Chemical Composition in the IRAS 16562–3959 High-mass Star-forming Region

Figure 1 shows the continuum image made from the widest spectral window (216.961–218.834 GHz, Table 1). The angular resolution is 032 × 025. The strongest continuum peak associates with the G345.5+1.47 MYSO, which is indicated as Core A in Figure 1. Another continuum peak corresponds to Core C, which we will identify later in this subsection. A small continuum peak, labeled as D in Figure 1, is detected at the north position from Core A, and a weak continuum emission has been detected at the Core B position.

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We define the position of the hypercompact (HC) H ii region and that of prominent molecular emission based on the H30α and CH₃OH (4_−2,3 − 3_−1,2 E) moment zero maps. These positions are shown in the continuum map (Figure 1) and in the moment zero maps of the H30α and of the methanol transition shown in Figure 2. Core A corresponds to the HCH ii region, and Cores B and C correspond to the strongest rich molecular cores identified in IRAS 16562–3959 by Cesaroni et al. (2017) and by Guzmán et al. (2018). Table 2 lists general properties of these cores based on 2D Gaussian fittings to the moment zero maps.

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Table 2.  Identification of Cores

Position	R.A. (J2000)	Decl. (J2000)	θmajor × θminor	Position Angle	Vsys (km s−1)a	Flux (mJy beam−1)b	${N}_{{{\rm{H}}}_{2}}$ (cm−2)c
Core A	16h59m41627	−40°03'4361	030 × 025	66°	−17.1	119.0 ± 1.6	${5.0}_{-2.6}^{+5.5}\times {10}^{24}$
Core B	16h59m41586	−40°03'4296	070 × 048	108°	−14.6	7.8 ± 1.6	${1.9}_{-1.0}^{+2.1}\times {10}^{23}$
Core C	16h59m41089	−40°03'3903	054 × 047	27°	−11.8	19.2 ± 1.6	${4.8}_{-2.4}^{+5.3}\times {10}^{23}$

Notes. Core A is identified based on the moment zero map of the H30α line by the 2D Gaussian fitting. Core B and Core C are identified by the 2D Gaussian fitting of the moment zero image of the CH₃OH (4_−2,3 − 3_−1,2 E) line.

^aObtained by the Gaussian fitting of the J = 12 − 11, K = 4 line of CH₃CN. ^bContinuum fluxes with beam sizes of 03 for Core A and 05 for the others, respectively. ^cErrors were derived by changes in assumed temperatures between 50 and 200 K.

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We derived the H₂ column density, ${N}_{{{\rm{H}}}_{2}}$, at each core using the following formula (Kauffmann et al. 2008):

$$\begin{eqnarray}&&\begin{array}{l}{N}_{{{\rm{H}}}_{2}}=2.02\times {10}^{20}\left({e}^{1.439{\left(\lambda /\mathrm{mm}\right)}^{-1}{\left(T/10{\rm{K}}\right)}^{-1}}-1\right)\\ \times \ {\left(\displaystyle \frac{{\kappa }_{\nu }}{0.01{\mathrm{cm}}^{2}{{\rm{g}}}^{-1}}\right)}^{-1}\left(\displaystyle \frac{{F}_{\nu }^{\mathrm{beam}}}{\mathrm{mJy}\ {\mathrm{beam}}^{-1}}\right){\left(\displaystyle \frac{{\theta }_{\mathrm{HPBW}}}{10^{\prime\prime} }\right)}^{-2}{\left(\displaystyle \frac{\lambda }{\mathrm{mm}}\right)}^{3},\end{array}\end{eqnarray} \tag{ 1 }$$

where

$$\begin{eqnarray}&&{\kappa }_{\nu }=0.1{\left(\displaystyle \frac{\nu }{1\mathrm{THz}}\right)}^{\beta }.\end{eqnarray} \tag{ 2 }$$

