INFALL AND OUTFLOW MOTIONS IN THE HIGH-MASS STAR-FORMING COMPLEX G9.62+0.19

Tie Liu; Yuefang Wu; Sheng-Yuan Liu; Sheng-Li Qin; Yu-Nung Su; Huei-Ru Chen; Zhiyuan Ren

doi:10.1088/0004-637X/730/2/102

1. INTRODUCTION

High-mass stars play a major role in the evolution of the Galaxy. They are the principal sources of heavy elements and UV radiation (Zinnecker & Yorke 2007). However, the formation and evolution of high-mass stars are still unclear. A possible evolution sequence of high-mass stars from infrared dark clouds to classic H ii regions has been suggested (Van der Tak & Menten 2005). But one of the major topics—whether high-mass stars form through accretion-disk outflow, like low-mass ones (Shu et al. 1987), or via collision coalescence (Wolfire & Cassinelli 1987; Bonnell et al. 1998)—is still far from being solved.

Yet more and more observations at various resolutions seem to support the accretion-disk-outflow models rather than the collision-coalescence models. Disks are detected in several high-mass star-forming regions (Patel et al. 2005; Jiang et al. 2005; Sridharan et al. 2005). Outflows are found with a high detection rate as in low-mass cores in single-dish surveys (Wu et al. 2004; Zhang et al. 2005; Qin et al. 2008b). High-resolution studies have also confirmed that molecular outflows are common in high-mass star-forming regions (Su et al. 2004; Qiu et al. 2007, 2009; Qin et al. 2008a, 2008c). Searching for inflow motions has also made large progress in recent years (Wu & Evans 2003; Fuller et al. 2005; Wyrowski et al. 2006; Klaassen & Wilson 2007; Wu et al. 2007, 2009; Furuya et al. 2011). Both infall and outflow motions in the massive core JCMT 18354−0649S are detected (Wu et al. 2005) and further confirmed by higher resolution observations (Liu et al. 2011). Although accretion-disk-outflow systems are found in high-mass star-forming regions, there may be differences between low- and high-mass formation.

The infall motion can be detected via "blue profile," a double-peaked profile with the blueshifted peak being stronger for optically thick lines and a single peak at the absorption part of optically thick lines for optically thin lines, which is caused by self-absorption of the cooler outer infalling gas toward the warmer central region (Zhou et al. 1993). In contrast, the "red profile" where the redshifted peak of a double-peaked profile is stronger for optically thick lines is suggested as an indicator for outflow motions. Mardones et al. (1997) defined the "blue excess" in a survey, E, as E = (N_B − N_R)/N_T, where N_T is the number of sources and N_B and N_R mark the number of sources with blue and red profiles, respectively. The blue excess seems to have no significant differences among the low-mass cores in different evolutionary phases. However, using the IRAM 30 m telescope, Wu et al. (2007) found that ultracompact (UC) H ii regions show a higher blue excess than their precursors, indicating fundamental differences between low- and high-mass star-forming conditions. The searches need to be expanded.

Located at a distance of 5.7 kpc (Hofner et al. 1994), G9.62+0.19 is a well-studied high-mass star-forming region containing a cluster of H ii regions, which are probably at different evolutionary stages. Multiwavelength Very Large Array observations have identified nine radio continuum sources (denoted from A to I; Garay et al. 1993; Testi et al. 2000); components C to I are very compact (<5'' in diameter; Garay et al. 1993; Testi et al. 2000). As revealed in NH₃ (4,4), (5,5), and CH₃CN (J = 6–5), component F is a hot molecular core (HMC) and hence likely the youngest source in the region (Cesaroni et al. 1994; Hofner et al. 1994, 1996). G9.62+0.19 E is a young massive star surrounded by a very small UC H ii region and a dusty envelope (Hofner et al. 1996), while G9.62+0.19 D is a small cometary UC H ii region excited by a B0.5 zero-age main-sequence star (Hofner et al. 1996; Testi et al. 2000). Both G9.62+0.19 E and G9.62+0.19 D seem to be at a more evolved stage than G9.62+0.19 F. Thus, G9.62 complex is an ideal sample to examine massive star-forming activities including outflow and infall motions.

Maser emissions of NH₃, H₂O, OH, and CH₃OH, as well as the strong thermal NH₃ emissions, were detected along a narrow region with projected length 20'' and width ⩽2'' (Hofner et al. 1994). A possible explanation for this alignment is compression of the molecular gas by shock front originating from an even more evolved H ii region to the west of the star-forming front (Hofner et al. 1994). High-velocity molecular outflows have also been detected in this region, and G9.62+0.19 F is believed to be the driving source (Gibb et al. 2004; Hofner et al. 2001; Su et al. 2005). However, most of the previous work was carried out at low frequencies, probing low excitation conditions. To examine the hot dust/gas environment and dynamical processes in this region, higher resolution studies at high frequencies are needed. In this paper, we report the results of the Submillimeter Array (SMA)⁶ observations toward the G9.62+0.19 region at 860 μm.

2. OBSERVATIONS

The observations of G9.62+0.19 with the SMA were carried out on 2005 July 9 with seven antennas in its compact configuration at 343 GHz for the lower sideband (LSB) and 353 GHz for the upper sideband (USB). The T_sys ranges from 210 to 990 K with a typical value of 380 K at both sidebands during the observations. The observations had two fields for the G9.62+0.19 complex to cover the entire region with emissions. One phase reference center was R.A. (J2000) = 18^h06^m1421 and decl. (J2000) = −20°31'46 farcs 2, and the other was R.A. (J2000) = 18^h06^m1500 and decl. (J2000) = −20°31'34 farcs 20. Uranus and Neptune were observed for antenna-based bandpass calibration. QSOs 1743-038 and 1911-201 were employed for antenna-based gain correction. Neptune was used for flux-density calibration. The frequency spacing across the spectra band was 0.8125 MHz corresponding to a velocity resolution of ∼0.7 km s⁻¹.

MIRIAD was employed for calibration and imaging (Sault et al. 1995). The imaging was done to each field separately and the mosaic continuum map was made using a linear mosaicing algorithm (task "linmos" in MIRIAD). The 860 μm continuum data were acquired by averaging over all the line-free channels in both sidebands. The spectral cubes were constructed using the continuum-subtracted spectral channels smoothed into a velocity resolution of 1 km s⁻¹. Additional self-calibration with models of the clean components from a previous imaging process was performed on the continuum data in order to remove residual errors due to phase and amplitude problems, and the gain solutions obtained from the continuum data were applied to the line data. The synthesized beam size of the continuum emission with robust weighting of 0.5 is 2 farcs 76 × 1 farcs 88 (P.A. = 21 fdg 4).

3. RESULTS

3.1. Continuum Emission

The 860 μm continuum image combining the visibility data from both sidebands is shown in Figure 1. Three submillimeter cores are detected. The known centimeter and millimeter continuum components (Testi et al. 2000) of B, C, D, E, F, G, H, and I are marked by plus signs. Water masers (Hofner & Churchwell 1996) are marked by open squares and methanol masers (Norris et al. 1993) by triangles. The near-IR sources (Persi et al. 2003; Testi et al. 1998; Linz et al. 2005) are marked by filled circles. IRAC sources are taken from the database of the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE)⁷ and labeled with asterisks. The northern core is located at the southeast of G9.62+0.19 C, and the middle core is associated with G9.62+0.19 E. The 860 μm continuum emission at the southern core is concentrated on the HMC G9.62+0.19 F and extends to G9.62+0.19 D in the south and to G9.62+0.19 G in the north.

