Extended HNCO, SiO, and HC₃N Emission in 43 Southern Star-forming Regions

Shocks are a ubiquitous phenomenon in the interstellar medium (ISM) of galaxies. They may be driven by supernova explosions, the pressure of photoionized gas, stellar winds, and collisions between fast-moving clumps of interstellar gas, where the fluid-dynamical disturbances proceed at a velocity that exceeds the local sound speed because of the presence of large pressure gradients (Draine & McKee 1993). The processes of birth, evolution, and death of stars are always associated with shocks (Gusdorf 2015). Therefore, a systematic study of shocks in regions of star formation is of great importance for our general understanding of the physical and chemical boundary conditions of star formation.

Isocyanic acid (HNCO), which was detected for the first time in the Galactic radio source Sgr B2 by Snyder & Buhl (1971), is a ubiquitous molecule. Based on the morphology of the emission, the abundance of HNCO with respect to H₂, and the relation to SiO emission, several authors tested the hypothesis that HNCO could be a good tracer of interstellar shocks (e.g., Zinchenko et al. 2000; Meier & Turner 2005; Minh & Irvine 2006). Later, Rodríguez-Fernández et al. (2010) tested this hypothesis by observing several transitions of HNCO toward a well-known shocked Galactic source, L1157–mm, and proposed that HNCO is a good shock tracer and that the gas-phase abundance of HNCO is achieved both by grain mantle evaporation through shock waves and by neutral–neutral reactions in the gas phase involving CN and O₂.

The silicon monoxide (SiO) molecule is an excellent tracer of molecular gas processed by the action of high-velocity (∼20–50 km s⁻¹) shocks in regions of star formation (Martin-Pintado et al. 1992; Schilke et al. 1997; Codella et al. 1999; Gusdorf et al. 2008a, 2008b; López-Sepulcre et al. 2016). Martin-Pintado et al. (1992) reported that this molecule is enhanced by large factors (in some cases by >10⁶) toward molecular outflows, which strongly suggests that grain disruption by shocks is the major mechanism for releasing SiO into the gas phase (Martin-Pintado et al. 1992). Furthermore, SiO emission can also trace photon-dominated regions (PDRs) with ∼10–20 km s⁻¹ shocks (e.g., Schilke et al. 2001). In the Galactic center region, the gas is characterized by large line widths, indicating a high degree of turbulence. There, SiO is widespread (see, e.g., the surveys in the J = 1–0 transition by Martín-Pintado et al. 1997 and Jones et al. 2013, and in the J = 2–1 transition by Jones et al. 2012), which was interpreted as evidence for large-scale or ubiquitous (fast) shocks. Hüttemeister et al. (1998) observed 33 targets in the Galactic center region and found that at least some SiO is detected in all cloud cores, mostly associated with cool gas. They proposed that this was due to shocks caused by local turbulence and/or cloud–cloud collisions. In the Galactic disk, quasithermal SiO emission is tightly correlated with high-temperature regions (e.g., Ziurys et al. 1989). In addition, observations indicate that the properties of molecular peaks in the Galactic center region are markedly different from those in the Galactic disk, where thermal SiO emission is confined to very small regions in the vicinity of outflows associated with star formation (Downes et al. 1982; Hüttemeister et al. 1998).

Interstellar cyanoacetylene (HC₃N) was first detected in Sgr B2 by Turner (1971). It is an excellent dense gas tracer (Morris et al. 1976; Morris & Snell 1977; Chung et al. 1991; Bergin et al. 1996). Both chemical models and observations indicate that the abundance of HC₃N is enhanced toward hot cores (through the gas-phase reaction C₂H₂ + CN  →  HC₃N + H, see van Dishoeck & Hogerheijde 1999 and Chapman et al. 2009 for details).

Shocks are often related to outflowing gas. The above-mentioned tracers, however, are not ideal when trying to study the entire velocity field encompassing quiescent, outflowing, and inflowing gas. For this purpose the HCO⁺ J = 1–0 line has been chosen. With its skewed line profiles in the case of outflow or inflow, it provides the required signature (Wu et al. 2007; He et al. 2015, 2016; Li et al. 2019).

The two shock tracers HNCO and SiO are chemically different. HNCO is thought to arise from sublimation of dust ice mantles under soft shocks (Meier & Turner 2005), while SiO can be significantly enhanced in the gas phase by strong shocks due to a partial/total evaporation of the SiO present in both the grain mantles and the grain cores (e.g., Schilke et al. 1997; Jiménez-Serra et al. 2005, 2008; Gusdorf et al. 2008b). Moreover, HNCO and HC₃N are easily dissociated by UV radiation and are tracers of hot core (Rodriguez-Franco et al. 1998; Martín et al. 2008; Miettinen 2014), while SiO is more robust against UV radiation. In past years, some HNCO or SiO molecular line surveys of massive galactic dense clumps have already been published (Harju et al. 1998; Zinchenko et al. 2000; Li et al. 2013; Csengeri et al. 2016). However, most of the line surveys of massive Galactic dense clumps performed so far are based on single-pointing observations, in which the spatial distribution of HNCO and SiO emissions cannot be explored. Furthermore, beside sources in the Galactic center region, the large-scale SiO J = 2–1 line emission has been measured only in a few regions of the Galactic disk, such as the filamentary infrared dark cloud (IRDC) G035.39−00.33 (Jiménez-Serra et al. 2010) and the three IRDCs G028.37+00.07, G034.43+00.24, and G034.77−00.55 (Cosentino et al. 2018), with the IRDC G034.77−00.55 being further studied in detail by using data from the Atacama Large Millimeter/submillimeter Array (Cosentino et al. 2019). Therefore, it is clear that high-sensitivity mapping observations of HNCO, SiO, and HC₃N are needed to characterize the spatial distributions of these species and their relation to physical properties of dense clumps. This paper presents a systematic study of a sample of 43 massive dense clumps. The spectral line mapping data used here were taken from the Millimetre Astronomy Legacy Team 90 GHz survey (Foster et al. 2011; Jackson et al. 2013). After describing the sample selection and observations in Section 2, the observational results are presented in Section 3. In Sections 4 and 5, we analyze and discuss the results, and we summarize the paper in Section 6.

2.1. Observations

2.2. Sample Selection

3.1. Dust Temperature

Combining the Herschel Hi-GAL observations at 160, 250, 350, 500 μm, and the APEX ATLASGAL observations at 870 μm, we obtained dust temperature maps pixel by pixel from a single-temperature graybody model of dust emission that is given by

$$\begin{eqnarray}&&{I}_{\nu }({T}_{d},{N}_{{{\rm{H}}}_{2}})={B}_{\nu }({T}_{d})\left(1-{e}^{R\times \mu {m}_{{\rm{H}}}{N}_{{{\rm{H}}}_{2}}{\kappa }_{\nu }}\right),\end{eqnarray} \tag{ 1 }$$

where I_ν is the observed intensity, B_ν (T_d ) is the Planck function at a dust temperature T_d , ${N}_{{{\rm{H}}}_{2}}$ is the column density of molecular hydrogen, R is the gas-to-dust mass ratio (assumed to be 100), μ = 2.8 is the mean molecular weight of the interstellar medium (Kauffmann et al. 2008), and κ_ν is the dust absorption coefficient. The curve of κ_ν is approximated by a power law ${\kappa }_{\nu }={\kappa }_{0}{(\nu /{\nu }_{0})}^{\beta }$, which depends on frequency (ν < 1 THz) and spectral index β (Hildebrand 1983). Guzmán et al. (2015) tested 14 IRAS sources selected from the 1.1 mm Bolocam Galactic Plane Survey (Rosolowsky et al. 2010; Ginsburg et al. 2013) and the ATLASGAL catalog, and found that $\overline{\beta }$ = 1.6 is in agreement with the absorption coefficient law of silicate-graphite grains, with 3 × 10⁴ yr of coagulation, and without ice coatings analyzed by Ormel et al. (2011). This choice of β is also consistent with previous studies (see, e.g., Battersby et al. 2011). The sources in this paper can also be considered as a subsample of the targets analyzed by Guzmán et al. (2015), who determined dust temperatures for all molecular clumps from the MALT90 survey using the aforementioned absorption coefficient law presented by Ormel et al. (2011). However, Guzmán et al. (2015) just provide average and peak dust temperatures for each source in their work, so we calculate dust temperature maps here. We follow their method without fitting β and directly use κ_ν  = 32.9, 13.9, 7.7, 4.3, and 1.7 cm² g⁻¹ for the 160, 250, 350, 500, and 870 μm data, which were extracted from Ormel et al. (2011). Before performing spectral energy distribution (SED) fitting, we made use of the routine ¹⁴ developed by Wang et al. (2015) to remove emission from the background and foreground. Next, all previously mentioned images are convolved to the SPIRE 500 μm data resolution (i.e., to 37'') using the kernels provided by Aniano et al. (2011), and then regridded to the same pixelization (∼12'').

The line optical thickness (τ_ν ) of HNCO 4₀₄–3₀₃ could not be derived through fitting the hyperfine structure, because the hyperfine structure is not spectrally resolved. With limited bandwidth, we also could not derive τ_ν of HNCO 4₀₄–3₀₃, SiO 2–1, and HC₃N 10–9 by comparing the intensities of two different isotopologues of the same species. Therefore, we calculate the value of optical depth using Equation (2). Assuming local thermodynamic equilibrium (LTE) conditions, all levels are populated according to the same excitation temperature (T_ex). In addition, we performed checks on the mean volume density of the H₂ molecule, which is given by ${n}_{{{\rm{H}}}_{2}}=\tfrac{{N}_{{{\rm{H}}}_{2}}}{\theta \times d}$, combining column density (${N}_{{{\rm{H}}}_{2}}$, see Section 3.2) with source size (θ, see Section 4), and find that about 75% of the sources are denser than 1.2 × 10⁴ cm⁻³, which indicates that the gas and dust are thermally coupled (see Merello et al. 2019 for details). Distances (d) for CMZ sources are assumed to be 8.15 kpc (Reid et al. 2019), and are taken from SIMBAD ¹⁵ for the remaining sources. Therefore we assumed that T_ex is equal to the temperature of the dust, T_d. The optical thickness of the line emission (τ) can then be derived from the equation (Hernandez et al. 2011)

$$\begin{eqnarray}&&{T}_{{\rm{mb}}}=\displaystyle \frac{h\nu }{{k}_{{\rm{B}}}}\left[F({T}_{{\rm{ex}}})-F({T}_{{\rm{bg}}})\right]\left(1-{e}^{-{\tau }_{\nu }}\right),\end{eqnarray} \tag{ 2 }$$

where T_mb is the main-beam brightness temperature, h is the Planck constant, k_B is the Boltzmann constant, ν is the transition frequency, T_ex is the line excitation temperature, T_bg is the background temperature, and F(T_ex) is the (average) photon occupation number, which is given by $F(T)=1/({e}^{h\nu /{k}_{{\rm{B}}}T}-1)$. With T_ex from T_d, and T_mb and T_bg being known from the observations, the optical depths τ_ν can also be determined.