The symbols of λ, T, κ_ν, ${F}_{\nu }^{\mathrm{beam}}$, and θ_HPBW in Equation (1) are the wavelength, temperature, dust opacity, flux of dust continuum emission, and half power beamwidth, respectively. The continuum data wavelength is approximately 1.37 mm. The flux values at each core are summarized in Table 2. These fluxes are beam-averaged values with beam sizes of 03 for Core A and 05 for the others, respectively. We corrected the continuum emission toward Core A by subtracting the free–free continuum emission calculated from the H30α spectra in Figure A1 as we describe in Appendix A. We subtracted this free–free component from the continuum flux and derived the dust continuum emission. H30α emission toward Core B and Core C was not detected. The dust opacity at the 1.37 mm (κ_ν) is calculated using Equation (2) (Planck Collaboration et al. 2011) with β = 1.6 (Sadavoy et al. 2013). The dust temperature (T) is assumed to be 100 K, because COMs have been detected toward all of the cores as shown in Section 3.2. If we change the temperature between 50 and 200 K, the derived ${N}_{{{\rm{H}}}_{2}}$ values change by a factor of ∼2. The derived ${N}_{{{\rm{H}}}_{2}}$ values are summarized in Table 2.

We made moment zero images of each molecular line integrating the whole velocity range where emission is detected. Figure 3 shows the spatial distributions of (a) C¹⁸O (2 − 1), (b) H₂CO (${3}_{\mathrm{2,2}}-{2}_{\mathrm{2,1}}$), (c) DCN (3 − 2), and (d) ³³SO (6₅ − 5₄). Their spatial distributions, except for ³³SO, are relatively extended compared to those of COMs (Figures 4 and 5). The red crosses indicate positions of Cores A to C (Figure 2 and Table 2). Table 3 summarizes species, transition, rest frequency, excitation energy, and peak intensity of each moment zero image analyzed in this work.

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Table 3.  Summary of Moment Zero Images

Panel	Species	Transition	Rest Frequency	Eu/k	Peak Intensity
			(GHz)	(K)	(Jy beam−1 × km s−1)
Figure 3
(a)	C18O	J = 2 − 1	219.5603541	15.8	0.60
(b)	H2CO	${J}_{{Ka},{Kc}}={3}_{\mathrm{2,2}}-{2}_{\mathrm{2,1}}$	218.475632	68.1	0.74
(c)	DCN	J = 3 − 2	217.2385378	20.9	0.33
(d)a	33SO	JN = 65 − 54, $F=\tfrac{9}{2}-\tfrac{7}{2}$	217.8271782	34.7	0.28
		JN = 65 − 54, $F=\tfrac{11}{2}-\tfrac{9}{2}$	217.8298337	34.7	⋯
		JN = 65 − 54, $F=\tfrac{13}{2}-\tfrac{11}{2}$	217.8317691	34.7	⋯
		JN = 65 − 54, $F=\tfrac{15}{2}-\tfrac{13}{2}$	217.8326422	34.7	⋯
Figure 4
(a)	CH3OH	4−2,3 − 3−1,2 E	218.440063	45.5	1.11
(b)	CH3OH	20−1,19 − 20−0,20 E	217.886504	508.4	0.40
(c)b	CH3CN	J = 12 − 11, K = 3	220.7090165	133.2	1.60
(d)a	CH3OCH3	224,19 − 223,20 EA	217.189668	253.4	0.15
		224,19 − 223,20 AE	217.189669	253.4	⋯
		224,19 − 223,20 EE	217.191400	253.4	⋯
		224,19 − 223,20 AA	217.193132	253.4	⋯
(e)a,b	(CH3)2CO	202,18 − 193,17 EE	218.1272074	119.1	0.13
		203,18 − 193,17 EE	218.1272074	119.1	⋯
		202,18 − 192,17 EE	218.1272074	119.1	⋯
		203,18 − 192,17 EE	218.1272074	119.1	⋯
(f)	H30α	⋯	231.900928	⋯	11.1
Figure 5
(a)	HC3N	v7 = 2, J = 24 − 23, l = 0	219.6751141	773.5	0.082
(b)	HC3N	v7 = 2, J = 24 − 23, l = 2e	219.7073487	776.8	0.095
(c)	HC13CCN	v = 0, J = 24 − 23	217.3985682	130.4	0.15
(d)	HCC13CN	v = 0, J = 24 − 23	217.4195740	130.4	0.11
(e)a	13CN	N = 2 − 1, $J=\tfrac{3}{2}-\tfrac{1}{2}$, F1 = 2 − 1, F = 1 − 0	217.296605	15.6	0.51
		N = 2 − 1, $J=\tfrac{5}{2}-\tfrac{3}{2}$, F1 = 2 − 2, F = 2 − 2	217.298937	15.7	⋯
		N = 2 − 1, $J=\tfrac{3}{2}-\tfrac{1}{2}$, F1 = 2 − 1, F = 2 − 1	217.301175	15.6	⋯
		N = 2 − 1, $J=\tfrac{3}{2}-\tfrac{1}{2}$, F1 = 2 − 1, F = 3 − 2	217.303191	15.6	⋯
(f)	DCN	J = 3 − 2	217.2385378	20.9	0.33
(g)a	HNCO	103,8 − 93,7	219.6567695	433.0	0.14
		103,7 − 93,6	219.6567708	433.0	⋯
(h)a	HNCO	102,9 − 92,8	219.7338500	228.3	0.56
		102,8 − 92,7	219.7371930	228.3	⋯