Gaussian fits were made to the continuum. The northern core seems to be a point-like source. The middle core is very compact with a deconvolved size of ∼1 farcs 4. The southern core is found to be elongated from north to south with an average size of 2 farcs 4, containing at least three sources, D, F, and G. F is at its peak position. The peak positions, sizes, peak intensities, and total fluxes of these three submillimeter cores are listed in Columns 2–5 in Table 1. The physical properties of these cores will be further discussed in Section 4.2.

Table 1. Parameters of 860 μm Continuum Emission

Name	R.A.	Decl.	Deconvolution Sizes	I_peak	S_ν	T_d^a	β^b	Mass	N $_{\rm H_{2}}$
	(J2000)	(J2000)	('' × '')	(Jy beam⁻¹)	(Jy)	(K)		(M_☉)	(10²⁴ cm⁻²)
Northern core	18:06:14.447	−20:31:28.253	Point source	0.20 ± 0.02	0.26	50	1.5	13
Middle core	18:06:14.668	−20:31:31.830	148 × 129 (P.A. = −378)	0.76 ± 0.04	1.07	92	1.2	30	1.2
Southern core	18:06:14.889	−20:31:40.149	471 × 126 (P.A. = −202)	0.95 ± 0.12	2.52	51	0.8	165	2.1

Notes. ^aThe dust temperature is assumed to be the same as the rotational temperature of H₂CS transitions. ^bThe opacity index β is obtained from Su et al. (2005).

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3.2. Line Emission

Tens of molecular transitions including hot molecular lines CH₃OH, HCOOCH₃, and CH₃OCH₃ are detected toward both the middle and southern submillimeter cores, indicating these two cores are hot and dense (Qin et al. 2010). Figure 2 presents the full LSB and USB spectra in the UV domain over the shortest baseline. The strongest lines are identified and labeled on the plots. Only HCN (4–3) and CS (7–6) line emissions are detected toward the northern core with our sensitivity. Thus, we mainly focus on the middle and southern cores in this paper. The systemic velocities of 2.1 km s⁻¹ for the middle core and 5.2 km s⁻¹ for the southern core are obtained by averaging the V_lsr of multiple singly peaked lines. Six transitions of the thioformaldehyde (H₂CS) and molecular transitions SO (8₇–7₇), CS (7–6), HC¹⁵N (4–3), and HCN (4–3) are analyzed here, while the others will be discussed in another paper. We have made Gaussian fits to the beam-averaged spectra and present the observed parameters of these lines in Table 2.

**Figure 2.** Full LSB and USB spectra in the UV domain over the shortest baseline. The strongest lines are identified and labeled on the plots.
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3.2.1. Line Emission at the Middle Submillimeter Core

The integrated intensity maps of four transitions of H₂CS toward the middle core are shown in the upper panels of Figure 3. From panels (a) to (d), the upper level energy of H₂CS transitions varies from ∼90 K to ∼400 K. The H₂CS emission is spatially coincident with continuum emission of the middle core very well. The position–velocity (P–V) diagram and first moment map of H₂CS (10_2,8–9_2,7) emission are presented in Figure 4. The P–V diagram is constructed across the peak of the continuum along the N–S direction. From the P–V diagram two emission peaks are clearly revealed. The velocities of the two emission peaks are 1 and 3 km s⁻¹ with 1 farcs 5 spatial separation, indicating a velocity gradient in the N–S direction. The first moment map also shows velocity changes in the N–S direction. The small velocity gradient detected in H₂CS (10_2,8–9_2,7) emission may indicate a disk with a low inclination along the line of sight, which requires further confirmation with higher angular resolution observations and other molecular line tracers.

**Figure 4.** P–V diagram (left) and first moment map (right) of H₂CS (10_2,8–9_2,7) emission at the middle core. (a) The contours of the P–V diagram are from 0.6 to 1.4 in steps of 0.2 Jy beam⁻¹ (1σ). (b) Contour plot of H₂CS (10_2,8–9_2,7) integrated intensity image overlaid on the first moment map. The contours are from 0.9 (3σ) in steps of 0.9 Jy beam⁻¹ km s⁻¹. The first moment map is constructed from the data after imposing a cutoff of 3σ.
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Table 2. Observed Parameters of the Lines

Molecule	Transition^a	Frequency	E_u	rms	V_lsr^b				Intensity^b				FWHM^b
		(GHz)	(K)	(Jy beam⁻¹)	(km s⁻¹)				(Jy beam⁻¹)				(km s⁻¹)
					D	E	F	G	D	E	F	G	D	E	F	G
H₂CS	10_0,10–9_0,9	342.946	90.6	0.3	5.5 ± 0.2	2.7 ± 0.2	5.9 ± 0.2	6.0 ± 0.5	1.9 ± 0.3	1.4 ± 0.2	1.7 ± 0.1	0.9 ± 0.2	2.7 ± 0.4	3.1 ± 0.5	4.3 ± 0.6	3.8 ± 1.1
	10_2,9–9_2,8^c	343.322	143.3	0.3	4.0 ± 0.5	2.2 ± 1.1		4.9 ± 0.3	0.9 ± 0.2	1.4 ± 0.3		1.1 ± 0.3	4.0 ± 0.6	4.4 ± 1.6		1.9 ± 0.6
	10_2,8–9_2,7	343.813	143.3	0.3	6.0 ± 0.3	2.5 ± 0.4	4.9 ± 0.5		1.3 ± 0.2	0.9 ± 0.1	0.8 ± 0.2		3.0 ± 0.7	5.5 ± 0.9	3.9 ± 1.3
	10_3,8–9_3,7^d	343.410	209.1	0.3	6.2 ± 0.8	2.4 ± 0.7	5.0 ± 0.3		1.1 ± 0.7	1.1 ± 0.1	2.1 ± 0.3		3.3 ± 0.7	4.1 ± 0.5	2.7 ± 0.6
	10_3,7–9_3,6^e	343.414	209.1	0.3	5.8 ± 2.8	2.6 ± 0.2	5.5 ± 0.3	5.8 ± 0.2	0.7 ± 0.2	2.0 ± 0.2	2.4 ± 0.3	1.5 ± 0.3	5.9 ± 0.8	4.0 ± 0.5	2.9 ± 0.6	2.3 ± 0.5
	10_5,6/5–9_5,5/4^f	343.203	419.2	0.2		1.8 ± 0.4	5.9 ± 0.3	5.9 ± 0.2		0.6 ± 0.2	0.7 ± 0.2	0.9 ± 0.2		3.2 ± 1.0	2.4 ± 0.8	1.5 ± 0.4
SO	8₈–7₇	344.311	87.5	0.2	4.9 ± 0.1	2.2 ± 0.1	5.0 ± 0.1	4.4 ± 0.1	2.9 ± 0.1	4.0 ± 0.1	5.5 ± 0.1	3.5 ± 0.1	3.9 ± 0.2	5.3 ± 0.2	9.0 ± 0.2	8.2 ± 0.3
HC¹⁵N ν = 0	4–3	344.200	41.3	0.2	6.1 ± 0.4	2.6 ± 0.1	5.2 ± 0.1	4.8 ± 0.3	0.6 ± 0.1	2.1 ± 0.1	2.8 ± 0.1	1.1 ± 0.1	3.6 ± 1.0	4.8 ± 0.3	8.0 ± 0.3	7.5 ± 0.8
CS	7–6	342.883	65.8	0.3
HCN ν = 0	4–3	354.505	42.5	0.3