3.2. Column Densities and Fractional Abundances

The beam-averaged column densities of the three molecules (the size of the beam is 38''), N_MOL, can be derived from Garden et al. (1991):

$$\begin{eqnarray}&&{N}_{\mathrm{MOL}}=\displaystyle \frac{8\pi {\nu }^{3}}{{c}^{3}}\displaystyle \frac{{Q}_{\mathrm{rot}}}{{g}_{{\rm{u}}}{A}_{\mathrm{ul}}}\displaystyle \frac{\exp ({E}_{{}_{l}}/{k}_{{\rm{B}}}{T}_{\mathrm{ex}})}{[1-\exp (-h\nu /{k}_{{\rm{B}}}{T}_{\mathrm{ex}})]}\int {\tau }_{\nu }\,{dv},\end{eqnarray} \tag{ 3 }$$

where g_u is the statistical weight of the upper level, A_ul is the Einstein coefficient for spontaneous emission, ${E}_{{}_{l}}$ is the energy of the lower state, and Q_rot is the partition function. All the above parameters of HNCO 4₀₄–3₀₃, SiO 2–1, and HC₃N 10–9 are presented in detail by Sanhueza et al. (2012). The calculated τ_ν and column densities of these three molecules are listed in Table 1, column (10) and Table 2, column (7), respectively.

