, USA
8Department of Astronomy, Peking University, 100871 Beijing, People’s Republic
of China
9Departamento de Astronom’ia, Universidad de Chile, Las Condes, 7591245
Santiago, Chile
10Department of Physics, University of Helsinki, PO Box 64, FI-00014 Helsinki,
Finland
11Korea Astronomy and Space Science Institute, 776 Daedeokdaero, Yuseong-gu,
Daejeon 34055, Republic of Korea
12University of Science and Technology, Korea (UST), 217 Gajeong-ro,Yuseong-
gu, Daejeon 34113, Republic of Korea
13School of Physics and Astronomy, Sun Yat-sen University, 2 Daxue Road,
Zhuhai, Guangdong 519082, People’s Republic of China
14Yunnan Observatories, Chinese Academy of Sciences, 396 Yangfangwang, Guandu
District, Kunming, 650216, People’s Republic of China
15Chinese Academy of Sciences South America Center for Astronomy, National
Astronomical Observatories, CAS, Beijing 100101, China
16Institute of Astronomy and Astrophysics, Academia Sinica, 11F of Astronomy-
Mathematics Building, AS/NTU No. 1, Section 4, Roosevelt Road., Taipei 10617,
Taiwan
17School of Space Research, Kyung Hee University, Yongin-Si, Gyeonggi-Do
17104, Republic of Korea
18Kavli Institute for Astronomy and Astrophysics, Peking University, 5
Yiheyuan Road, Haidian District, Beijing 100871, People’s Republic of China
19Department of Physics, Anhui Normal University, Wuhu, Anhui 241002, People’s
Republic of China
20Institute of Astrophysics, School of Physics and Electronical Science,
Chuxiong Normal University, Chuxiong, 675000, People’s Republic of China
21Institute of Astronimy and Astrophysics, Anqing Normal University, Anqing,
246133, People’s Republic of China
22College of Science, Yunnan Agricultural University, Kunming 650201, People’s
Republic of China
24Physical Research Laboratory, Navrangpura, Ahmedabad-380 009, India
25Indian Institute of Space Science and Technology, Thiruvananthapuram 695
547, India

##  Appendix A Supplementary material

###  A.1 Spectra in SPW 7

Figure 7: Spectra in SPW 7 for the 60 hot cores. The observed spectra are
shown in black curves and the XCLASS modelled spectra are coded in color.
Figure 8: continued Figure 9: continued Figure 10: continued Figure 11:
continued Figure 12: continued Figure 13: continued Figure 14: continued
Figure 15: continued Figure 16: continued Figure 17: continued Figure 18:
continued

###  A.2 Spectra in SPW 8

Figure 19: Spectra in SPW 8 for the 60 hot cores. The observed spectra are
shown in black curves and the XCLASS modelled spectra are coded in color.
Figure 20: continued Figure 21: continued Figure 22: continued Figure 23:
continued Figure 24: continued Figure 25: continued Figure 26: continued
Figure 27: continued Figure 28: continued Figure 29: continued Figure 30:
continued

###  A.3 Images of the continuum and organic molecular lines for externally
heated hot cores

Figure 31: Images of the continuum and organic molecular lines. In the left
panels, the background shows the three-color image composed by H40$\alpha$
(red), SiO (green) and Spitzer 4.5 $\micron$ (blue), and the white contours
represent the 3 mm continuum; the green rectangles mark the imaging regions of
the right panels. In the right panel, the background shows the 3 mm continuum.
The red, cygn and yellow contours represent the integrated intensities of
C2H5CN, CH3OH and CH3OCHO, respectively. The contour levels are 10 to 90
percent (stepped by 20 percent) of the peak values. The innermost contour has
a level of 95 percent of the peak value. Figure 31: – continued Figure 31: –
continued Figure 31: – continued Figure 31: – continued

###  A.4 Images of the continuum and organic molecular lines for internally
heated hot cores

Figure 32: Images of the continuum and organic molecular lines. In the left
panels, the background shows the three-color image composed by H40$\alpha$
(red), SiO (green) and Spitzer 4.5 $\micron$ (blue), and the white contours
represent the 3 mm continuum; the green rectangles mark the imaging regions of
the right panels. In the right panel, the background shows the 3 mm continuum.
The red, cygn and yellow contours represent the integrated intensities of
C2H5CN, CH3OH and CH3OCHO, respectively. The contour levels are 10 to 90
percent (stepped by 20 percent) of the peak values. The innermost contour has
a level of 95 percent of the peak value. Figure 32: – continued Figure 32: –
continued Figure 32: – continued Figure 32: – continued