Notes. ^aNot all the detected lines are listed in this table. The others will be presented in another paper. ^bThe V_lsr, intensity, and FWHM of each transition are derived from single Gaussian fit toward the beam-averaged spectra. ^cBlended with H¹³₂CO (5_1,4–4_1,4) at 343.325713 GHz. ^dBlended with H₂CS (10_3,7–9_3,6). ^eBlended with H₂CS (10_3,8–9_3,7). ^fThe two transitions of H₂CS (10_5,6–9_5,6) and (10_5,6–9_5,6) have the same frequency, line strength, and permanent dipole moment. Therefore, they have the same contributions to the observed line profile.

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The spectra and integrated intensity maps of HC¹⁵N (4–3) and SO (8₇–7₇) are presented in Figure 5. The two spectra seem to be symmetric and their cores are associated with that of the continuum emission very well.

Figure 6 presents the spectra and P–V diagrams of HCN (4–3) and CS (7–6) emissions of the middle core. HCN (4–3) and CS (7–6) show asymmetric profiles. The blue and red emission peaks of HCN (4–3) are around 0 and 6 km s⁻¹, respectively. The blueshifted emission of CS (7–6) peaks around 1 km s⁻¹, while the redshifted one around 4 km s⁻¹. We can see that the blueshifted emission of both HCN (4–3) and CS (3-2) is always stronger than the redshifted emission and the absorption is also redshifted, which are blue profiles (see Section 1). Besides the "blue profile," some weak absorption dips are found around 10 km s⁻¹ in both the spectra and P–V diagrams of HCN (4–3) and CS (7–6), and further observations are needed to determine the properties of these absorption dips. In this paper, we only pay attention to the "blue profile" found in CS (7–6) and HCN (4–3) emissions.

The integrated intensity maps of HCN (4–3) and CS (7–6) toward the middle core are presented in Figure 7. The HCN (4–3) and CS (7–6) are associated with the dust emission.

3.2.2. Line Emission at the Southern Core

The integrated intensity maps of four transitions of H₂CS at the southern core are shown in the lower panels of Figure 3. The upper level energy of H₂CS transitions varies from ∼90 K to ∼400 K from panels (e) to (h). As the upper level energy increases, the emission peak of the different transitions of H₂CS moves from southeast to northwest, indicating a temperature gradient in the southern core.

Averaged spectra of SO (8₇–7₇), HC¹⁵N (4–3), HCN (4–3), and CS (7–6) at the southern core are presented in Figure 8. The spectra of SO (8₇–7₇) and HC¹⁵N are averaged over a region of 4'', while HCN (4–3) and CS (7–6) are averaged over a region of 6''. SO (8₇–7₇) emission has a total velocity extent of larger than 20 km s⁻¹. From a Gaussian fit to the spectrum, the peak velocity of SO emission is 5.1 ± 0.1 km s⁻¹, coinciding very well with the systemic velocity of 5.2 km s⁻¹. HC¹⁵N (4–3) has a velocity extent of about 15 km s⁻¹. The velocity extents of CS (7–6) and HCN (4–3) are as high as 40 and 60 km s⁻¹, respectively. Emission wings are clearly detected from the spectra of the four lines. A "red profile" is significantly exhibited in the spectra of CS (7–6) and HCN (4–3), of which the redshifted emission is always stronger than the blueshifted emission with an absorption dip at the blueshifted side of the systemic velocity (5.2 km s⁻¹). This profile is caused by absorption of the colder blueshifted gas in front of the hot core, indicating outflow motions. The "red profile" is consistent with that detected using single-dish observations (see Figure 6 of Hofner et al. 2001).

The integrated intensity maps of HC¹⁵N (4–3) and SO (8₇–7₇) at the southern core are presented in Figure 9. To avoid the influence of outflow motions, both the maps are integrated from 2 to 8 km s⁻¹. Both the emissions of HC¹⁵N (4–3) and SO (8₇–7₇) coincide with the centimeter/millimeter component F, and extend from D to G.

As shown in the left panels of Figure 10, the high-velocity gas of HC¹⁵N (4–3) and SO (8₇–7₇) can be identified by the vertically dashed lines in the P–V diagrams. For SO (8₇–7₇), we integrate from −4 km s⁻¹ ⩽ V ⩽ 0 km s⁻¹ for the blue wing and 10 km s⁻¹ ⩽ V ⩽ 14 km s⁻¹ for the red wing and present the contour map in panel (c) of Figure 10. For HC¹⁵N (4–3), only the red wing emission is presented in panel (d) of Figure 10. The high-velocity emission of both HC¹⁵N (4–3) and SO (8₇–7₇) is associated with core F, indicating that core F is the driven source of the outflow. The blue and red wings of SO (8₇–7₇) overlap to a large extent in the contour maps, and hence the molecular outflow revealed by SO (8₇–7₇) is observed close to its flow axis.

Figure 11 presents the channel maps of CS (7–6) emission. The redshifted high-velocity gas seems to be elongated from northeast to southwest, while the blueshifted high-velocity gas from north to south. The high-velocity gas revealed by CS (7–6) is also very obvious in the P–V diagram in Figure 13(d). As shown in the P–V diagram, the blueshifted high-velocity gas extends about 8'' from north to south. The high-velocity emission integrated over the wings (−12 km s⁻¹ ⩽ V ⩽ −5 km s⁻¹ for the blue wing and 15 km s⁻¹ ⩽ V ⩽ 22 km s⁻¹ for the red wing) is presented in Figure 13(e).

Figure 12 shows the channel maps of HCN (4–3) emission. The maximum of absorptions appears at around 0 km s⁻¹. The redshifted high-velocity gas seems to be elongated from west to east, while the blueshifted high-velocity gas is from north to south. At very high velocity channels (V ⩽ −16 km s⁻¹), the blueshifted emission is totally located at the southeast. By comparing the channel maps and P–V diagrams (see Figure 13) of HCN (4–3) and CS (7–6) at velocity intervals −12 km s⁻¹ ⩽ V ⩽ −5 km s⁻¹ and 15 km s⁻¹ ⩽ V ⩽ 22 km s⁻¹, we find similar structures in CS (7–6) and HCN (4–3) emissions. The high-velocity emission of HCN (4–3) integrated from −12 to −5 km s⁻¹ for the blue wing and from 15 to 22 km s⁻¹ for the red wing is presented in panel (b) of Figure 13. As of CS (7–6), the blueshifted gas revealed by HCN (4–3) is elongated from north to south with the emission center located between G9.62+0.19 F and G9.62+0.19 D, while the redshifted gas is from northeast to southwest. Two clumps are found in the blueshifted high-velocity emission of HCN (4–3), which are located at northwest and southeast of F, respectively.