Table 2. Derived Parameters for Sources

Source a	l	b	MOL	vrange	∫Tmb dv	N	${N}_{{{\rm{H}}}_{2}}$	x	Comment
	(deg)	(deg)		(km s−1)	(K km s−1)	(1013 cm−2)	(1022 cm−2)	(10−10)
(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
G000.067−00.077	0.065	−0.078	HNCO	16.86, 96.65	70.65 ± 1.54	88.15 ± 1.92	18.13 ± 1.56	48.62 ± 4.34	CMZ
	0.065	−0.078	SiO	26.22, 91.03	48.09 ± 1.32	11.84 ± 0.32		6.53 ± 0.59
	0.065	−0.078	HC3N	32.59, 69.68	51.26 ± 0.98	25.16 ± 0.48		13.88 ± 1.23
G000.104−00.080	0.107	−0.085	HNCO	32.75, 73.68	59.60 ± 0.65	74.70 ± 0.81	16.72 ± 2.11	44.68 ± 5.66	CMZ
	0.107	−0.085	SiO	17.44, 76.46	60.75 ± 0.79	15.01 ± 0.19		8.98 ± 1.13
	0.107	−0.085	HC3N	26.07, 72.01	76.95 ± 0.76	37.60 ± 0.37		22.49 ± 2.84
G000.106−00.001	0.104	−0.004	HNCO	27.84, 84.77	41.18 ± 0.67	52.09 ± 0.84	8.76 ± 1.22	59.44 ± 8.34	CMZ
	0.104	−0.004	SiO	30.31, 71.54	28.39 ± 0.53	7.06 ± 0.13		8.05 ± 1.13
	0.104	−0.004	HC3N	34.31, 72.77	42.33 ± 0.49	20.50 ± 0.23		23.39 ± 3.27
G000.110+00.148	0.113	0.152	HNCO	80.21, 121.25	13.96 ± 0.56	17.50 ± 0.70	4.81 ± 0.54	36.37 ± 4.39	CMZ
	0.113	0.152	SiO	87.88, 113.59	6.85 ± 0.52	1.69 ± 0.12		3.52 ± 0.48
	0.113	0.152	HC3N	90.06, 110.86	8.82 ± 0.39	4.31 ± 0.19		8.96 ± 1.09
G000.314−00.100	0.314	−0.096	HNCO	47.32, 104.77	21.71 ± 0.82	27.97 ± 1.05	7.09 ± 0.40	39.46 ± 2.69	CMZ
	0.314	−0.096	SiO	37.29, 112.58	21.95 ± 1.07	5.52 ± 0.26		7.79 ± 0.58
	0.314	−0.096	HC3N	51.79, 116.48	13.82 ± 0.86	6.58 ± 0.40		9.29 ± 0.78
G000.497+00.021	0.497	0.018	HNCO	3.51, 53.42	69.41 ± 0.72	86.21 ± 0.89	16.59 ± 2.84	51.96 ± 8.93	CMZ
	0.497	0.018	SiO	0.70, 51.31	22.14 ± 0.73	5.44 ± 0.17		3.28 ± 0.57
	0.497	0.018	HC3N	2.81, 53.42	43.80 ± 0.66	21.60 ± 0.32		13.02 ± 2.24
G000.645+00.027	0.647	0.032	HNCO	21.76, 81.95	112.17 ± 0.84	138.69 ± 1.03	15.67 ± 1.34	88.49 ± 7.62	CMZ
	0.647	0.032	SiO	22.45, 93.99	30.90 ± 0.81	7.56 ± 0.19		4.83 ± 0.43
	0.647	0.032	HC3N	26.92, 77.82	60.69 ± 0.76	30.08 ± 0.37		19.19 ± 1.66
G000.892+00.143	0.894	0.142	HNCO	57.39, 98.18	19.97 ± 0.52	24.25 ± 0.63	3.30 ± 0.41	73.46 ± 9.52	CMZ
	0.894	0.142	SiO	54.08, 108.93	16.16 ± 0.61	3.91 ± 0.14		11.84 ± 1.56
	0.894	0.142	HC3N	54.36, 92.40	14.11 ± 0.49	7.14 ± 0.24		21.62 ± 2.84
G000.908+00.116	0.919	0.116	HNCO	51.24, 88.10	12.73 ± 0.66	15.39 ± 0.79	3.53 ± 0.32	43.58 ± 4.60	CMZ
	0.919	0.116	SiO	41.22, 93.71	9.49 ± 0.69	2.29 ± 0.16		6.48 ± 0.76
	0.919	0.116	HC3N	56.05, 88.90	5.82 ± 0.46	2.96 ± 0.23		8.38 ± 1.01
G001.226+00.059	1.233	0.061	HNCO	77.97, 136.29	16.64 ± 0.67	19.76 ± 0.79	5.43 ± 1.05	36.39 ± 7.20	CMZ
	1.233	0.061	SiO	79.95, 147.82	41.61 ± 0.73	9.92 ± 0.17		18.27 ± 3.55
	1.233	0.061	HC3N	88.52, 131.35	22.60 ± 0.48	11.76 ± 0.24		21.66 ± 4.22
G001.344+00.258	1.348	0.262	HNCO	75.10, 131.08	26.14 ± 0.71	30.63 ± 0.83	5.65 ± 0.52	54.24 ± 5.25	CMZ
	1.348	0.262	SiO	66.91, 136.99	55.98 ± 0.86	13.23 ± 0.20		23.43 ± 2.20
	1.348	0.262	HC3N	76.01, 124.25	35.37 ± 0.60	18.75 ± 0.31		33.21 ± 3.13
G001.381+00.201	1.381	0.187	HNCO	75.30, 123.23	10.08 ± 0.61	11.86 ± 0.71	2.68 ± 0.17	44.31 ± 3.94	CMZ
	1.381	0.187	SiO	60.05, 133.58	26.93 ± 0.76	6.38 ± 0.18		23.84 ± 1.69
	1.381	0.187	HC3N	69.85, 121.05	11.24 ± 0.62	5.92 ± 0.32		22.11 ± 1.89
G001.510+00.155	1.522	0.144	HNCO	64.27, 95.39	28.51 ± 0.61	32.83 ± 0.70	3.04 ± 0.40	107.93 ± 7.35	CMZ
	1.522	0.144	SiO	66.14, 98.90	15.81 ± 0.63	3.69 ± 0.14		12.15 ± 1.70
	1.522	0.144	HC3N	66.37, 92.11	19.75 ± 0.61	10.76 ± 0.33		35.36 ± 4.88
G001.610−00.172	1.607	−0.174	HNCO	29.14, 55.45	26.24 ± 0.49	29.96 ± 0.55	4.53 ± 0.50	66.07 ± 7.42	CMZ
	1.607	−0.174	SiO	30.13, 55.94	6.29 ± 0.42	1.46 ± 0.09		3.22 ± 0.41
	1.607	−0.174	HC3N	34.80, 55.94	7.47 ± 0.37	4.13 ± 0.20		9.10 ± 1.10
G001.655−00.062	1.661	−0.053	HNCO	36.36, 62.59	46.96 ± 0.47	52.08 ± 0.52	6.23 ± 0.61	83.63 ± 8.29	CMZ
G1.87−SMM 1*
	1.661	−0.053	SiO	38.79, 64.65	10.48 ± 0.44	2.39 ± 0.10		3.84 ± 0.41
	1.661	−0.053	HC3N	39.36, 60.72	14.98 ± 0.39	8.76 ± 0.22		14.06 ± 1.43
G001.694−00.385	1.694	−0.388	HNCO	−50.97, −22.28	25.33 ± 0.69	28.92 ± 0.78	6.38 ± 0.83	45.31 ± 6.03	CMZ
	1.694	−0.388	SiO	−61.15, −15.18	19.66 ± 0.93	4.57 ± 0.21		7.16 ± 0.99
	1.694	−0.388	HC3N	−47.89, −20.12	24.02 ± 0.70	13.27 ± 0.38		20.79 ± 2.77
G001.699−00.366	1.700	−0.367	HNCO	−53.14, −19.61	26.66 ± 0.55	30.31 ± 0.62	6.70 ± 0.40	45.26 ± 2.91	CMZ
	1.700	−0.367	SiO	−59.97, −10.30	19.35 ± 0.63	4.48 ± 0.14		6.70 ± 0.46
	1.700	−0.367	HC3N	−49.41, −10.85	38.80 ± 0.54	21.60 ± 0.30		32.25 ± 2.02
G001.734−00.410	1.737	−0.412	HNCO	−57.64, −24.24	36.06 ± 0.43	40.15 ± 0.47	11.75 ± 1.53	34.18 ± 4.49	CMZ
	1.737	−0.412	SiO	−54.63, −20.53	18.80 ± 0.44	4.30 ± 0.10		3.66 ± 0.48
	1.737	−0.412	HC3N	−49.99, −25.17	30.04 ± 0.36	17.41 ± 0.20		14.82 ± 1.94
G001.883−00.062	1.887	−0.063	HNCO	26.73, 49.74	13.22 ± 0.36	15.36 ± 0.41	4.07 ± 0.38	37.69 ± 3.69	CMZ
G1.87−SMM 23*
	1.887	−0.063	SiO	30.10, 44.78	5.29 ± 0.38	1.24 ± 0.08		3.05 ± 0.36
	1.887	−0.063	HC3N	28.52, 43.79	6.74 ± 0.34	3.62 ± 0.18		8.89 ± 0.94
G003.240+00.635	3.241	0.635	HNCO	29.37, 56.47	6.47 ± 0.45	7.23 ± 0.50	3.17 ± 0.41	22.80 ± 3.40	CMZ
	3.241	0.635	SiO	23.88, 59.90	13.07 ± 0.52	3.00 ± 0.11		9.44 ± 1.30
	3.241	0.635	HC3N	28.68, 56.81	7.44 ± 0.45	4.28 ± 0.25		13.48 ± 1.95
G003.338+00.419	3.339	0.425	HNCO	9.00, 55.42	20.86 ± 0.72	23.13 ± 0.79	8.73 ± 0.30	26.51 ± 1.29	CMZ
	3.339	0.425	SiO	1.87, 60.94	35.05 ± 0.74	7.99 ± 0.16		9.16 ± 0.37
	3.339	0.425	HC3N	6.09, 56.72	34.77 ± 0.74	20.32 ± 0.43		23.29 ± 0.94
G359.445−00.054	359.446	−0.058	HNCO	−121.35, −84.20	24.52 ± 0.58	31.45 ± 0.74	6.51 ± 1.02	48.32 ± 7.70	CMZ
	359.446	−0.058	SiO	−121.35, −92.22	7.07 ± 0.47	1.77 ± 0.11		2.72 ± 0.46
	359.446	−0.058	HC3N	−112.13, −92.52	9.24 ± 0.42	4.42 ± 0.20		6.79 ± 1.11
G359.453−00.112	359.455	−0.112	HNCO	−65.95, −42.46	17.49 ± 0.49	22.33 ± 0.62	14.37 ± 1.09	15.53 ± 1.26	CMZ
	359.455	−0.112	SiO	−62.96, −43.10	8.70 ± 0.41	2.18 ± 0.10		1.51 ± 0.13
	359.455	−0.112	HC3N	−66.38, −42.46	14.00 ± 0.46	6.72 ± 0.22		4.68 ± 0.38
G359.565−00.161	359.551	−0.166	HNCO	−75.68, −43.94	17.28 ± 0.53	22.26 ± 0.68	3.79 ± 0.18	58.67 ± 3.44	CMZ
	359.551	−0.166	SiO	−70.39, −35.12	7.73 ± 0.57	1.94 ± 0.14		5.12 ± 0.45
	359.551	−0.166	HC3N	−72.51, −43.23	9.99 ± 0.50	4.76 ± 0.23		12.54 ± 0.88
G359.868−00.085	359.866	−0.082	HNCO	−9.51, 23.01	63.64 ± 0.54	80.13 ± 0.67	35.98 ± 2.52	22.27 ± 1.57	CMZ
	359.866	−0.082	SiO	−14.66, 30.87	43.22 ± 0.69	10.71 ± 0.17		2.98 ± 0.21
	359.866	−0.082	HC3N	−11.95, 22.20	61.99 ± 0.62	30.16 ± 0.30		8.38 ± 0.59
G359.895−00.069	359.892	−0.076	HNCO	−4.15, 35.34	81.24 ± 0.55	101.83 ± 0.68	26.93 ± 3.12	37.81 ± 4.40	CMZ
	359.892	−0.076	SiO	−12.52, 37.35	62.86 ± 0.58	15.53 ± 0.14		5.77 ± 0.67
	359.892	−0.076	HC3N	−7.83, 35.67	89.48 ± 0.52	43.73 ± 0.25		16.24 ± 1.88
G359.977−00.072	359.980	−0.070	HNCO	26.79, 63.94	55.09 ± 0.79	73.64 ± 1.05	21.39 ± 0.87	34.43 ± 1.48	CMZ
	359.980	−0.070	SiO	19.59, 76.07	70.96 ± 0.93	18.29 ± 0.23		8.55 ± 0.36
	359.980	−0.070	HC3N	21.87, 71.52	97.20 ± 0.89	44.99 ± 0.41		21.03 ± 0.87
G008.671−00.357	8.677	−0.360	HNCO	31.47, 40.86	4.59 ± 0.27	5.65 ± 0.33	18.69 ± 7.45	3.02 ± 1.21	Bubble
	8.677	−0.360	SiO	28.63, 41.97	4.50 ± 0.35	1.10 ± 0.08		0.59 ± 0.23
	8.677	−0.360	HC3N	31.47, 42.47	9.71 ± 0.31	4.84 ± 0.15		2.59 ± 1.03
G010.473+00.028	10.480	0.030	HNCO	58.39, 71.82	4.37 ± 0.28	5.55 ± 0.35	25.54 ± 6.74	2.17 ± 1.15	Bubble
	10.480	0.030	SiO	55.54, 84.44	4.57 ± 0.47	1.14 ± 0.11		0.45 ± 0.23
	10.480	0.030	HC3N	59.21, 76.30	6.23 ± 0.35	3.00 ± 0.16		1.18 ± 0.62
G322.159+00.635†	322.159	0.637	HNCO	−59.93, −51.67	1.93 ± 0.25	2.93 ± 0.37	24.45 ± 3.76	1.20 ± 0.24	Bubble
	322.159	0.637	SiO	−63.59, −45.31	9.83 ± 0.32	2.76 ± 0.08		1.13 ± 0.17
	322.159	0.637	HC3N	−61.83, −52.08	13.57 ± 0.25	5.88 ± 0.10		2.40 ± 0.37
G326.653+00.618	326.640	0.615	HNCO	−46.18, −34.80	3.87 ± 0.30	4.87 ± 0.37	25.46 ± 3.52	1.91 ± 0.30	Bubble
	326.640	0.615	SiO	−48.69, −28.43	5.32 ± 0.37	1.32 ± 0.09		0.52 ± 0.08
	326.640	0.615	HC3N	−43.34, −36.30	6.44 ± 0.23	3.13 ± 0.11		1.23 ± 0.17
G327.293−00.579†	327.296	−0.577	HNCO	−51.43, −39.94	3.74 ± 0.28	5.07 ± 0.37	59.52 ± 10.28	0.85 ± 0.30	Bubble
	327.296	−0.577	SiO	−53.44, −35.71	8.43 ± 0.38	2.19 ± 0.09		0.37 ± 0.12
	327.296	−0.577	HC3N	−50.62, −36.72	19.24 ± 0.30	8.82 ± 0.13		1.48 ± 0.51
G345.004−00.224†	345.007	−0.220	HNCO	−34.85, −20.90	3.70 ± 0.28	4.92 ± 0.37	19.83 ± 8.50	2.48 ± 1.08	Bubble
	345.007	−0.220	SiO	−43.62, −11.96	12.96 ± 0.54	3.33 ± 0.13		1.68 ± 0.72
	345.007	−0.220	HC3N	−35.63, −19.17	11.49 ± 0.32	5.34 ± 0.14		2.69 ± 1.15
G350.101+00.083†	350.110	0.096	HNCO	−74.50, −62.16	4.93 ± 0.36	6.41 ± 0.46	16.87 ± 3.76	3.80 ± 0.89	Bubble
	350.110	0.096	SiO	−77.85, −58.98	5.65 ± 0.45	1.43 ± 0.11		0.85 ± 0.20
	350.110	0.096	HC3N	−75.74, −62.16	4.83 ± 0.34	2.28 ± 0.16		1.35 ± 0.31
G351.443+00.659	351.451	0.654	HNCO	−8.29, 2.03	6.05 ± 0.31	7.76 ± 0.39	45.35 ± 10.38	1.71 ± 0.78	Bubble
	351.451	0.654	SiO	−16.13, 5.85	22.42 ± 0.48	5.62 ± 0.12		1.24 ± 0.56
	351.451	0.654	HC3N	−11.73, 2.41	33.35 ± 0.35	15.95 ± 0.16		3.52 ± 1.61
G351.582−00.352	351.579	−0.355	HNCO	−100.73, −90.92	5.18 ± 0.25	7.35 ± 0.35	31.87 ± 8.80	2.31 ± 0.64	Bubble
	351.579	−0.355	SiO	−104.31, −92.48	3.60 ± 0.35	0.97 ± 0.09		0.30 ± 0.08
	351.579	−0.355	HC3N	−101.04, −90.92	7.30 ± 0.30	3.26 ± 0.13		1.02 ± 0.28
G351.775−00.537	351.773	−0.539	HNCO	−8.04, 1.36	4.73 ± 0.35	7.62 ± 0.56	50.86 ± 6.09	1.50 ± 0.37	Bubble
	351.773	−0.539	SiO	−13.53, 11.97	25.98 ± 0.59	7.58 ± 0.17		1.49 ± 0.35
	351.773	−0.539	HC3N	−8.53, 3.80	13.09 ± 0.40	5.57 ± 0.17		1.09 ± 0.26
G329.030−00.202†	329.031	−0.201	HNCO	−47.95, −39.93	3.50 ± 0.27	4.43 ± 0.34	22.03 ± 3.86	2.01 ± 0.38	NMSFR
	329.031	−0.201	SiO	−56.70, −36.64	8.90 ± 0.37	2.21 ± 0.09		1.00 ± 0.18
	329.031	−0.201	HC3N	−55.25, −38.65	10.63 ± 0.35	5.15 ± 0.16		2.34 ± 0.41
G331.708+00.583	331.709	0.582	HNCO	−69.26, −62.52	1.61 ± 0.21	1.91 ± 0.24	8.32 ± 1.88	2.30 ± 0.60	NMSFR
	331.709	0.582	SiO	−74.56, −59.08	3.72 ± 0.35	0.89 ± 0.08		1.07 ± 0.26
	331.709	0.582	HC3N	−70.41, −61.80	3.59 ± 0.25	1.87 ± 0.13		2.25 ± 0.53
G331.709+00.602	331.709	0.602	HNCO	−70.63, −60.74	3.26 ± 0.30	3.94 ± 0.36	10.13 ± 1.53	3.89 ± 0.68	NMSFR
	331.709	0.602	SiO	−71.11, −58.98	2.63 ± 0.33	0.63 ± 0.07		0.63 ± 0.12
	331.709	0.602	HC3N	−70.79, −63.77	3.59 ± 0.25	1.83 ± 0.12		1.80 ± 0.30
G335.586−00.289†	335.584	−0.288	HNCO	−49.02,−41.51	2.79 ± 0.24	3.75 ± 0.32	17.33 ± 3.99	2.16 ± 0.53	NMSFR
	335.584	−0.288	SiO	−56.53, −41.92	6.02 ± 0.31	1.56 ± 0.08		0.90 ± 0.21
	335.584	−0.288	HC3N	−52.31, −42.13	7.58 ± 0.28	3.50 ± 0.12		2.02 ± 0.47
G348.754−00.941†	348.761	−0.948	HNCO	−22.22, −10.53	3.81 ± 0.35	4.46 ± 0.41	9.80 ± 3.42	4.55 ± 1.64	NMSFR
	348.761	−0.948	SiO	−16.89, −6.91	3.34 ± 0.35	0.79 ± 0.08		0.81 ± 0.29
	348.761	−0.948	HC3N	−16.54, −4.68	5.02 ± 0.32	2.66 ± 0.16		2.71 ± 0.96
G351.157+00.701	351.155	0.709	HNCO	−9.69, −2.76	2.86 ± 0.27	3.99 ± 0.37	23.82 ± 5.66	1.67 ± 0.42	NMSFR
	351.155	0.709	SiO	−9.18, −2.38	3.54 ± 0.25	0.94 ± 0.06		0.39 ± 0.09
	351.155	0.709	HC3N	−10.82, −1.87	13.08 ± 0.28	5.90 ± 0.12		2.47 ± 0.59

Notes. The columns are as follows: (1) MALT90 name; (2) and (3) Galactic longitude and latitude of the yellow crosses on maps of Figures A1 to A22; (4) molecular species; (5) velocity range; (6) integrated intensity is derived by integrating over the velocity range indicated in column (5); (7) beam-averaged column density of the molecules discussed in this paper; (8) beam-averaged H₂ column density; (9) fractional abundance relative to H₂;  (10) source category (see Section 2.2).