Notes. Rest frequency and excitation energy are taken from the CDMS (Müller et al. 2005).

^aThese lines were not resolved and detected as one line. ^bRest frequency and excitation energy are taken from the JPL catalog (Pickett et al. 1998).

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The spatial distributions of C¹⁸O and H₂CO show arc-like structures with strong peaks at Core B. There is also weaker emission associated with Core C. In Section 4.2, we discuss the formation mechanisms of H₂CO by comparing between the spatial distributions of C¹⁸O and H₂CO.

The DCN spatial distribution (panel (c) of Figure 3) is different from the above two species. A strong peak is located at Core B and a filamentary structure can be seen at the northeastern region, corresponding to the northeast outflow cavity wall (NEC-wall) in Guzmán et al. (2018). Other emission regions are located nearby and to the west of Core C.

Panel (d) of Figure 3 shows a close-up moment zero image of ³³SO (6₅ − 5₄) emission at G345.5+1.47. One ³³SO peak is associated with Core B, and other peaks are located near Core A. The ³³SO emission seems to surround Core A. We discuss the ³³SO spatial distribution around Core A in detail in Section 4.3.

Figures 4 and 5 show moment zero maps of various molecular emission lines' more compact morphology. The information of lines and peak intensities for each panel is listed in Table 3. Most of the molecular emissions are associated with both Cores B and C as shown in Figures 4 and 5.

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Figure 4 shows the spatial distributions of COMs in panels (a)–(e) and H30α in panel (f). The CH₃OH (4_−2,3 − 3_−1,2 E) line in panel (a) shows more extended spatial distribution than the CH₃OH (20_−1,19 − 20_−0,20 E) line in panel (b). This is caused by their different upper energy levels; E_u/k = 44.5 K for panel (a) and 508.4 K for panel (b), respectively (Table 3).

The CH₃CN (12 − 11, K = 3) line comes from Cores A, B, and C, while the lines of oxygen-bearing COMs mainly come from Core B and Core C (Figure 4). The upper energy level of the CH₃CN line of panel (c) is 133.2 K, which is an intermediate value compared to those of panels (a) and (b). Therefore, the different spatial distributions between CH₃OH and CH₃CN are not brought by the different upper energy level of the lines. The different morphologies between the CH₃OH (4_−2,3 − 3_−1,2 E) and the CH₃CN line seem to reflect different origins. As indicated in Figure 6, the CH₃OH (4_−2,3 − 3_−1,2 E) line has been detected at Core A, although there is no emission peak at Core A (panel (a) of Figure 4). Since low excitation energy lines of CH₃OH trace shock regions (e.g., Taniguchi et al. 2020), the CH₃OH line around Core A is possibly originated in a shock region. Shock regions are induced by several star formation phenomena, such as jets and molecular outflows. Guzmán et al. (2011) identified the SE–NW molecular outflow and the central knot of the jet/outflow system is consistent with the Core A position (Guzmán et al. 2010). Alternatively, the CH₃OH line may trace a remnant gas scattered by the HCH ii region. The CH₃CN emission, on the other hand, does peak at Core A, which implies thermal sublimation from dust grains.