**Figure 13.** P–V diagrams and integrated intensity maps of HCN (4–3) (upper panels) and CS (7–6) (lower panels) at the southern core. The P–V diagrams are cut along the N–S direction. The vertical solid line in P–V diagrams labels the systemic velocity. The dashed and solid contours in the right panels show the redshifted and blueshifted emissions, respectively. The blueshifted and redshifted emissions in the integrated maps are integrated from −12 to −5 km s⁻¹ and 15 to 22 km s⁻¹, respectively, in panels (b) and (e). (a) The P–V diagram of HCN (4–3). The contours are from 0.9 (3σ) in steps of 1.2 Jy beam⁻¹. (b) Integrated intensity maps of HCN (4–3) at line wings. The contours are 1.5 (5σ), 4.5, 7.5, and 10.5 Jy beam⁻¹ km s⁻¹. (c) The integrated intensity maps of HCN (4–3) at extremely high velocities. The blueshifted and redshifted emissions in the integrated maps are integrated from −20 to −13 km s⁻¹ and 23 to 39 km s⁻¹, respectively. The contours are from 1.5 (5σ) in steps of 3 Jy beam⁻¹ km s⁻¹ for both blueshifted and redshifted emissions. (d) The P–V diagram of CS (7–6). The contours are from 0.9 (3σ) in steps of 1.2 Jy beam⁻¹. (e) Integrated intensity maps of CS (7–6) at line wings. The contours are 1.5 (5σ), 4.5, 7.5, and 10.5 Jy beam⁻¹ km s⁻¹.
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In order to reveal the very high velocity emission traced by HCN (4–3) but not CS (7–6), we integrate over the wings at much higher velocities (−20 km s⁻¹ ⩽ V ⩽ −13 km s⁻¹ for the blue wing and 23 km s⁻¹ ⩽ V ⩽ 39 km s⁻¹ for the red wing) and present the integrated emission map in Figure 13(c). The redshifted emission is elongated from northeast to southwest with the emission center located between G9.62+0.19 F and G9.62+0.19 G, while the blueshifted emission center is located between G9.62+0.19 F and G9.62+0.19 D. It is clearly seen that the high-velocity gas traced by SO (8₇–7₇), CS (7–6), and HCN (4–3) have different spatial distributions, which should be caused by the complicated interactions between the outflow and the ambient gas. It may also indicate a change of the outflow axis. The change of outflow axis is also found in IRAS 20126+4104 (Su et al. 2007) and JCMT 18354−0649S (Liu et al. 2011).

From the integrated emission maps of SO (8₇–7₇), HCN (4–3), and CS (7–6) high-velocity gas, it is clearly seen that G9.62+0.19 F is located at the middle of the redshifted and blueshifted lobes, suggesting that G9.62+0.19 F is the outflow driving source.

4. DISCUSSION

4.1. Rotational Temperature of H₂CS Transitions

Six transitions of H₂CS have been detected in the middle and southern cores, enabling us to estimate the rotational temperature. Under the assumptions that the gas is optically thin under local thermodynamic equilibrium and the gas emission fills the beam, the rotation temperature and beam-averaged column density can be estimated using the rotational temperature diagram (RTD) by (Cummins et al. 1986; Turner 1991; Liu et al. 2002)

$\begin{eqnarray} &&\hbox{ln}\bigg(\frac{N_{u}}{g_{u}}\bigg) = \hbox{ln}\bigg(\frac{N_{T}}{Q_{{\rm rot}}}\bigg)-\frac{E_{u}}{T_{{\rm rot}}}\nonumber\\ &&= \hbox{ln}\Bigg[2.04\times 10^{20}\frac{\int I({\rm Jy\,beam}^{-1})dv(\rm km\, s^{-1})}{\theta _{a}\theta _{b}({\rm arcsec}^{2})g_{I}g_{K}\nu ^{3}({\rm GHz}^{3})S\mu ^{2}({\rm debye}^{2})}\Bigg],\nonumber\\ \end{eqnarray} \tag{ 1 }$

where N_u is the observed column density of the upper energy level, g_u is the degeneracy factor in the upper energy level, N_T is the total beam-averaged column density, Q_rot is the rotational partition function, E_u is the upper level energy in K, T_rot is the rotation temperature, ∫Idv is the integrated intensity of the specific transition, θ_a and θ_b are the FWHM beam size, g_K is the K-ladder degeneracy, g_I is the degeneracy due to nuclear spin, ν is the rest frequency, S is line strength, and μ is the permanent dipole moment. For H₂CS, the interchangeable nuclei are spin $\frac{1}{2}$ , leading to ortho and para forms with g_I equaling $\frac{3}{4}$ and $\frac{1}{4}$ , respectively (Blake et al. 1987; Turner 1991). The partition function Q_rot of H₂CS is (Blake et al. 1987)

$\begin{equation} Q_{{\rm rot}} = 2\left[\frac{\pi (kT_{{\rm rot}})^{3}}{h^{3}{\it ABC}}\right]^{\frac{1}{2}}, \end{equation} \tag{ 2 }$

where k and h are the Boltzmann and Planck constants, respectively, and A, B, and C are the rotation constants. Thus, the rotation temperature T_rot and total column density N_T can be estimated by least-squares fitting to the multiple transitions. We applied the RTD method toward D, E, F, and G (see Figure 14), and the fitting results are listed in the second and third columns of Table 3. The rotational temperature of the middle core (E) is 83 ± 21 K. In the southern core, the rotational temperature estimated decreases from G (91 K) to F (83 K) and D (43 K), suggesting the temperature gradient in the southern core. The total column density of H₂CS ranges from 1.3 × 10¹⁵ (G) to 3.8 × 10¹⁵ cm⁻² (D).

**Figure 14.** Population diagrams of H₂CS toward four centimeter/millimeter cores. The names of the cores are labeled in the upper right corners of each panel. Open circles in blue represent the observed data. The vertical bars present 3σ errors of ln(N_u/g_u) due to the uncertainties of integrated intensities. The solid line shows the linear least-squares fitting using the RTD method. Crosses in red mark the weighted mean results from the PD analysis. The inferred parameters from the PD analysis are presented in the upper right corners of each panel.
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Table 3. Physical Parameters of H₂CS Transitions Obtained with the Rotational Temperature Diagram (RTD) Method and Population Diagram (PD) Analysis