^a * indicates the source name is adopted from Miettinen (2014). † indicates a source is identified as an IRDC by Peretto & Fuller (2009).

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The hydrogen column density, ${N}_{{{\rm{H}}}_{2}}$, can be derived from the ATLASGAL dust continuum maps smoothed to the MALT90 resolution of 38'' (Kauffmann et al. 2008):

$$\begin{eqnarray}&&{N}_{{{\rm{H}}}_{2}}=\displaystyle \frac{{S}_{\nu }R}{{B}_{\nu }({T}_{{\rm{d}}}){\rm{\Omega }}{\kappa }_{\nu }\mu {m}_{{\rm{H}}}},\end{eqnarray} \tag{ 4 }$$

where S_ν is the 870 μm continuum flux, Ω is the beam solid angle (which has been smoothed to the MALT90 resolution of 38''), and κ_ν is the dust absorption coefficient (taken as 1.7 cm² g⁻¹, see Section 3.1).

The fractional abundances of the molecules were calculated by dividing the beam-averaged molecular column density by the hydrogen column density, x_MOL = N_MOL/${N}_{{{\rm{H}}}_{2}}$. The calculated fractional abundances of HNCO, SiO, and HC₃N are listed in Table 2, column (9).

Figures A1–A22 show the diagrams of the HNCO, SiO, and HC₃N 3 × 3 arcmin² integrated intensity maps superposed on the ATLASGAL 870 μm emission images and dust temperature maps, the beam-averaged spectra of HNCO 4₀₄–3₀₃, SiO 2–1, and HC₃N 10–9 at the position marked by yellow crosses on the maps, and the trends of normalized abundance ratios N_HNCO/N_SiO, N_HNCO/N_HC3N, and N_HC3N/N_SiO along Galactic longitude and latitude passing through the yellow cross for the 43 southern sources. The HNCO lines, and a part of the strong SiO and HC₃N lines, which show single Gaussian profiles, were fitted with a single Gaussian using CLASS. Sources showing more than one velocity were fitted using two Gaussian components. For instance, CMZ sources G359.977−00.072, G359.895−00.069, G359.868−00.085, G359.453−00.112, G000.067−00.077, G000.104−00.080, G000.106−00.001, G000.908+00.116, G001.655−00.062, and G001.883−00.062, Bubble sources G345.004−00.224, G351.443+00.659, and G351.775−00.537, and NMSFR sources G329.030−00.202 and G335.586−00.289 showing more than one velocity component were fitted with two Gaussian components (the two Gaussian components and total fits are shown with red and green lines in Figures A1–A22). The Gaussian fitting velocity components with FWHM line width (ΔV) of HNCO 4₀₄–3₀₃, SiO 2–1, and HC₃N 10–9 emission are presented in Table 1. The integrated line intensity (∫T_mb dv) is derived by integrating the area underneath the lines over the velocity range indicated in Table 2, column (6). The details of the 43 individual sources are described in the Appendix. In most CMZ sources, we find that the abundance ratios N_HNCO/N_SiO and N_HNCO/N_HC3N show a decreasing trend toward the yellow cross, which is similar to the dust temperature, while N_HC3N/N_SiO moves in the opposite direction. In Bubble and NMSFR category sources, dust temperature and N_HC3N/N_SiO roughly follow a similar trend toward the center, where stars are forming (see Section 5.4).

We characterize the size of HNCO 4₀₄–3₀₃, SiO 2–1, and HC₃N 10–9 emission using a beam deconvolved angular diameter of a circle with the same area as the half-peak intensity, which is given by $\theta =2{\left(\tfrac{A}{\pi }-\tfrac{{\theta }_{\mathrm{beam}}^{2}}{4}\right)}^{1/2}$, where A is the area within the half-peak intensity and θ_beam is the FWHM beam size. The angular diameters of HNCO range from 25'' to 200'', with a mean value of 102''± 45'', the error being the standard deviation of an individual measurement. Nearly all the angular diameters of HNCO are well above the beam size. The angular diameters of SiO range from 14'' to 196'', with a mean value of 97''± 48''. Molecular lines of HNCO 4₀₄–3₀₃ and SiO 2–1 show similar sizes for most sources. The angular diameters of HC₃N range from 18'' to 151'', with a mean value of 81''± 37''. Nearly half of the angular diameters of HC₃N are smaller than the telescope beam size. The average angular diameters of NMSFR, Bubble, and CMZ sources are 63''± 23'', 61''± 24'', and 124''± 38'' for HNCO, 43''± 7'', 51''± 20'', and 122''± 37'' for SiO, and 39''± 10'', 48''± 24'', and 100''± 28'' for HC₃N, respectively. That HC₃N turns out to be more compact than HNCO and SiO cannot be an effect of relatively weak HC₃N lines, which are not detected in the outskirts of the clouds due to the limited sensitivity of our data. On the contrary, the line tending to be weakest in our study is SiO, thus suggesting that HC₃N really originates from a smaller volume than HNCO and SiO. Besides, these three species show more spatially extended emission toward the CMZ sources than the other two categories. The distributions of the angular diameters for HNCO 4₀₄–3₀₃, SiO 2–1, and HC₃N 10–9 are shown in Figure 1.

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5.1. Previous Studies

5.2. Molecular Line Properties and Column Densities

Figure 2 shows a comparison of line widths between HNCO and SiO, HNCO and HC₃N, as well as SiO and HC₃N. We find that SiO has the largest line widths, which agrees with previous research of Zinchenko et al. (2000). The line widths of HNCO and HC₃N are similar. The mean values of the HNCO line widths are 3.81 ± 1.34, 5.71 ± 1.63, and 18.19 ± 7.46 km s⁻¹ for the NMSFR, Bubble, and CMZ samples, respectively. Separately for SiO and HC₃N, the corresponding values are 4.86 ± 1.64, 8.80 ± 2.08, and 23.83 ± 9.11 km s⁻¹, and 3.87 ± 0.57, 5.40 ± 1.26, and 17.36 ± 7.53 km s⁻¹. For NMSFR and CMZ sources, the SiO line widths are about 1.3 times larger than those of HNCO and HC₃N, while for sources in the Bubble category they are 1.4 to 1.6 times larger. SiO has the largest line widths, since it is tracing the strongest shocks.

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Figure 3 shows a comparison of integrated intensities (in K km s⁻¹) averaged over a circular area positioned on the yellow cross of each map with a diameter corresponding to the ∼38'' Mopra beamwidth. We find a tight correlation between SiO and HC₃N (r = 0.89), and a linear fitting relationship:

$$\begin{eqnarray}&&{I}_{{\mathrm{HC}}_{3}{\rm{N}}}=(1.15\pm 0.09)\times {I}_{\mathrm{SiO}}+(1.08\pm 2.45).\end{eqnarray} \tag{ 5 }$$

${I}_{{\mathrm{HC}}_{3}{\rm{N}}}$ and, to a lesser extent, I_SiO also show linear relationships with the integrated intensity of HNCO, with linear Pearson correlation coefficients of r = 0.81 and 0.66, respectively. The relationships are

$$\begin{eqnarray}&&{I}_{{\mathrm{HC}}_{3}{\rm{N}}}=(0.75\pm 0.08)\times {I}_{\mathrm{HNCO}}+(6.00\pm 2.92)\end{eqnarray} \tag{ 6 }$$

$$\begin{eqnarray}&&{I}_{\mathrm{SiO}}=(0.47\pm 0.08)\times {I}_{\mathrm{HNCO}}+(8.49\pm 2.89).\end{eqnarray} \tag{ 7 }$$

Applying Equation (2), Figure 4 plots optical depths. We find that the three molecular lines studied here are all optically thin (τ ≪ 1) in our star-forming regions, which agrees with the previous study of Miettinen (2014). For sources in the CMZ, the optical depths of the HNCO lines are similar to those of the HC₃N lines. The optical depths of the HC₃N and SiO lines appear to be greater than those of HNCO in NMSFR and Bubble sources.

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Using the equations in Section 3.2, column densities and abundances of HNCO, SiO, and HC₃N are derived and presented in columns (7) and (9) of Table 2. The column densities of HNCO, SiO, and HC₃N for their whole sample to be compared with those by other groups, summarized in Section 5.1, lie in the ranges 1.91 × 10¹³–1.39 × 10¹⁵ cm⁻², 6.30 × 10¹²–1.83 × 10¹⁴ cm⁻², and 1.83 × 10¹³–4.50 × 10¹⁴ cm⁻², with mean values of 2.93 × 10¹⁴ cm⁻², 4.83 × 10¹³ cm⁻², and 1.19 × 10¹⁴ cm⁻², respectively. The corresponding median values are 1.98 × 10¹⁴ cm⁻², 3.00 × 10¹³ cm⁻², and 5.92 × 10¹³ cm⁻². The mean abundances of HNCO, SiO, and HC₃N are 3.13 × 10⁻⁹, 5.24 × 10⁻¹⁰, and 1.14 × 10⁻⁹, respectively. The corresponding median values are 3.44 × 10⁻⁹, 3.28 × 10⁻¹⁰, and 8.96 × 10⁻¹⁰. Sanhueza et al. (2012) derived an SiO abundance comparable to ours for our whole sample.