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The spatial distribution of the CH₃OCH₃ (22_4,19 − 22_3,20) lines in panel (d) is similar to that of (CH₃)₂CO (20 − 19) lines in panel (e). The upper energy levels of the lines in panels (d) and (e) are 253.4 K and 119.1 K, respectively (Table 3). Both of these lines likely trace hot core regions. In addition, their distributions at Core B are more compact than that of the CH₃OH (4_−2,3 − 3_−1,2 E; E_u/k = 44.5 K) line in panel (a), and similar to the CH₃OH (20_−1,19 − 20_−0,20 E; E_u/k = 508.4 K) line in panel (b). These different spatial distributions seem to reflect lower abundances of CH₃OCH₃ and (CH₃)₂CO compared to the CH₃OH abundance (Section 3.4) and different upper energy levels.

Figure 5 shows the spatial distributions of HC₃N and its ¹³C isotopologues (24 − 23) in panels (a)–(d), ¹³CN (2 − 1) in panel (e), DCN (3 − 2) in panel (f), and HNCO (10 − 9) in panels (g) and (h). The vibrationally excited lines of HC₃N (v₇ = 2, J = 24 − 23) mainly come from Core B, while the ground vibrational state lines (v = 0, J = 24 − 23) of the ¹³C isotopologues associate with Cores A, B, and C, as shown in panels (a)–(d). The two ¹³C isotopologues of HC₃N, HC¹³CCN and HCC¹³CN, show the same spatial distributions in panels (c) and (d). The more extended distributions of the ¹³C isotopologues are caused by the lower energy levels of the observed lines. In fact, these vibrationally excited lines have extremely high upper energy levels of ∼775 K, compared to those of the ground vibrational state lines (130.4 K) in panels (c) and (d). The HC₃N spatial distributions are similar to that of CH₃CN (12 − 11), a typical hot core tracer.

The ¹³CN (2 − 1) emission in panel (e) is associated with Core B and its weak emission comes from Core C, while the DCN (3 − 2) distribution in panel (f) shows a more extended structure. The upper energy levels are 15.6 K and 20.9 K for the ¹³CN and DCN lines, respectively. Because these are quite similar to each other, different upper energy transition levels is not the main cause of the different spatial distributions of these two species. This difference is more likely to be caused by the different environments traced by each species. The CN/HCN ratio is enhanced in high ultraviolet (UV) flux regions, because HCN is destroyed by the UV radiation forming CN (e.g., Riaz et al. 2018). A possible explanation for the nondetection of the ¹³CN line at Core A, at which the UV flux is likely strongest in this region, is the selective photodissociation. In order to investigate the effect of the selective photodissociation, we need to observe the H¹³CN lines and lines of normal species of CN and HCN.

Four HNCO (10 − 9) lines also show similar spatial distributions to CH₃CN and HC₃N; the HNCO lines associate with Cores A, B, and C as shown in panels (g) and (h). The differences in spatial distribution between panels (g) and (h) arises from the different upper energy levels. The upper energy levels of lines in panel (g) are 433.0 K and higher than those in panel (h) (228.3 K), and therefore the spatial distribution of panel (h) is more extended.

The critical density is another important factor to determine spatial distributions of each line. Since we could not derive accurate densities at each position, we do not discuss the critical density. However, we note that most molecular lines have been detected at Core B, at which the derived H₂ column density is lower than the other cores. This suggests the upper energy level of the transition is more relevant than the critical density to explain the emission and make the compositions presented in this study.

Figures 6 and 7 show spectra of the 217.0–218.6 GHz and 219.56–219.76 GHz bands toward Cores A, B, and C, respectively. The bottom three panels are a zoom of the top three. We identified lines using the CASSIS software with the Cologne Database for Molecular Spectroscopy (CDMS; Müller et al. 2005) and the Jet Propulsion Laboratory (JPL) catalog (Pickett et al. 1998). Since the velocity resolution of the final spectra of Figure 6 is low (3 km s⁻¹), some lines could not be identified without ambiguity. At the position of Core C, the ³³SO line is detected as a spurious-like line (Figure 6). Such a spurious-like feature is considered to be brought by very low-velocity resolution (3 km s⁻¹). We confirm that it does not affect other regions. We then neglect the position of Core C in its moment zero image. We did not apply Gaussian fitting for detected lines due to the low-velocity resolution. Table 4 summarizes species, transition, rest frequency, and excitation energy of detected lines at each core position.