Core	RTD		PD
	T_rot (K)	N_tot (10¹⁵ cm⁻²)	T_rot (K)	N_tot (10¹⁶ cm⁻²)	f	τ

						(10_0,10–9_0,9)	(10_2,9–9_2,8)	(10_2,8–9_2,7)	(10_3,8–9_3,7)	(10_3,7–9_3,6)	(10_5,6/5–9_5,6/4)
D	43 ± 9	3.8 ± 2.9	42 ± 34	4.2 ± 2.9	0.46 ± 0.24	6.4 ± 4.4	0.7 ± 0.5	0.9 ± 0.6	0.4 ± 0.5	0.2 ± 0.3
E	83 ± 21	2.5 ± 1.6	92 ± 74	3.6 ± 3.0	0.26 ± 0.23	4.1 ± 4.3	0.7 ± 0.7	0.6 ± 0.6	0.7 ± 0.8	0.7 ± 0.8	0.1 ± 0.1
F	83 ± 7	2.6 ± 0.6	51 ± 23	4.0 ± 2.9	0.34 ± 0.23	3.9 ± 3.1		1.1 ± 0.9	1.1 ± 1.2	1.1 ± 1.2	0.0 ± 0.1
G	91 ± 17	1.3 ± 0.7	105 ± 37	3.7 ± 3.1	0.12 ± 0.18	2.5 ± 2.7	2.4 ± 2.3			2.4 ± 2.1	0.3 ± 0.3

Note. The rotational temperature and total column density of H₂CS transitions derived from the RTD analysis are presented in the second and third columns, while those derived from the PD analysis are shown in the fourth and fifth columns. The sixth column gives the filling factor of each source inferred from the PD analysis. The last six columns exhibit the optical depth of each transition using the PD analysis.

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However, the filling factor and the optical depth correction were not taken into account in the RTD method. To investigate their effect we applied the population diagram (PD) analysis (Goldsmith & Langer 1999; Wang et al. 2010). In the PD analysis, we have

$\begin{equation} \hbox{ln}\bigg(\frac{\hat{N_{u}}}{g_{u}}\bigg) = \hbox{ln}\left(\frac{N_{T}}{Q_{{\rm rot}}}\right)-\frac{E_{u}}{T_{{\rm rot}}}+\hbox{ln}(f)-\hbox{ln}\left(\frac{\tau }{1-e^{-\tau }}\right), \end{equation} \tag{ 3 }$

where $\hat{N_{u}}$ is the inferred column density of the upper energy level from the PD analysis, f is the source filling factor, and τ is the optical depth. The optical depth τ can be expressed by (Remijan et al. 2004)

$\begin{equation} \tau = \frac{8\pi ^{3}S\mu ^{2}\nu }{3k\Delta v T_{{\rm rot}}}\frac{N_{T}}{Q_{{\rm rot}}}e^{-\frac{E_{u}}{T_{{\rm rot}}}}, \end{equation} \tag{ 4 }$

where Δ v is the FWHM line width. Under LTE, the upper-level populations, $\hat{N_{u}}$ , can be predicted according to the right-hand side of Equation (3) for a given set of total column density, N_T, rotational temperature, T_rot, and source filling factor, f. The expected $\hat{N_{u}}$ were evaluated for the parameter space of T_rot = 10–500 K, N_T = 10¹⁴–10¹⁷ cm⁻², and f between 0.01 and 1.0. To compare the observed N_u and the inferred $\hat{N_{u}}$ , we calculate the χ² as

$\begin{equation} \chi ^{2} = \sum \left(\frac{N_{u}-\hat{N_{u}}}{\delta N_{u}}\right)^{2}, \end{equation} \tag{ 5 }$

where δN_u is the 1σ error of observed upper-state column density. Although the χ² is a good representation of the goodness of fit, the parameter set with the lowest χ² may not actually represent physical parameters very well due to the uncertainties of the observed data. In order to find a representative parameter set, we compute a weighted mean and standard deviation for all the parameters, with the weights being the inverse of the χ². All the parameter sets where the inferred upper-level population $\hat{N_{u}}$ corresponds with the observed upper-level population N_u within 3σ are used to compute the weighted means and standard deviations. The derived rotational temperature, total column density, and filling factor of each component are listed in Columns 3–5 of Table 3. The inferred optical depths of each line transition are listed in the last six columns of Table 3. The rotational temperatures of D, E, F, and G are estimated to be 42 ± 34, 92 ± 74, 51 ± 23, and 105 ± 37 K, respectively. A temperature gradient in the southern core is also revealed as in the RTD method. The four components D, E, F, and G have similar total column density as high as 4×10¹⁶ cm⁻², about an order-of-magnitude higher than those obtained from RTD method, which are mainly due to the small source filling factor (<0.5). The optical depths of H₂CS (10_0,10–9_0,9) at the four components are all much larger than one, while the other transitions are always optically thin except H₂CS (10_2,9–9_2,8) line at G.

4.2. Core Properties

In the optically thin case, the total dust and gas masses of the three submillimeter cores can be obtained with the formula M = S_νD²/κ_νRB_ν(T_d) (Hildebrand 1983), where S_ν is the flux at 860 μm, D is the distance, R = 0.01 is the mass ratio of dust to gas, and κ_ν is dust opacity per unit dust mass. B_ν(T_d) is the Planck function at a dust temperature of T_d. We assume that T_d equals the rotational temperature of H₂CS. For the northern core, since only CS (7–6) (upper energy E_u = 65.8 K) and HCN (4–3) (E_u = 42.5 K) exhibit strong emission lines, we assume T_d to be 50 K. Together with the measurements at centimeter and millimeter wavelengths, Su et al. (2005) extrapolated the ionized gas emission at millimeter/submillimeter wavelengths and found that the 0.85 mm continuum associated with components D, E, and F is dominated by thermal dust emission. They have derived an opacity index β of components E and F to be 1.2 and 0.8, respectively. For the northern submillimeter core, β = 1.5 is assumed. Using the above dust opacity indexes, we adopt κ_ν = 2.0, 1.8, and 1.5 cm² g⁻¹ for the northern, middle, and southern cores, respectively (Ossenkopf & Henning 1994). At the distance of 5.7 kpc, we get the total dust and gas masses for these three cores and list all the parameters in Table 1. The deduced masses for the northern, middle, and southern cores are 13, 30, and 165 M_☉, respectively. The column density of H₂ is 1.2×10²⁴ and 2.1×10²⁴ cm⁻² for the middle and southern submillimeter cores, respectively.

4.3. Infall Properties in the Middle Core

In the middle core, both CS(7–6) and HCN(4–3) emissions exhibit "blue-profile" features, indicating infall motions of the gas envelope toward the central star (Keto et al. 1988; Zhou et al. 1993; Zhang et al. 1998; Wu & Evans 2003; Wu et al. 2005, 2007; Fuller et al. 2005; Wyrowski 2008; Sun & Gao 2008). The velocity difference (0.9 km s⁻¹) between the absorption dip in the CS (7–6) spectrum (3 km s⁻¹) and the systemic velocity (2.1 km s⁻¹) is taken as the infall velocity V_in. Since both HCN (4–3) and CS (7–6) emissions are not resolved toward the middle core, we simply take the dust core size as the radius of the infall region, which may underestimate the infall rate derived below. The kinematic mass infall rate can be calculated using dM/dt = 4πR²_innmV_in. n = 1.5×10⁷ cm⁻³ is the number density of this dust core. Taking helium into account, the mean molecular mass m is 1.36 times the H₂ molecule mass. The infall rate calculated is 4.3 × 10⁻³ M_☉ yr⁻¹. For comparison, the V_in from pure free-infall assumption is also derived using the formula V²_in = 2GM/R_in. The pure free-infall velocity is V_in = 3.6 km s⁻¹ and thus the "gravitational" mass infall rate is 1.7 × 10⁻² M_☉ yr⁻¹, which is larger than the kinematic infall rate.