The mean HNCO column density we find, 2.93 × 10¹⁴ cm⁻², is quite similar to those derived by Miettinen (2014) toward their IRDC sources. Our median column density of 1.98 × 10¹⁴ cm⁻² is about five times higher than the median value found by Sanhueza et al. (2012). Because most sources of our work are part of the CMZ or associated with bubbles, the surrounding environment may cause this difference. The average HNCO column densities of NMSFR, Bubble, and CMZ are 3.75 × 10¹³, 5.81 × 10¹³, and 4.37 × 10¹⁴ cm⁻², respectively. The corresponding median values are 3.97 × 10¹³, 5.60 × 10¹³, and 3.03 × 10¹⁴ cm⁻². We find that the average and median column densities of HNCO for each category increase as in the case of SiO, monotonically from the NMSFR sources to the Bubble sources to the CMZ sources. This suggests that shocks in the Bubble category are strong enough to evaporate a significant amount of HNCO from grain mantles without dissociating them in comparison with sources of the NMSFR category. HNCO in the CMZ is much more abundant than the other two species, SiO and HC₃N, and seems to react more sensitively to its extreme environment.

The mean values of SiO peak column densities of NMSFR, Bubble, and CMZ clouds we derive are 9.58 × 10¹² cm⁻², 2.88 × 10¹³ cm⁻², and 6.97 × 10¹³ cm⁻², respectively. The corresponding median values are 8.15 × 10¹² cm⁻², 1.89 × 10¹³ cm⁻², and 4.76 × 10¹³ cm⁻². The NMSFR values are similar to those found by Sakai et al. (2010) for their sample of clumps within IRDCs, and by Miettinen (2014) for their sample of massive clumps (see Section 5.1).

For the HC₃N peak column densities of NMSFR, Bubble, and CMZ sources we derive mean values of 3.49 × 10¹³, 5.81 × 10¹³, and 1.60 × 10¹⁴ cm⁻², and median values of 3.08 × 10¹³, 5.09 × 10¹³, and 1.18 × 10¹⁴ cm⁻², respectively. Again column densities are rising from the NMSFR to the CMZ sources. The median HC₃N column density found by Miettinen (2014) (11 out of their 12 detections are in the CMZ) is quite similar to our value for the CMZ clouds. Our average value is almost an order of magnitude higher than the highest value found by Sakai et al. (2008) toward massive clumps associated with IRDCs located in the Galactic disk. Our median column density in NMSFR (3.08 × 10¹³ cm⁻²) exceeds the value found by Sanhueza et al. (2012) by a factor of five. From the foregoing, we find that the CMZ contains large reservoirs of SiO, HNCO, and HC₃N gas, with almost one order of magnitude higher column densities than in the disk, especially when being compared to the sources in the NMSFR category.

Figure 5 confirms that column densities of our three molecules increase from NMSFR, to Bubble, and on to sources with CMZ classification. A particularly strong correlation coefficient r = 0.88 is found for the column densities of SiO and HC₃N for the entire sample. However, the differences between the Pearson correlation coefficients are not that large, so this is not a finding we will emphasize. Least-squares fitting yields

$$\begin{eqnarray}&&\mathrm{log}({N}_{{\mathrm{HC}}_{3}{\rm{N}}})=(2.24\pm 0.98)+(0.86\pm 0.07)\times \mathrm{log}({N}_{\mathrm{SiO}}).\end{eqnarray} \tag{ 8 }$$

Moreover, a slightly weaker correlation is obtained between the column densities of HNCO and SiO (r = 0.71) for the whole sample, as well as between the column densities of HNCO and HC₃N (r = 0.79). Results given by linear fitting are

$$\begin{eqnarray}&&\mathrm{log}({N}_{\mathrm{SiO}})=(5.33\pm 1.26)+(0.58\pm 0.09)\times \mathrm{log}({N}_{\mathrm{HNCO}}),\end{eqnarray} \tag{ 9 }$$

and

$$\begin{eqnarray}\begin{array}{rcl}\mathrm{log}({N}_{{\mathrm{HC}}_{3}{\rm{N}}}) & = & (4.97\pm 1.07)+(0.63\pm 0.08)\\ & & \times \mathrm{log}({N}_{\mathrm{HNCO}}).\end{array}\end{eqnarray} \tag{ 10 }$$

The high correlations between SiO and HC₃N in integrated intensities and column densities above indicate that there is a close relationship during the process of their chemical evolution. This argument will contribute to determining the dominant chemical model through numerical simulations of star-forming regions, where large-scale shocks are generated, e.g., due to external star-forming activities or outflows from embedded young stellar objects.

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5.3. The Ratios of Integrated Intensity and Abundances

Figure 6 presents a comparison of the fractional abundances of HNCO, SiO, and HC₃N. Two obvious clusters are seen between the CMZ sources and the other two categories, implying that the chemical properties of sources in the CMZ are different from those of sources far from the Galactic center. A high correlation (right panel of Figure 6) is obtained between the fractional abundances of SiO and HC₃N (r = 0.93) for the whole sample, possibly because their parent species Si and C₂H₂ are both released from dust grains due to a sudden increase in temperature or because of the erosion of dust grains via sputtering or grain–grain collisions in shocks. The linear fitting result (red dashed line) is

$$\begin{eqnarray}&&\mathrm{log}({x}_{{\mathrm{HC}}_{3}{\rm{N}}})=(-0.94\pm 0.50)+(0.86\pm 0.05)\times \mathrm{log}({x}_{\mathrm{SiO}}).\end{eqnarray} \tag{ 11 }$$

The upper cluster only contains CMZ sources, while the lower cluster contains NMSFR and Bubble sources. Moreover, Figure 6 shows that HNCO is the most abundant molecule in the CMZ category, while the fractional abundance of SiO tends to be the lowest in the whole sample. For sources in the Bubble and NMSFR categories, the fractional abundances of HNCO and HC₃N present a linear distribution with a slope of almost unity, while SiO is slightly less abundant.

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In order to have a brief understanding of the chemical properties of the three molecules HNCO, SiO, and HC₃N, we computed integrated intensity and abundance ratios. I_HNCO/I_SiO, ${I}_{\mathrm{SiO}}/{I}_{{\mathrm{HC}}_{3}{\rm{N}}}$, ${I}_{\mathrm{HNCO}}/{I}_{{\mathrm{HC}}_{3}{\rm{N}}}$, N_HNCO/N_SiO, ${N}_{\mathrm{SiO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$, and ${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$ are listed for the whole sample in Table 3. Statistics provided in Table 4 include the mean, median, and standard deviation (std), as well as minimum and maximum values of the sample.

Table 3. The Integrated Intensity and Abundance Ratios of HNCO, SiO, and HC₃N

Source	IHNCO/ISiO	${I}_{\mathrm{HNCO}}/{I}_{{\mathrm{HC}}_{3}{\rm{N}}}$	${I}_{\mathrm{SiO}}/{I}_{{\mathrm{HC}}_{3}{\rm{N}}}$	NHNCO/NSiO	${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$	${N}_{\mathrm{SiO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$	Comment
G000.067−00.077	1.47 ± 0.05	1.38 ± 0.04	0.94 ± 0.03	7.45 ± 0.26	3.50 ± 0.10	0.47 ± 0.02	CMZ
G000.104−00.080	0.98 ± 0.02	0.77 ± 0.01	0.79 ± 0.01	4.98 ± 0.08	1.99 ± 0.03	0.40 ± 0.01	CMZ
G000.106−00.001	1.45 ± 0.04	0.97 ± 0.02	0.67 ± 0.01	7.38 ± 0.18	2.54 ± 0.05	0.34 ± 0.01	CMZ
G000.110+00.148	2.04 ± 0.17	1.58 ± 0.09	0.78 ± 0.07	10.36 ± 0.84	4.06 ± 0.24	0.39 ± 0.03	CMZ
G000.314−00.100	0.99 ± 0.06	1.57 ± 0.11	1.59 ± 0.13	5.07 ± 0.31	4.25 ± 0.30	0.84 ± 0.06	CMZ
G000.497+00.021	3.14 ± 0.11	1.58 ± 0.03	0.51 ± 0.02	15.85 ± 0.52	3.99 ± 0.07	0.25 ± 0.01	CMZ
G000.645+00.027	3.63 ± 0.10	1.85 ± 0.03	0.51 ± 0.01	18.35 ± 0.48	4.61 ± 0.07	0.25 ± 0.01	CMZ
G000.892+00.143	1.24 ± 0.06	1.42 ± 0.06	1.15 ± 0.06	6.20 ± 0.27	3.40 ± 0.14	0.55 ± 0.03	CMZ
G000.908+00.116	1.34 ± 0.12	2.19 ± 0.21	1.63 ± 0.18	6.72 ± 0.58	5.20 ± 0.48	0.77 ± 0.08	CMZ
G001.226+00.059	0.40 ± 0.02	0.74 ± 0.03	1.84 ± 0.05	1.99 ± 0.09	1.68 ± 0.08	0.84 ± 0.02	CMZ
G001.344+00.258	0.47 ± 0.01	0.74 ± 0.02	1.58 ± 0.04	2.32 ± 0.07	1.63 ± 0.05	0.71 ± 0.02	CMZ
G001.381+00.201	0.37 ± 0.02	0.90 ± 0.07	2.40 ± 0.15	1.86 ± 0.12	2.00 ± 0.16	1.08 ± 0.07	CMZ
G001.510+00.155	1.80 ± 0.08	1.44 ± 0.05	0.80 ± 0.04	8.90 ± 0.39	3.05 ± 0.11	0.34 ± 0.02	CMZ
G001.610−00.172	4.17 ± 0.29	3.51 ± 0.19	0.84 ± 0.07	20.52 ± 1.32	7.25 ± 0.38	0.35 ± 0.03	CMZ
G001.655−00.062	4.48 ± 0.19	3.13 ± 0.09	0.70 ± 0.03	21.79 ± 0.94	5.95 ± 0.16	0.27 ± 0.01	CMZ
G001.694−00.385	1.29 ± 0.07	1.05 ± 0.04	0.82 ± 0.05	6.33 ± 0.34	2.18 ± 0.09	0.34 ± 0.02	CMZ
G001.699−00.366	1.38 ± 0.05	0.69 ± 0.02	0.50 ± 0.02	6.77 ± 0.25	1.40 ± 0.03	0.21 ± 0.01	CMZ
G001.734−00.410	1.92 ± 0.05	1.20 ± 0.02	0.63 ± 0.02	9.34 ± 0.24	2.31 ± 0.04	0.25 ± 0.01	CMZ
G001.883−00.062	2.50 ± 0.19	1.96 ± 0.11	0.78 ± 0.07	12.39 ± 0.86	4.24 ± 0.24	0.34 ± 0.03	CMZ
G003.240+00.635	0.50 ± 0.04	0.87 ± 0.08	1.76 ± 0.13	2.41 ± 0.19	1.69 ± 0.15	0.70 ± 0.05	CMZ
G003.338+00.419	0.60 ± 0.02	0.60 ± 0.02	1.01 ± 0.03	2.89 ± 0.11	1.14 ± 0.05	0.39 ± 0.01	CMZ
G359.445−00.054	3.47 ± 0.24	2.65 ± 0.14	0.77 ± 0.06	17.77 ± 1.18	7.12 ± 0.36	0.40 ± 0.03	CMZ
G359.453−00.112	2.01 ± 0.11	1.25 ± 0.05	0.62 ± 0.04	10.24 ± 0.55	3.32 ± 0.14	0.32 ± 0.02	CMZ
G359.565−00.161	2.24 ± 0.18	1.73 ± 0.10	0.77 ± 0.07	11.47 ± 0.90	4.68 ± 0.27	0.41 ± 0.04	CMZ
G359.868−00.085	1.47 ± 0.03	1.03 ± 0.01	0.70 ± 0.01	7.48 ± 0.13	2.66 ± 0.03	0.36 ± 0.01	CMZ
G359.895−00.069	1.29 ± 0.01	0.91 ± 0.01	0.70 ± 0.01	6.56 ± 0.07	2.33 ± 0.02	0.36 ± 0.01	CMZ
G359.977−00.072	0.78 ± 0.02	0.57 ± 0.01	0.73 ± 0.01	4.03 ± 0.08	1.64 ± 0.03	0.41 ± 0.01	CMZ
G008.671−00.357	1.02 ± 0.10	0.47 ± 0.03	0.46 ± 0.04	5.14 ± 0.48	1.17 ± 0.08	0.23 ± 0.02	Bubble
G010.473+00.028	0.96 ± 0.12	0.70 ± 0.06	0.73 ± 0.09	4.87 ± 0.56	1.85 ± 0.15	0.38 ± 0.04	Bubble
G322.159+00.635	0.20 ± 0.03	0.14 ± 0.02	0.72 ± 0.03	1.06 ± 0.14	0.50 ± 0.06	0.47 ± 0.02	Bubble
G326.653+00.618	0.73 ± 0.08	0.60 ± 0.05	0.83 ± 0.06	3.69 ± 0.38	1.56 ± 0.13	0.42 ± 0.03	Bubble
G327.293−00.579	0.44 ± 0.04	0.19 ± 0.01	0.44 ± 0.02	2.32 ± 0.19	0.57 ± 0.04	0.25 ± 0.01	Bubble
G345.004−00.224	0.29 ± 0.02	0.32 ± 0.03	1.13 ± 0.06	1.48 ± 0.13	0.92 ± 0.07	0.62 ± 0.03	Bubble
G350.101+00.083	0.87 ± 0.09	1.02 ± 0.10	1.17 ± 0.12	4.48 ± 0.47	2.81 ± 0.28	0.63 ± 0.07	Bubble
G351.443+00.659	0.27 ± 0.01	0.18 ± 0.01	0.67 ± 0.02	1.38 ± 0.08	0.49 ± 0.02	0.35 ± 0.01	Bubble
G351.582−00.352	1.44 ± 0.16	0.71 ± 0.04	0.49 ± 0.05	7.58 ± 0.79	2.25 ± 0.14	0.30 ± 0.03	Bubble
G351.775−00.537	0.18 ± 0.01	0.36 ± 0.03	1.98 ± 0.08	1.01 ± 0.08	1.37 ± 0.11	1.36 ± 0.05	Bubble
G329.030−00.202	0.39 ± 0.03	0.33 ± 0.03	0.84 ± 0.04	2.00 ± 0.17	0.86 ± 0.07	0.43 ± 0.02	NMSFR
G331.708+00.583	0.43 ± 0.07	0.45 ± 0.07	1.04 ± 0.12	2.15 ± 0.33	1.02 ± 0.15	0.48 ± 0.05	NMSFR
G331.709+00.602	1.24 ± 0.19	0.91 ± 0.10	0.73 ± 0.11	6.25 ± 0.90	2.15 ± 0.24	0.34 ± 0.04	NMSFR
G335.586−00.289	0.46 ± 0.05	0.37 ± 0.03	0.79 ± 0.05	2.40 ± 0.24	1.07 ± 0.10	0.45 ± 0.03	NMSFR
G348.754−00.941	1.14 ± 0.16	0.76 ± 0.08	0.67 ± 0.08	5.65 ± 0.77	1.68 ± 0.18	0.30 ± 0.03	NMSFR
G351.157+00.701	0.81 ± 0.10	0.22 ± 0.02	0.27 ± 0.02	4.24 ± 0.48	0.68 ± 0.06	0.16 ± 0.01	NMSFR