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Table 4.  Summary of Detected Lines

Species	Transition	Rest Frequency	Eu/k	Core Aa	Core Ba	Core Ca
		(GHz)	(K)
(CH3)2CO	193,16 − 184,15, 194,16 − 183,15	217.0225	115.5	N	Y	(Y)
13CN	N = 2 − 1, J = 3/2 − 3/2, F1 = 1 − 1, F = 1 − 2	217.046988	15.7	N	Y	(Y)
	N = 2 − 1, J = 3/2 − 3/2, F1 = 1 − 1, F = 2 − 1	217.0728010	15.7	N	Y	(Y)
SiO	v = 0, J = 5 − 4	217.104980	31.3	Y	(Y)	Y
CH3OCH3b	224,19 − 223,20 EA	217.189668	253.4	N	Y	Y
	224,19 − 223,20 AE	217.189669	253.4	N	Y	Y
	224,19 − 223,20 EE	217.191400	253.4	N	Y	Y
	224,19 − 223,20 AA	217.193132	253.4	N	Y	Y
DCN	J = 3 − 2	217.2385378	20.9	Y	Y	Y
13CN	N = 2 − 1, J = 3/2 − 1/2, F1 = 1 − 0, F = 0 − 1	217.264639	15.7	N	Y	N
	N = 2 − 1, J = 5/2 − 3/2	217.296605	15.7	N	Y	Y
	N = 2 − 1, J = 5/2 − 3/2, F1 = 2 − 2, F = 2 − 3	217.315147	15.7	N	Y	N
HC13CCN	J = 24 − 23	217.3985682	130.4	Y	Y	Y
HCC13CN	J = 24 − 23	217.419574	130.4	Y	Y	Y
13CN	N = 2 − 1, J = 5/2 − 3/2, F1 = 2 − 1, F = 3 − 2	217.4285632	15.7	N	Y	(Y)
(CH3)2CO	1812,6 − 1713,4	217.5921139	141.3	N	N	(Y)
	2320,3 − 2221,1b	217.6552	248.3	N	Y	(Y)
	3712,25 − 3711,26, 3713,25 − 3712,26b	217.6553	493.2	N	Y	(Y)
33SOb	JN = 65 − 54, $F=\tfrac{9}{2}-\tfrac{7}{2}$	217.8271782	34.7	Y	Y	(Y)
	JN = 65 − 54, $F=\tfrac{11}{2}-\tfrac{9}{2}$	217.8298337	34.7	Y	Y	(Y)
	JN = 65 − 54, $F=\tfrac{13}{2}-\tfrac{11}{2}$	217.8317691	34.7	Y	Y	(Y)
	JN = 65 − 54, $F=\tfrac{15}{2}-\tfrac{13}{2}$	217.8326422	34.7	Y	Y	(Y)
CH3OH	20−1,19 − 20−0,20 E	217.886504	508.4	N	Y	Y
(CH3)2CO	202,18 − 193,17, 203,18 − 192,17	218.0914	119.2	N	Y	(Y)
	3222,10 − 3219,13	218.1059	438.9	N	Y	(Y)
	202,18 − 193,17, 203,18 − 193,17, 202,18 − 192,17	218.1272	119.1	N	Y	(Y)
	202,18 − 193,17, 203,18 − 192,17	218.1629	119.0	N	Y	(Y)
H2CO	30,3 − 20,2	218.2222	21.0	Y	Y	Y
CH3OCHO	173,14 − 163,13	218.2809	99.7	N	Y	Y
	173,14 − 163,13	218.2979	99.7	N	Y	Y
HC3N	v = 0, J = 24 − 23	218.3247	131.0	Y	Y	Y
CH3OH	4−2,3 − 3−1,2	218.440063	45.5	Y	Y	Y
H2CO	32,2 − 22,1	218.4756	68.1	Y	Y	Y
CH3OCH3	233,21 − 232,22	218.4898	263.8	N	Y	(Y)
	233,21 − 232,22	218.4924	263.8	N	Y	(Y)
	233,21 − 232,22	218.495	263.8	N	Y	(Y)
(CH3)2CO	1912,8 − 1813,5	218.5439	154.3	N	Y	(Y)
C18O	J = 2 − 1	219.5603541	15.8	Y	Y	Y
HNCOb	103,8 − 93,7	219.6567695	433.0	N	Y	N
	103,7 − 93,6	219.6567708	433.0	N	Y	N
HC3N	v7 = 2, J = 24 − 23, l = 0	219.67465	769.7	N	Y	N
	v7 = 2, J = 24 − 23, l = 2e	219.70689	772.3	N	Y	N
HNCO	102,9 − 92,8	219.7338500	228.3	Y	Y	(Y)
	102,8 − 92,7	219.7371930	228.3	Y	Y	(Y)
HC3N	v7 = 2, J = 24 − 23, l = 2f	219.74174	772.3	N	Y	N