4.4. Outflow Properties in the Southern Core

4.4.1. Shock Chemistry in the Outflow Region of the Southern Core

Observations have suggested that there are important differences in molecular abundances in different outflow regions (Bachiller & Perez Gutiérrez 1997; Choi et al. 2004; Jörgensen et al. 2004; Codella et al. 2005). Significant abundance enhancements are found in the shocked region for sulfur-bearing molecules (Bachiller & Perez Gutiérrez 1997; Jörgensen et al. 2004), and the abundance of HCN in outflow regions is related to atomic carbon abundance (Choi 2002). However, previous studies of the chemical impact of outflows are confined to the well-collimated outflows around Class 0 sources, while such studies, especially high-resolution studies on massive outflows, are rare (Bachiller & Perez Gutiérrez 1997; Jörgensen et al. 2004; Arce et al. 2007).

A red and bright IRAC source is found to be associated with the southern core. The magnitudes of the IRAC source at 3.6, 4.5, and 5.8 μm are 10.102 ± 0.093, 8.361 ± 0.108, and 7.778 ± 0.302 mag, respectively. The [3.6 − 4.5] color is as large as 1.74, indicating shocked emission in the southern core (Takami et al. 2010). Maser emissions of NH₃, H₂O, OH, and CH₃OH as well as the strong thermal NH₃ emissions also uncover the existence of the shocked gas (Hofner et al. 1994). Outflows can be revealed from shocked H₂ emission probed by the strong and extended emission at the 4.5 μm band (Qiu et al. 2008; Takami et al. 2010). Thus, the massive outflow in the southern core of G9.62 complex provides an ideal sample to study shock chemistry.

The fractional abundance of a certain molecule is defined as $\chi = N_{T}/N_{\rm H_{2}}$ , where N_T is the total column density of a specific molecule and $N_{\rm H_{2}}$ is the H₂ column density. Assuming that the gas is optically thin and the emission fills the beam, the beam-averaged total column density of a specific molecule can be obtained from

$\begin{equation} N_{T} = 2.04\times 10^{20}\frac{\int I({\rm Jy\, beam}^{-1})dv({\rm km \,s}^{-1})Q_{{\rm rot}}e^{E_{u}/T_{{\rm rot}}}}{\theta _{a}\theta _{b}({\rm arcsec}^{2})g_{I}g_{K}\nu ^{3}({\rm GHz}^{3})S\mu ^{2}{\rm (debye}^{2})}. \end{equation} \tag{ 6 }$

Assuming that T_rot of HC¹⁵N equals that of H₂CS and the gas is optically thin, N_T of HC¹⁵N is calculated to be 3.0 × 10¹³ cm⁻² at the core region. At the galactocentric distance of 3 kpc for G9.62+0.19 (Scoville et al.1987; Hofner et al. 1994), the abundance ratio [¹⁴N]/[¹⁵N] ≈350 (Wilson and Rood. 1994). Thus, the total column density of HCN at the core region should be 1.1 × 10¹⁶ cm⁻². Therefore, the fractional abundance of HCN relative to H₂ at the core region is 5.2 × 10⁻⁹. HCN appears to be greatly enhanced in the outflow regions of the L1157 (Bachiller & Perez Gutiérrez 1997), while it has similar abundances in the outflow region and the ambient cloud of NGC 1333¨CIRAS 2A (Jörgensen et al. 2004). Owing to the lack of a direct estimation of the H₂ column density toward the outflow region, the fractional abundance of HCN in the outflow region is also assigned to 5.2 × 10⁻⁹ in calculating the outflow parameters. Since the HC¹⁵N emission traces outflowing gas at much lower velocity than HCN, perhaps HCN could be more enhanced in the high-velocity component. With the possibility of higher opacity and the lack of direct H₂ column density measurement, the derived fractional abundance is perhaps a lower limit anyway. Su et al. (2007) estimate an HCN abundance of ∼1–2 × 10⁻⁸ in the massive outflow lobes of IRAS 20126+4104, which is comparable to our estimation here.

Since the blueshifted outflow gas traced by CS (7–6) and HCN (4–3) suffers self-absorption, the abundance ratios among SO (8₇–7₇), CS (7–6), and HCN (4–3) were inferred from the beam-averaged spectra taken from the redshifted outflow lobe. The abundance ratio as a function of flow velocity (the outflow velocity relative to the systemic velocity) of [CS/SO] is obtained assuming five different excitation temperatures in the left panel of Figure 15. It can be seen that the abundance ratio of [CS/SO] increases with the excited temperature. At each excitation temperature, the abundance ratio of [CS/SO] has lower values at flow velocities less than 6 km s⁻¹, and higher values when V_flow larger than 8 km s⁻¹, whereas the abundance ratio seems to be constant at flow velocities between 6 and 8 km s⁻¹. There are two reasons for the lower abundance ratio when V_flow< 6 km s⁻¹: first, the flux missing of CS (7–6) due to the interferometer is more serious than SO (8₇–7₇); second, CS (7–6) may be more optically thick at lower flow velocities than SO (8₇–7₇). As shown in the P–V diagrams, the emission region of CS (7–6) is much larger than SO (8₇–7₇) at high velocities. The higher abundance ratio when V_flow> 8 km s⁻¹ is due to the smaller filling factor of SO (8₇–7₇) emission. We propose that the mean observed value between 6 and 8 km s⁻¹ can represent the actual abundance ratio of [CS/SO]. Assuming a typical excitation temperature of T_ex = 30 K (Wu et al. 2004), the abundance ratio of [CS/SO] at the redshifted lobe is inferred as 0.7. Nilsson et al. (2000) find that the [SO/CS] abundance ratios are strongly enhanced in the Orion A and NGC 2071 outflows where the [SO/CS] ratios are estimated to be about 24 and 2.2, respectively. However, the [SO/CS] abundance ratio in the outflow of G9.62+0.19 is found to be 1.4, much lower than that found in Orion A outflow.

As shown in the right panel of Figure 15, the abundance ratio of [CS/HCN] decreases linearly with the flow velocity. To avoid the missing flux difficulty, the abundance ratio is calculated at high flow velocities larger than 7 km s⁻¹. The decreasing of the abundance ratio with velocity is because the emission region traced by CS (7–6) is always smaller than HCN (4–3), leading to a smaller filling factor for CS (7–6), which can be verified easily by comparing the channel maps between CS (7–6) in Figure 11 and HCN (4–3) in Figure 12 at high velocities. We fitted the observed data with a linear function, and adopted the value at flow velocity of 10 km s⁻¹ as the actual abundance ratio of [CS/HCN] in the outflow region, which is [CS/HCN] = 1.2. Since HCN fractional abundance is 5.2 × 10⁻⁹, the fractional abundances of CS and SO are deduced to be 6.2 × 10⁻⁹ and 8.9 × 10⁻⁹, respectively.