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Table 4. Statistics of Integrated Intensity and Abundance Ratios

All sources
Quantity	Mean	Std a	Median	Min.	Max.
IHNCO/ISiO	1.36	1.07	1.14	0.18	4.48
${I}_{\mathrm{HNCO}}/{I}_{{\mathrm{HC}}_{3}{\rm{N}}}$	1.07	0.77	0.90	0.14	3.51
${I}_{\mathrm{SiO}}/{I}_{{\mathrm{HC}}_{3}{\rm{N}}}$	0.92	0.46	0.78	0.27	2.40
NHNCO/NSiO	6.82	5.33	5.65	1.01	21.79
${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$	2.58	1.70	2.15	0.49	7.25
${N}_{\mathrm{SiO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$	0.45	0.24	0.39	0.16	1.36
CMZ sources
Quantity	Mean	Std a	Median	Min.	Max.
IHNCO/ISiO	1.76	1.15	1.45	0.37	4.48
${I}_{\mathrm{HNCO}}/{I}_{{\mathrm{HC}}_{3}{\rm{N}}}$	1.42	0.75	1.25	0.57	3.51
${I}_{\mathrm{SiO}}/{I}_{{\mathrm{HC}}_{3}{\rm{N}}}$	0.98	0.49	0.78	0.50	2.40
NHNCO/NSiO	8.79	5.71	7.38	1.86	21.79
${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$	3.33	1.69	3.05	1.14	7.25
${N}_{\mathrm{SiO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$	0.46	0.22	0.39	0.21	1.08
Bubble sources
Quantity	Mean	Std a	Median	Min.	Max.
IHNCO/ISiO	0.64	0.43	0.59	0.18	1.44
${I}_{\mathrm{HNCO}}/{I}_{{\mathrm{HC}}_{3}{\rm{N}}}$	0.47	0.29	0.42	0.14	1.02
${I}_{\mathrm{SiO}}/{I}_{{\mathrm{HC}}_{3}{\rm{N}}}$	0.86	0.47	0.73	0.44	1.98
NHNCO/NSiO	3.30	2.21	3.00	1.01	7.58
${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$	1.35	0.78	1.27	0.49	2.81
${N}_{\mathrm{SiO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$	0.50	0.33	0.40	0.23	1.36
NMSFR sources
Quantity	Mean	Std a	Median	Min.	Max.
IHNCO/ISiO	0.75	0.38	0.64	0.39	1.24
${I}_{\mathrm{HNCO}}/{I}_{{\mathrm{HC}}_{3}{\rm{N}}}$	0.51	0.27	0.41	0.22	0.91
${I}_{\mathrm{SiO}}/{I}_{{\mathrm{HC}}_{3}{\rm{N}}}$	0.72	0.25	0.76	0.27	1.04
NHNCO/NSiO	3.78	1.87	3.32	2.00	6.25
${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$	1.24	0.56	1.05	0.68	2.15
${N}_{\mathrm{SiO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$	0.36	0.12	0.39	0.16	0.48

Note.

^a Standard deviation of the mean.

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From the left panel of Figure 7, we deduce no correlation between the integrated intensities of I_HNCO/I_SiO and ${I}_{\mathrm{SiO}}/{I}_{{\mathrm{HC}}_{3}{\rm{N}}}$. It is interesting to find from Figures 7 and 8 that the ratios I_HNCO/I_SiO, ${I}_{\mathrm{HNCO}}/{I}_{{\mathrm{HC}}_{3}{\rm{N}}}$, N_HNCO/N_SiO, and ${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$ for Bubble sources present two separate groups, one of which (G322.159+00.635, G327.293−00.579, G345.004−00.224, G351.443+00.659, and G351.775−00.537) has smaller ratios than the NMSFR sources, while the other group (G008.671−00.357, G010.473+00.028, G326.653+00.618, G350.101+00.083, and G351.582−00.352) has higher ones. Inflow and outflow activities have been simultaneously detected in three (G345.004−00.224, G351.443+00.659, G351.775−00.537) out of five sources in the group with smaller ratio (see Section 5.4). This may indicate that the combined action of star formation activities and H ii region (bubble) feedback will enhance the abundances of SiO and HC₃N. The middle panel of Figure 7 plots I_HNCO/I_HC3N as a function of I_HNCO/I_SiO. A strong positive correlation is found, and the fitted linear relationship is of the form $\mathrm{log}({I}_{\mathrm{HNCO}}/{I}_{{\mathrm{HC}}_{3}{\rm{N}}})\,=(-0.08\pm 0.03)+(0.78\pm 0.08)\times \mathrm{log}({I}_{\mathrm{HNCO}}/{I}_{\mathrm{SiO}})$, with a Pearson's r = 0.84. This is due to the good linear correlation between I_SiO and ${I}_{{\mathrm{HC}}_{3}{\rm{N}}}$ shown in Figure 3. Moreover, the integrated intensity ratios of our three molecules, shown in the middle panel of Figure 7, increase from the Bubble sources with small ratios to the NMSFR sources, then to the Bubble sources with higher ratios and finally to the CMZ sources. The integrated intensity ratio ${I}_{\mathrm{HNCO}}/{I}_{{\mathrm{HC}}_{3}{\rm{N}}}$ shows no obvious correlation with ${I}_{\mathrm{SiO}}/{I}_{{\mathrm{HC}}_{3}{\rm{N}}}$ in the right panel of Figure 7. As shown in the left panel of Figure 8, there is a weak hint that the abundance ratio ${N}_{\mathrm{SiO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$ decreases with increasing N_HNCO/N_SiO. Instead, a high correlation coefficient, 0.84, is obtained for the abundance ratio ${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$ versus N_HNCO/N_SiO for the whole sample in the middle panel of Figure 8. The red dashed line indicates the least-squares fit expressed as $\mathrm{log}({N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}})=(-0.19\pm 0.06)+(0.72\,\pm 0.07)\times \mathrm{log}({N}_{\mathrm{HNCO}}/{N}_{\mathrm{SiO}})$ with r = 0.84. The abundance ratio ${N}_{\mathrm{SiO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$ shows no obvious trend as a function of ${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$.