Notes. Rest frequency and excitation energy are taken from the CDMS (Müller et al. 2005) and the JPL catalog (Pickett et al. 1998).

^a"Y" and "N" mean detection and nondetection at each core, respectively. The symbol (Y) means tentative detection. ^bThese lines were not resolved and detected as one line.

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Core B is the most line-rich position, where various oxygen-bearing COMs, e.g., CH₃OCH₃ and CH₃OCHO, have been detected. Furthermore, vibrational-excited lines of HC₃N, whose upper energy levels are extremely high (E_u/k ≈ 775 K), have been detected.

As shown later, the excitation temperature of CH₃CN is derived to be above 200 K at Core C. Furthermore, the abundances of COMs are high. For example, the CH₃OH abundance is around 10⁻⁶ (Figure 9). This abundance can be reproduced only after the temperature reaches above 200 K (Taniguchi et al. 2019). These results suggest active hot core chemistry at Core C. Guzmán et al. (2018) also concluded that Core C is associated with a second hot molecular core within IRAS 16562–3959, linked to an MYSO less massive than G345.5+1.47. In summary, all of these results imply that a very young star is embedded at Core C.

We analyzed spectra at the three cores using the CASSIS software (Caux et al. 2011). In the analyses presented here, we have used the local thermodynamic equilibrium (LTE) model available in the CASSIS spectrum analyzer by assuming lines are optically thin. The rotational diagram fittings of CH₃CN do not show any systematic shifts as shown in Figure 8. Thus, the assumption of the optically thin regime seems to be reasonable in our case.

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Except for CH₃CN, only one or a few lines with similar excitation energies have been detected for each species. We then applied the Markov Chain Monte Carlo (MCMC) method, which is an interactive process that goes through all of the parameters with a random walk and heads into the solutions space, and χ² minimization gives the final solution.

We derive the excitation temperatures and column densities of CH₃CN using its K−ladder lines of the J = 12 − 11 transition. Left panels of Figure 8 show spectra of the eight K−ladder lines of the J = 12 − 11 transition (K = 0 − 7) toward the three cores. The lines from right to left correspond to from K = 0 to K = 7. We fitted the spectra with a Gaussian profile. We cannot fit the K = 7 line toward Core A, and we omitted it from the fitting.

Right panels of Figure 8 show the rotational diagram using the Gaussian fitting results of the left panels. The derived column densities and excitation temperatures are summarized in Table 5. The column densities and excitation temperatures are (1.0 ± 0.1) × 10¹⁶ cm⁻² and 279 ± 57 K; (1.4 ± 0.2) × 10¹⁶ cm⁻² and 420 ± 117 K; and (1.3 ±0.2) × 10¹⁵ cm⁻² and 213 ± 25 K at Cores A, B, and C, respectively. Although we assumed that the CH₃CN lines are optically thin, we cannot rule out the optically thick case due to the scatter. The plots for the first three K−ladder lines (K = 0 − 2) are coincident with each other within their errors, but the plots for the K = 0 line may be slightly lower than the other two plots. If the lines are optically thick, the derived rotational temperature should be overestimated (Beltrán et al. 2011; Furuya et al. 2011).

We derived the column densities and excitation temperatures of other species using the MCMC method and the LTE model in the CASSIS software. We considered the following two cases:

• 1.  
  assume excitation temperatures between 50 and 200 K, and
• 2.  
  assume a range of temperatures around the excitation temperatures derived by the CH₃CN fitting.