4.4.2. Properties of the Bipolar Outflow Traced by SO (8₇–7₇) Emission

The SO (8₇–7₇) emission in the southern core shows line wings, suggesting outflow motions. From the integrated intensity map in Figure 10(c), we find the outflow lobes revealed by SO (8₇–7₇) emission peak at different positions with different position angles compared with previously reported H₂S (2_2,0–2_1,1) (Gibb et al. 2004) and HCO⁺ (1–0) data (Hofner et al. 2001). But in the same sense, the blue and red lobes revealed by SO overlap to a large extent as well as HCO⁺ (1–0) and H₂S (2_2,0–2_1,1) data, consistent with the argument of the outflow being viewed pole-on (Hofner et al. 2001).

The total mass of each outflow lobe is given by

$\begin{equation} M_{{\rm flow}} = 1.04 \times 10^{-4}D^{2}\frac{Q_{{\rm rot}}e^{E_{u}/T_{{\rm rot}}}}{\chi \nu ^{3}S\mu ^{2}}\int \frac{\tau }{1-e^{-\tau }}S_{\nu }dv, \end{equation} \tag{ 7 }$

where M_flow, D, S_ν, χ, and τ are the outflow gas mass in M_☉, source distance in kpc, line flux density in Jy, relative abundance to H₂, and optical depth, respectively. The other parameters have the same units as in Equation (1). The fractional abundance of SO is taken as 8.9 × 10⁻⁹ (see Section 4.4.1). Assuming an excitation temperature of 30 K and the outflowing gas as optically thin, the inferred outflow masses are 13 M_☉ for each of the redshifted and blueshifted lobes. Thus, the momentum can be calculated by P = ∑M(v)dv, and the energy by $E=\sum {\frac{1}{2}}M(v)v^{2}dv$ , where v is the flow velocity. The derived parameters are listed in Table 4. The momentum and energy of the red lobe are 82 M_☉ km s⁻¹ and 5.4 × 10⁴⁵ erg, respectively. For the blue lobe, the momentum and energy are calculated to be 86 M_☉ km s⁻¹ and 5.8 × 10⁴⁵ erg, respectively. The dynamical timescale t_dyn is estimated as R/V_char, where R (∼ 0.06 pc) is adopted as the mean size of the outflow lobes assuming a collimation factor of unity, and V_char (∼5.5 km s⁻¹) is assumed as the mass-weighted mean velocity. Thus, the dynamic timescale is estimated to be 1 × 10⁴ yr, which may be underestimated due to the uncertainty of the outflow scale. The mechanical luminosity L and the mass-loss rate $\dot{M}$ are calculated as L = E/t and $\dot{M} = P/(tV_{w})$ , respectively, where the wind velocity V_w is assumed to be 500 km s⁻¹ (Lamers et al. 1995). The mechanical luminosity L and the total mass-loss rate are estimated to be 9.3 L_☉ and 3.6 × 10⁻⁵ M_☉ yr⁻¹, respectively.

Table 4. Outflow Parameters of the Southern Core

Molecule	Velocity Interval		M		P		E
Component	(km s⁻¹)		(M_☉)		(M $_{\odot } \, \rm km\, s^{-1}$ )		(10⁴⁵ erg)
	Blue	Red	Blue	Red	Blue	Red	Blue	Red
SO	[−4,0]	[10,14]	13	13	86	82	5.8	5.4
CS	[−12,−5]	[15,22]	3.7	5.5	47	68	6.0	8.7
HCN	[−20,−5]	[15,39]	5.4	17.6	85	294	14.1	54.6

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4.4.3. Very High Velocity Gas Detected in CS (7–6) Emission

The CS (7–6) emission at the southern core shows a "red profile" with wide wings. We take 6.2 × 10⁻⁹ as the fractional abundance of CS relative to H₂ along the outflow lobes. Assuming T_ex = 30 K, we derive the parameters for the CS outflow (Table 4) with the same method used for SO (8₇–7₇). The outflow masses at very high velocities (v_flow>10 km s⁻¹) are 3.7 M_☉ and 5.5 M_☉ for the blueshifted and redshifted lobes, respectively. The momentum and energy of the blueshifted lobe at very high velocities are calculated to be 47 M_☉ km s⁻¹ and 6.0 × 10⁴⁵ erg, respectively. For the redshifted lobe, the momentum and energy at extremely high velocities are calculated to be 68 M_☉ km s⁻¹ and 8.7 × 10⁴⁵ erg, respectively, which are similar to those of the blueshifted lobe.

4.4.4. Very High Velocity Gas Detected in HCN (4–3) Emission

As discussed before, HCN (4–3) has a velocity extent of at least 60 km s⁻¹, which traces extremely high velocity gas. Adopting an excited temperature of 30 K, and an HCN-to-H₂ abundance ratio of 5.2 × 10⁻⁹, the parameters of the outflow are calculated and listed in Table 4. The outflow mass at very high velocities (v_flow>10 km s⁻¹) are 5.2 M_☉ and 17.6 M_☉ for the blueshifted and redshifted lobes, respectively. The momentum and energy of the blueshifted lobe at very high velocities are 85 M_☉ km s⁻¹ and 1.4 × 10⁴⁶ erg, respectively. For the redshifted lobe, the momentum and energy at very high velocities are 294 M_☉ km s⁻¹ and 5.5 × 10⁴⁶ erg, respectively, which are larger than those of the blueshifted lobe.

4.4.5. Mass–Velocity Diagrams

A broken power law, dM(v)/dv ∝ v^−γ, usually exhibits in molecular outflows near young stellar objects (Chandler et al. 1996; Lada & Fich 1996; Ridge & Moore 2001; Su et al. 2004; Qiu et al. 2007, 2009). The slope γ, typically ranging from 1 to 3 at low outflow velocities, often steepens at velocities larger than 10 km s⁻¹—with γ as large as 10 in some cases (Arce et al. 2007). Assuming optically thin, the mass–velocity diagrams of the outflow at the southern core of the G9.62+0.19 complex are shown in Figure 16. SO (8₇–7₇), CS (7–6), and HCN (4–3) results were all used in the mass spectra. We calculate the outflow mass traced by CS (7–6) and HCN (4–3) from V_flow of 10 km s⁻¹ to avoid the absorption of the spectra. Instead of broken power-law appearance, the mass–velocity diagram of the blueshifted lobe can be well fitted by a single power law with a power index of 2.28 ± 0.23. The mass–velocity diagram of the redshifted lobe can be well fitted by a single power law with a power index of 1.70 ± 0.17 even though the mass drops more rapidly after 25 km s⁻¹. As marked by the dashed ellipse in the right panel, the outflow mass revealed by CS (7–6) is much lower than that revealed by HCN (4–3) at very high velocities. Despite the CS data, the mass–velocity diagram of the redshifted lobe at velocities smaller than 25 km s⁻¹ can be fitted by a single power law with a much smaller power index of 1.08 ± 0.09. However, no significant slope changes are found in both the redshifted and blueshifted lobes of the outflow at the southern core, which are very different from those previous works.