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The average abundance ratios N_HNCO/N_SiO and ${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$ we find in the CMZ and the Bubble sources are 8.79 and 3.33, and 3.30 and 1.35, respectively. These are quite similar to those derived by Aladro et al. (2015) toward starburst galaxies. But their average ${N}_{\mathrm{SiO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$ abundance ratio is almost three times lower than the corresponding values in the CMZ. Interestingly, abundance ratios in the AGN NGC 1068 appear to be similar to those of the NMSFR sources. Beside the central region of our Galaxy we suggest that star formation regions having extended SiO and HNCO emissions in the Galactic disk can also be testbeds for studies of the chemistry of the interstellar medium in the nuclear regions of galaxies. It would be worthwhile to carry out high-sensitivity observations and to perform large sample statistics in the future.

We find that the mean I_HNCO/I_SiO ratios and their standard deviations obtained from the Bubble, NMSFR, and CMZ sources are 0.64 ± 0.43, 0.75 ± 0.38, and 1.76 ± 1.15, respectively. As already mentioned (Section 5.1), Kelly et al. (2017) found that the mean ratios I_HNCO/I_SiO are 0.35 ± 0.13 and 2.5 ± 1.3 in the heavily shocked East Knot and mildly shocked West Knot of NGC 1068, respectively. In the IRDC clumps studied by Miettinen (2014), the I_HNCO/I_SiO ratio was found to be 2.37 ± 0.74, comparable to our average value within the error limits toward the CMZ category. Moreover, the median abundance ratio N_HNCO/N_SiO, 6.47, found by Miettinen (2014), is very similar to 7.38 derived in this work toward the CMZ group of sources. This is explained by the fact that most of the clumps with simultaneously detected HNCO and SiO lines (by Miettinen 2014) are also located in the Galactic center region.

In order to characterize the integrated intensity ratios I_HNCO/I_SiO, ${I}_{\mathrm{HNCO}}/{I}_{{\mathrm{HC}}_{3}{\rm{N}}}$, ${I}_{{\mathrm{HC}}_{3}{\rm{N}}}/{I}_{\mathrm{SiO}}$ and their relationship with hydrogen column density and gas kinetic temperature, we have applied a RADEX analysis (van der Tak et al. 2007). We separately analyzed sources in the NMSFR, Bubble, and CMZ categories, using averaged HNCO line widths of 3.92, 5.61, and 17.82 km s⁻¹ , respectively. The corresponding input values for the HNCO column densities are 3.75 × 10¹³, 5.81 × 10¹³, and 4.37 × 10¹⁴ cm⁻² as derived from the results of Table 2. The corresponding column densities for SiO and HC₃N are 9.58 × 10¹² and 3.49 × 10¹³, 2.88 × 10¹³ and 5.81 × 10¹³, and 6.97 × 10¹³ and 1.60 × 10¹⁴ cm⁻². Our grid of models includes hydrogen number densities varying from 10³ cm⁻³ to 10⁶ cm⁻³ and temperatures varying from 10 K to 60 K. The results are displayed in Figure 9. In each column of Figure 9, we find that the integrated intensity ratios in different classes show similar trends. I_HNCO/I_SiO and ${I}_{\mathrm{HNCO}}/{I}_{{\mathrm{HC}}_{3}{\rm{N}}}$, shown in the left and middle column of Figure 9, are most sensitive to the hydrogen volume density n, while ${I}_{{\mathrm{HC}}_{3}{\rm{N}}}/{I}_{\mathrm{SiO}}$, shown in the right column of Figure 9, is more sensitive to the kinetic temperature T_kin.

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The Central Molecular Zone contains, for its limited volume, a large amount of molecular gas, existing under extreme physical conditions with respect to pressure and kinetic temperature (Ginsburg et al. 2016). From the Mopra telescope HNCO, SiO, and NH₃ mapping data toward the Central Molecular Zone of our Galaxy, Ott et al. (2014) found that there exists a distinct pattern of alternating strong SiO or HNCO lines. They suggested that the difference is due to the strength of the shocks. In order to quantitatively study this phenomenon, we plot the integrated intensity ratio I_HNCO/I_SiO (black squares) and abundance ratios N_HNCO/N_SiO (red points) for seven separate segments of Galactic longitude. The averaged I_HNCO/I_SiO indicated by cyan segments in Figure 10 from left to right are 2.51, 0.41, 1.29, 3.38, 1.31, and 2.57, respectively. The corresponding averaged N_HNCO/N_SiO ratios indicated by green segments are 12.29, 2.06, 6.46, 17.10, 6.66, and 13.16. This variation is compatible with alternating stronger and weaker shocks affected by the motion of clouds and the dynamics of the x₁ and x₂ orbits in the Central Molecular Zone (e.g., Dale et al. 2019), and is also compatible with the data from Ott et al. (2014).

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5.4. Origin of the SiO Emission: Outflow Activity versus H ii/SNR-induced Shocks

SiO is an excellent tracer of recent shock activity (e.g., active outflows). When Si is liberated from dust grains, it reacts with other species observable from the ground to form SiO within ∼10⁴ yr (Pineau des Forets et al. 1997). Widmann et al. (2016) indeed proposed that most detected SiO emission is likely to be a result of outflow activity. However, models and observations also suggest that SiO emission could be caused by the UV illumination of ices in PDRs (Schilke et al. 2001; Shepherd et al. 2004) or by the expansion of H ii regions or supernova remnants (SNRs; Cosentino et al. 2019, 2020), and not by an outflow. In Schilke et al. (2001), narrow SiO emission (average line width of 2.0 km s⁻¹) with abundances of the order of 10⁻¹¹–10⁻¹⁰ is produced directly by the photodesorption of a small fraction of silicon from the mantles of dust grains by the intense UV radiation field originating from the massive stars in the Trapezium. However, here we propose a different scenario: molecular gas affected by the expansion of an H ii region or an SNR could induce low-velocity shocks in the surrounding molecular environment that inject SiO into the gas phase. This is more in line with the scenario proposed by Cosentino et al. (2019) for spectrally narrow SiO emission (average line width of 1.6 km s⁻¹) detected toward IRDC G034.77−00.55, where the observed narrow SiO emission is produced by low-velocity shocks induced by the expansion of a supernova remnant. As star formation properties in the CMZ are complicated, here we focus on dynamical processes (i.e., inflow and outflow activities) in our Bubble and NMSFR sources.

To estimate the fraction of sources where SiO is produced by outflow activity, we use HCO⁺, which is a suitable species to trace both inflow and outflow activities by analyzing its line profile (Rawlings et al. 2004; Fuller et al. 2005; He et al. 2015, 2016). To identify HCO⁺ outflow features, we followed the method that was presented by Wu et al. (2005). HCO⁺ (1–0) spectra near the source position should have high-velocity wings, and position–velocity (P–V) diagrams should show the intensity of the wing emission decreasing smoothly toward the edge of the mapped region. Also, we have checked the optically thin H¹³CO⁺ 1–0 or HNCO 4₀₄–3₀₃ lines to avoid multiple velocity components causing wing emission. As a result, we identify four outflow candidates (G008.671−00.357, G345.004−00.224, G351.443+00.659, G351.775−00.537) from 10 sources in the Bubble category and four outflow candidates (G329.030−00.202, G331.708+00.583, G331.709+00.602, G335.586−00.289) from the six sources in the NMSFR category. The detection rates of outflow candidates in the Bubble and NMSFR categories are 40% and 67%, respectively. The difference between 40% and 67% is not significant in a statistical sense and may be an effect of sensitivity limitations of the Mopra observations. The wing emission is weak and it may not have been detected in all sources. Therefore, it is necessary to investigate the outflow detection rate on the basis of a larger sample and with higher sensitivity to check whether surrounding bubbles or cloud heating by shock waves can yield not only outflow activity but also an enhancement of SiO emission.

The mean SiO abundances in the Bubble and NMSFR categories are 9.37 × 10⁻¹¹ and 8.70 × 10⁻¹¹, respectively. In combination with the abundances derived by Schilke et al. (2001) from SiO 2–1 in the PDR of the Orion Bar, in the range (3−7) × 10⁻¹¹, this suggests that the pressure of photoionized gas from the surrounding bubbles or cloud heating by shock waves yields an enhancement of SiO emission for sources in the Bubble category besides the outflow activity. The HCO⁺ (1–0) P–V diagrams of these eight outflow candidates showing distinct wing emission are shown in Figures 11 and 12. Among these, G345.004-00.224 was identified by Yu & Wang (2014), while G008.671−00.357, G329.030−00.202, G331.708+00.583, G331.709+00.602, G335.586−00.289, G351.443+00.659, and G351.775−00.537 are newly identified in this work. Since the HCO⁺ spectrum in G345.004−00.224 exhibits an obvious self-absorption dip, the wide SiO 2–1 line emission has been added to show outflow activities in Figure 11. All the P–V diagrams were cut along the l direction in the Galactic coordinate system except for that of G351.443+00.659, which was cut along Galactic latitude where the HCO⁺ 1–0 wing emission is more pronounced.

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Ren et al. (2012) proposed that inflow and outflow motions should be closely related and interact with each other throughout the star formation process. We followed the inflow identification criterion used in He et al. (2015). Inflow motions have been studied by investigating the profile of the optically thick HCO⁺ 1–0 line, which would then show a double peak with a brighter blue peak or a skewed single blue peak. Meanwhile, the optically thin single peak of the H¹³CO⁺ 1–0 line should be located at the dip of the optically thick HCO⁺ 1–0 line to rule out a double peak caused by two velocity components along the line of sight. After checking the spatial variation of optically thick line profile asymmetries across the mapped region, we identified eight reliable inflow candidates. The extracted spectra of HCO⁺ 1–0 and H¹³CO⁺ 1–0 from the yellow cross of each source are shown in the lower panels of Figures 11 and 12. Five (G008.671−00.357, G331.708+00.583, G331.709+00.602, G335.586−00.289, and G351.443+00.659) of them have been identified as inflow candidates by He et al. (2015, 2016) and the other three (G329.030−00.202, G345.004−00.224, and G351.775−00.537) are newly identified in this work. Interestingly, all these inflow candidates have been detected to simultaneously show outflow activities. Therefore, our results are also consistent with the coexistence of inflowing and outflowing molecular gas.

We have performed a study of 43 southern star-forming regions in HNCO 4₀₄–3₀₃, SiO 2–1, and HC₃N 10–9, based on the MALT90 survey, the 870 μm ATLASGAL survey, and Herschel 160, 250, 350, and 500 μm data. Our sample was divided into three categories: 27 sources in the Central Molecular Zone of the Galaxy, 10 sources associated with expanding bubbles of ionized gas, and six "normal" star-forming clouds. The spatial distributions of the three measured molecular lines and their properties are analyzed. Integrated intensity ratios, abundance ratios, outflow activity, and inflow activity are discussed. Our main results can be summarized as follows.