We assume the excitation temperature range of the first case taking the typical temperature at the hot core of 100 K into consideration. For the second case, we tested the following temperature ranges: 220–340 K for Core A, 250–540 K for Core B, and 185–240 K for Core C, respectively.

We fixed source sizes, or size of the emitting region, to 03 for Core A and 05 for Core B and Core C, respectively (Table 2). Because the cores appear to be resolved, we assume a filling factor of unity. The line width (FWHM) was treated as a free parameter, constrained between 1 and 10 km s⁻¹. The line widths derived by the fitting are ∼2–4 km s⁻¹ for Core A and Core B, and ∼3–5 km s⁻¹ for Core C, respectively. Derived line widths agree well with those of typical hot cores.

Table 6 summarizes the derived column densities and excitation temperatures of the detected species except for CH₃CN toward each core. "Case 1" and "Case 2" represent the different assumed excitation temperature ranges, as described above. The excitation temperatures of H₂CO and CH₃OH are consistent within their standard deviation errors in all cases. Other oxygen-bearing COMs (CH₃OCHO, CH₃OCH₃, and (CH₃)₂CO) have similar excitation temperatures, except for (CH₃)₂CO assuming Case 1 toward Core C. Although there is a large uncertainty of the CH₃CN excitation temperature at Core B, the excitation temperatures of all the species are coincident with each other at each core in Case 2. We will discuss comparisons of the chemical compositions between Cores B and C in Section 4.1 with the results of Case 2.

We derived fractional abundances, X(molecules) = N(molecules)/${N}_{{{\rm{H}}}_{2}}$, of each species toward Cores B and C, and compare them as shown in Figure 9. We took the errors from both N(molecules) and ${N}_{{{\rm{H}}}_{2}}$ into consideration.

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The observed H₂CO and CH₃OH abundances at Cores B and C can be reproduced after their thermal desorption from dust grains (Figure B1 in Appendix B and Figure 6 of Taniguchi et al. 2019). The thermal desorption of H₂CO and CH₃OH occurs at ∼50 K and ∼100 K, respectively. Abundances of other COMs (∼10⁻⁸–10⁻⁷) are reproduced in a hot core model with temperature above 100 K (Garrod 2013). These results suggest hot core chemistry is taking place at both Core B and Core C.

All of the species have larger fractional abundances at Core B compared to Core C. While the fractional abundances of CH₃OH and H₂CO at Core B are higher than those at Core C by a factor of ≃100, the differences in fractional abundances of other species are around one order of magnitude. Methanol (CH₃OH) is important for formation of oxygen-bearing COMs (Öberg et al. 2009). Hence, oxygen-bearing COMs except for H₂CO and CH₃OH are expected to be formed later than H₂CO and CH₃OH. These results suggest that Core B is more chemically rich or a more evolved hot core with enough time for molecules to sublimate from dust grains and newly form in the hot gas.

Most COMs are generally abundant in hot core regions with temperatures above 100 K where ice mantles sublimate. As shown in Figures 3 and 4, the spatial distribution of H₂CO is different from distributions of other oxygen-bearing COMs; the distribution of H₂CO shows an arc-like structure with a strong peak at Core B, while other COMs are concentrated at Core B and Core C. Such different spatial distributions may suggest that the main formation mechanism of H₂CO is different from those of other COMs. In this subsection, we discuss possible main formation mechanisms of H₂CO.

According to Taniguchi et al. (2019), the formation mechanisms of H₂CO are highly sensitive to the physical evolution of the cores. Based on this, we can clearly distinguish three major formation processes of H₂CO (Figure B1 in Appendix B):

• 1.  
  formation in the gas-phase through the reaction CH₃ + O → H₂CO + H, active in cold starless core phase when T ≈ 10 K and n_H ≈ 10⁴–10⁷ cm⁻³;
• 2.  
  nonthermal desorption of H₂CO ice formed through the successive hydrogenation of CO ice (ice–H + ice–CO → ice–HCO, ice-HCO + ice–H → ice–H₂CO) active in the lukewarm stage when 10 K < T < 25 K and n_H ≈ 10⁷ cm⁻³;
• 3.  
  thermal evaporation of H₂CO ice, active in hot core stage when T > 50 K and n_H ≈ 10⁷ cm⁻³.