**Figure 16.** Mass–velocity relationships for the outflow lobes. Left: blueshifted lobe; right: redshifted lobe. The solid lines in both panels show the power-law fit toward all the data. The dashed line in the right panel shows the power-law fit toward the HCN and SO data up to V_flow = 25 km s⁻¹. The fitting results are presented in the lower left corners.
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4.5. Different Evolutionary Stages of the Three Dust Cores

The northern core has the smallest diameter and mass among the three cores. It seems likely to be a point source after deconvolution. It is located south of the nominal radio UC H ii region G9.62+0.19 C. In this region, eight near-IR sources are detected in a diffuse near-IR nebulosity at the west of the radio emission peak (Persi et al. 2003). The reddest one, c7 (18^h06^m1434,−20°31'25 farcs 0), is located within 1'' of the radio peak, while the faintest one, c8 (18^h06^m1442,−20°31'27 farcs 4), seems to be associated with the submillimeter core detected in SMA observation. Source c8 is too faint to be detected even at H band and also shows no emission at 12.5 μm. In contrast to the bright, rich molecular spectrum forest in the middle and southern submillimeter cores, the northern submillimeter core lacks strong molecular emissions. There is also no other early star-forming signature such as masers associated with it. Since it is with near-IR emission and at the edge of the UC H ii region G9.62+0.19 C, the northern core may be just a remnant core in the envelope of the UC H ii region G9.62+0.19 C, which needs further observations.

The middle core is associated with the hypercompact H ii region G9.62+0.19 E (Garay et al. 1993; Kurtz & Franco 2002). OH, H₂O, and NH₃ (5,5) masers have been detected near the radio emission peak (Forster & Caswell 1989; Hofner et al. 1994; Hofner & Churchwell 1996). Periodic class II methanol masers are also found in G9.62+0.19 E (van der Walt et al. 2009; Goedhart et al. 2005; Norris et al. 1993). Methanol masers are believed to be a good tracer of young massive star-forming regions at stages earlier than relatively evolved UC H ii regions (Longmore et al. 2007). No infrared source coincides with G9.62+0.19 E (Persi et al. 2003). Hot molecular CH₃CN lines are detected in this region, and a kinematic temperature of T_k = 108 K was obtained from CH₃CN emission with a large velocity gradient model (Hofner et al. 1996), which is coincident with the rotational temperature (T_rot = 92 K) obtained from H₂CS emission. A spectrum forest including hot molecular lines, such as CH₃OH, is detected toward G9.62+0.20 E, suggesting that this core is in a hot phase. Infall motions are traced by CS (7–6) and HCN (4–3) lines indicating active star forming in this region. All of the above suggest that G9.62+0.20 E is forming a massive young star.

The 860 μm dust emission of the southern core peaks at G9.62+0.19 F and extends from north to south. A hump structure is found to the southeast of the emission peak, indicating another possible submillimeter core. The previously recognized millimeter/centimeter cores (G9.62+0.19 D, G) are at the edges of the southern core. G9.62+0.19 G is a weak radio source (Testi et al. 2000), while G9.62+0.19 D is consistent with an isothermal UC H ii region excited by a B0.5 star (Hofner et al. 1996). Weaker radio emission was found at core F. H₂O and OH masers are found across the whole submillimeter core from north to south (Forster & Caswell 1989; Hofner & Churchwell 1996). A near-IR source with large NIR excess is found to be associated with G9.62+0.19 F (Testi et al. 1998; Persi et al. 2003). With higher resolution observations, Linz et al. (2005) found four near-IR objects (F1–F4) in this core. F4 has little emission at K band but becomes redder at longer wavelengths, which seems to correspond to the bright IRAC source with a large excess at 4.5 μm. This object is the dominating and closest associated source of core F. Core F is also confirmed to be the driving source of an active outflow. All of the above imply that G9.62+0.19 F is a very young massive star-forming region.

4.6. Blue Excess in High-mass Star-forming Regions

Wu et al. (2007) found that UC H ii regions show a higher blue excess than UC H ii precursors with the IRAM 30 m telescope. Wyrowski et al. (2006) also detected a large blue excess in UC H ii regions. A "blue profile" was detected with CS (7–6) and HCN (4–3) lines in the UC H ii region G9.62+0.19 E, while, a "red profile" was detected in the HMC G9.62+0.19 F, which coincides with their argument. The detection of infall signature in G9.62+0.19 E also coincides with the interpretation that material is still accreted during the UC H ii phase (Wu et al. 2007; Keto 2002). Around younger cores, the outflow is more active and cold than UC H ii regions, which leads to a more "red profile," while in UC H ii regions, the outflows become weak, the surrounding gas of UC H ii regions is thermalized, and the temperature gradient toward the central star is more likely to cause a "blue profile," which results in the higher blue excess than UC H ii precursors.

5. SUMMARY

We have observed the G9.62+0.19 complex with the SMA both in the 860 μm continuum and molecular line emission. The main results of this study are as follows.

1.
Dust continuum at 860 μm reveals three submillimeter cores in the G9.62+0.19 star-forming complex. With H₂CS as the rotational temperature prober, the temperatures of E and F are estimated to be 92 ± 74 and 51 ± 23 K, respectively. The masses calculated are 13, 30, and 165 M_☉ for the northern, middle, and southern cores, respectively.
2.
In the middle core, HCN (4–3) and CS (7–6) spectra exhibit infall signature. The infall rate calculated is 4.3 × 10⁻³ M_☉ yr⁻¹. The detection of infall signature in G9.62+0.19 E coincides with the interpretation that material is still accreted after the onset of the UC H ii phase (Wu et al. 2007).
3.
In the southern core, high-velocity gas is detected in SO (8₈–7₇), CS (7–6), and HCN (4–3) lines. A bipolar outflow with a total mass about 26 M_☉ and a mass-loss rate of 3.6 × 10⁻⁵ M_☉ yr⁻¹ is revealed in SO (8₈–7₇) line wing emission. G9.62+0.19 F is confirmed to be the driving source of the outflows in the southern submillimeter core. The abundance ratios of [CS/SO] and [CS/HCN] in the outflow region are found to be 0.7 and 1.2, respectively. The abundance ratio [CS/HCN] decreases with the flow velocity, indicating smaller outflow regions revealed by CS (7–6) than that revealed by HCN (4–3). The mass–velocity diagrams of the blueshifted and redshifted outflow lobes can be well fitted by a single power law. The power indexes for the blueshifted and redshifted lobes are 2.28 ± 0.23 and 1.70 ± 0.17, respectively. No significant slope changes are found in the mass–velocity diagrams.
4.
The evolutionary sequence of the centimeter/millimeter cores in this region is also analyzed. The northern core may be just a remnant core in the envelope of the UC H ii region G9.62+0.19 C, which needs further observations. The middle core (G9.62+0.19 E) is in a hypercompact H ii region. Core G9.62+0.19 F is confirmed to be an HMC.
5.
The detection of blue profiles at the hypercompact H ii region E and the red profiles at the HMC F supports the results of single-dish observations that UC H ii regions have a higher blue excess than their precursors.

We are grateful to the SMA staff for making the observations. This work is funded by grants of NSFC nos. 10733030 and 10873019.

INFALL AND OUTFLOW MOTIONS IN THE HIGH-MASS STAR-FORMING COMPLEX G9.62+0.19

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ABSTRACT

1. INTRODUCTION

2. OBSERVATIONS