Figure 10 shows the spatial distributions of C¹⁸O (color) and H₂CO (white contours). Their spatial distributions show a similar structure: an arc-like extended structure and a strong peak at Core B. The extended structure seems to suggest that heating sources are not necessary for formation of the gas-phase H₂CO. This means that either gas-phase formation or nonthermal desorption is important for H₂CO. The similar spatial distributions of C¹⁸O and H₂CO support the grain-surface formation and nonthermal desorption. Still, this does not mean that gas-phase formation is irrelevant for H₂CO. To distinguish the two reactions, we need further observations, e.g., O i distribution.

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The H₂CO abundance with respect to H₂ at Core B is derived to be around 10⁻⁵ (Figure 9). This abundance can be reproduced only after thermal evaporation of H₂CO (Figure B1) with temperatures above 50 K. Since many COMs have been detected at Core B, the dust temperature is expected to be above 100 K. Thus, the thermal evaporation seems to work efficiently at this position.

Guzmán et al. (2014) showed the spatial distributions of sulfur-bearing species, SO, SO₂, CS, and OCS, with an angular resolution of ∼23 × 13. They found that the emission lines of these species come from a molecular core with a size of around 3000 au at the G345.5+1.47 MYSO. Guzmán et al. (2014) suggested that the observed SO emission and morphology at Core B can be understood qualitatively using the predictions of hot gaseous phase chemical models (e.g., van der Tak et al. 2003). Panel (d) of Figure 3 shows a ring-like distribution of ³³SO around Core A. Such a structure has been found for the first time in this source owing to the high spatial resolution. In this subsection, we discuss this ³³SO structure.

Figure 11 shows the spatial distributions of ³³SO (color) and H30α (magenta contours) emissions. The strong emission peaks of ³³SO are located at the outer edge of H30α emission. There are two possible scenarios for such a ring-like structure around the HCH ii region; one is molecular destruction by the UV radiation from the central star, and the other is the gas-phase formation of SO at this position. If such a ring-like structure has been formed by molecular destruction by the UV radiation from the central star, other molecules are expected to show similar structures. However, we do not find ring-like structures in moment zero images of other molecules (Figures 4 and 5). This implies that the destruction did not produce the ring-like structure of ³³SO emission by the UV radiation from the central star.

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Sulfur monoxide (SO) is considered to be a shock tracer. At Core A, shocks can be produced by a molecular outflow and an expanding motion of the HCH ii region. However, the morphology of the ³³SO emission does not match the orientation of the molecular outflow nor the jet reported by Guzmán et al. (2011). Hence, the molecular outflow does not seem to be an origin of the ³³SO emission feature. In summary, ³³SO is expected to be enhanced in shock regions produced by an interaction between a thick cloud and an expanding motion of the HCH ii region. In shock regions, SO is considered to be formed by the following reactions (Esplugues et al. 2014):

$$\begin{eqnarray}&&{{\rm{O}}}_{2}+{\rm{S}}\to \mathrm{SO}+{\rm{O}},\end{eqnarray} \tag{ 3 }$$

and

$$\begin{eqnarray}&&\mathrm{OH}+{\rm{S}}\to \mathrm{SO}+{\rm{H}}.\end{eqnarray} \tag{ 4 }$$

Esplugues et al. (2014) suggested that reaction (3) is efficient just after the shock passes and the temperature is still high (T ≥ 1000 K), while reaction (4) becomes more efficient in cool gas regions. According to the Kinetic Database for Astrochemistry (KIDA: http://kida.astrophy.u-bordeaux.fr/), the α values for the reaction rate coefficients, defined as k = α(T/300)^βexp(−γ/T) cm³ s⁻¹, are 2.1 × 10⁻¹² and 6.6 × 10⁻¹¹ for reactions (3) and (4), respectively. The γ values for both of the reactions are reported as zero (Vidal et al. 2017). Therefore, the reaction (4) is expected to proceed faster than the reaction (3), and could be a main formation pathway of SO around the observed HCH ii region.