• (i)  
  The integrated intensity images show that the distributions of all three molecular lines are compact and consistent with condensed dust structures except for SiO in G000.110+00.148, G000.497+00.021, G001.610–00.172, G001.655–00.062, G001.694–00.385, and G003.240+00.635. The derived angular diameters of the three species indicate that HC₃N traces denser gas than HNCO and SiO.
• (ii)  
  The dust temperature images show that all the 27 CMZ sources are cold with  ∼14 K toward the central 870 μm continuum emission regions, while the dust is warmer in the more diffuse surrounding regions. For CMZ sources, the variations of I_HNCO/I_SiO and N_HNCO/N_SiO as a function of Galactic longitude are compatible with alternating stronger and weaker shocks affected by the motion of clouds and the dynamics of the x₁ and x₂ orbits in the CMZ.
• (iii)  
  From analyzing the center of each cold dense clump in Section 4, the dust temperature T_d and the abundance ratios N_HNCO/N_SiO and ${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$ show a decreasing trend with increasing column densities, while ${N}_{{\mathrm{HC}}_{3}{\rm{N}}}/{N}_{\mathrm{SiO}}$ reveals the opposite. This characteristic property appears to be most common for sources in the CMZ. In Bubble and NMSFR category sources, dust temperature and N_HC3N/N_SiO roughly follow a similar trend toward the center, where stars are forming.
• (iv)  
  The line widths of HNCO and HC₃N correlate well with each other and ratios are close to unity for the 43 star-forming regions. SiO lines tend to be wider than lines from HNCO and HC₃N, apparently being most sensitive to the presence of outflowing gas. HC₃N provides, on average, the most compact spatial distributions. SiO and HC₃N may have similar excitation mechanisms, as is suggested by the good correlation between line widths, integrated intensities, column densities, and fractional abundances of these two species, though the critical density of SiO is about four times higher than that of HC₃N at 20 K.
• (v)  
  Eight star-forming sources—G008.671−00.357, G345.004−00.224, G351.443+00.659, and G351.775−00.537 in the Bubble category, and G329.030−00.202, G331.708+00.583, G331.709+00.602, and G335.586−00.289 in the NMSFR category (see Section 2.2)—are found to have simultaneous outflow and inflow motions. And seven new outflow candidates (G008.671−00.357, G329.030−00.202, G331.708+00.583, G331.709+00.602, G335.586−00.289, G351.443+00.659, and G351.775−00.537) and three new inflow candidates (G329.030−00.202, G345.004−00.224, and G351.775−00.537) have been identified in this work.

We thank the referee for careful comments on this paper. This work was mainly funded by the National Natural Science foundation of China (NSFC) under grant 11703073. It was also partially funded by the NSFC under grants 11433008, 11973076, 11703074, and 11603063, the CAS "Light of West China" Program 2016-QNXZ-B-22, 2018-XBQNXZ-B-024, the "TianShan Youth Plan" under grant 2018Q084, and the Heaven Lake Hundred-Talent Program of Xinjiang Uygur Autonomous Region of China. C. Henkel has been funded by Chinese Academy of Sciences President's International Fellowship Initiative with grant No. 2021VMA0009. Moreover, 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. A.S. acknowledges funding through Fondecyt Regular (project code 1180350) and Chilean Centro de Excelencia en Astrofísica y Tecnologías Afines (CATA) BASAL grant AFB-170002.

This research has made use of the data products from the MALT90 survey, the SIMBAD database, operated at CDS, Strasbourg, France, the data from Herschel, a European Space Agency space observatory with science instruments provided by European-led consortia, and the ATLASGAL survey, which is a collaboration between the Max-Planck-Gesellschaft, the European Southern Observatory (ESO), and the Universidad de Chile.

Software: GILDAS/CLASS (Pety 2005; Gildas Team 2013), Matplotlib (Hunter 2007), astropy (Astropy Collaboration et al. 2013).

Below we give comments on individual sources. Figures A1–A14, A15–A19, and A20–A22 show the CMZ, Bubble, and NMSFR samples, respectively. It should be understood that in the following the four cardinal directions, north, south, east, and west, are meant with respect to the Galactic coordinates (l^II, b^II), and not with respect to R.A. and decl.

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G359.977−00.072 and G359.895−00.069. G359.977−00.072 is well known under the name +50 km s⁻¹ cloud. For the spectral line maps of G359.977−00.072 and G359.895−00.069 shown in Figure A1, there are single cores toward the region of lower dust temperature, but these have a slight offset from the maximum of the dust continuum emission. The SiO spectra in G359.977−00.072 are broader than those of HNCO and HC₃N. The abundance ratio ${N}_{{\mathrm{HC}}_{3}{\rm{N}}}/{N}_{\mathrm{SiO}}$ in these two sources shows a reverse trend to that of the dust temperature.

G359.868−00.085. From the top panel of Figure A2, it can be seen that there is no obvious molecular emission core at the centrally located brightest clump, which is associated with water masers (Sjouwerman et al. 2002). The SiO and HC₃N molecular emissions clearly show two cores on each side of the central clump. The abundance ratios ${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$ and ${N}_{{\mathrm{HC}}_{3}{\rm{N}}}/{N}_{\mathrm{SiO}}$ both show a similar trend to the dust temperature.

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G359.565−00.161. Molecular line emissions show similar morphologies in the lower panels of Figure A2. Weak molecular line emission is found toward the northeastern region of high dust temperature. All molecular line peaks have an offset from the maximum of the 870 μm dust continuum emission. The SiO 2–1 data are noisy. Near the yellow cross position, ${N}_{{\mathrm{HC}}_{3}{\rm{N}}}/{N}_{\mathrm{SiO}}$ reaches maxima, while N_HNCO/N_SiO and ${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$ and the dust temperature are low.

G359.453−00.112. As shown in the top panel of Figure A3, SiO, HNCO, and HC₃N exhibit a single core elongated from northwest to southeast in the velocity-integrated intensity maps, consistent with the cometary shape of the low-temperature dust continuum emission. In the northwestern high-temperature corner there is the H ii region G359.4−0.1 (i.e., Sgr C) (Kuchar & Clark 1997). An additional velocity component can be distinguished at the eastern side of the main component in all spectra. The ${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$ abundance ratio (green lines) is closely related to changes in the dust temperature.

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G359.445−00.054. G359.445−00.054 is part of the Sgr C Filament (Linden et al. 2011). We find one isolated core in HNCO and HC₃N and two compact cores in the SiO line map. Moreover, the cores are slightly offset from the maximum of dust continuum emission as shown in the lower panels of Figure A3. In the case of the SiO 2–1 emission, the structure is complicated, possibly because of low signal-to-noise ratios. There are two obviously saturated areas in the Hi-GAL data (visible in gray in the image shown in the lower left panel of G359.445−00.054), where the bubble MWP1G359450−000200S (Simpson et al. 2012) and the H ii region Gal 359.43−00.09 (Giveon et al. 2002) are found to be located on either side of the low-temperature filamentary structure of the dust. In addition, the velocity range appears to be widest for the HNCO 4₀₄–3₀₃ spectrum. With respect to temperature variations, the changes in abundance ratios are similar to source G001.734−00.410. Near the yellow cross position, ${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$ and dust temperature reach minima, while N_HNCO/N_SiO and ${N}_{{\mathrm{HC}}_{3}{\rm{N}}}/{N}_{\mathrm{SiO}}$ are high.

G000.067−00.077. As seen in the top panels of Figure A4, the emissions of the HNCO 4₀₄–3₀₃, SiO 2–1, and HC₃N 10–9 lines show a similar morphology with a centrally condensed structure. The emission peaks coincide with the 870 μm continuum emission. At this position, the dust temperature is relatively low. In addition, there are two regions of higher dust temperature, to the northeast and southwest of the 870 μm emission peak. The HC₃N spectra have a simple Gaussian shape. However, the HNCO and SiO lines toward the yellow cross position show non-Gaussian wing emission, indicative of outflows/shocks. HNCO and SiO are each fitted with two Gaussians. N_HNCO/N_SiO and ${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$ show a decreasing trend toward the central region of lower dust temperature.

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G000.104−00.080. The integrated intensity maps of the HNCO 4₀₄–3₀₃, SiO 2–1, and HC₃N 10–9 lines show a similar morphology, and coincide well with the ATLASGAL 870 μm emission where the dust temperature is relatively low. As in G000.067–00.077, two regions of higher dust temperature are located in the northeast and southwest of the source. The HNCO, SiO, and HC₃N lines all show an asymmetry and possess wing emission. The ${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$ and dust temperature curves are similar toward the central region in the direction of Galactic longitude shown in the last panel of Figure A4.

G000.106−00.001. All of the molecular line emissions have an offset of ∼30'' from the peak of the ATLASGAL 870 μm continuum emission and show a large gradient toward the northwestern region of higher dust temperature, which might be caused by external pressure from the arched filament H ii complex (Lang et al. 2001). The SiO integrated intensity (white contours in the lower left panel of the upper half of Figure A5) and cold dust structures have a similar morphology. HNCO and HC₃N show a line wing on the redshifted side, and are each fitted with two Gaussians. In the central part of G000.106−00.001, ${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$ and dust temperature curves are similar.

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G000.110+00.148. From the lower panels of Figure A5, HC₃N and HNCO emission peaks toward the region of lower dust temperature, but is offset to the northeast of the maximum of the 870 μm dust continuum emission (∼36''). The structure is complex in the case of SiO 2–1, perhaps because of low signal-to-noise ratios. A region of high dust temperature appears in the lower left corner. The HNCO 4₀₄–3₀₃ line shows a single Gaussian shape, while SiO 2–1 also shows wing emission.

G000.314−00.100. The upper panels of Figure A6 demonstrate that the HNCO 4₀₄–3₀₃ morphology shows a centrally condensed structure, with the emission peak coinciding with the 870 μm emission, which shows low dust temperatures. The SiO 2–1 and HC₃N 10–9 emissions also have a centrally condensed structure and show similar distributions, but the emission peaks are offset (∼20'') from those of the HNCO 4₀₄–3₀₃ and 870 μm emission. The spectral profiles have low signal-to-noise ratios in the HNCO 4₀₄–3₀₃ and SiO 2–1 lines. HNCO, SiO, and HC₃N are each fitted with two Gaussians. Abundance ratios ${N}_{{\mathrm{HC}}_{3}{\rm{N}}}/{N}_{\mathrm{SiO}}$ show an increasing trend with respect to N_HNCO/N_SiO and ${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$ toward the central region of low dust temperature.

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G000.497+00.021 and G000.645+00.027. From the lower panels of Figure A6 and the upper panels of Figure A7, the SiO emission shows a complex morphology in the observed area, while the other two molecular emission lines reveal compact cores in the integrated intensity map. In addition, extended SiO emission is seen particularly around the central core of HNCO emission in source G000.497+00.021. As in G000.314−00.100, the dust emission regions are cold, and the abundance ratio ${N}_{{\mathrm{HC}}_{3}{\rm{N}}}/{N}_{\mathrm{SiO}}$ shows an increasing trend toward the central region.

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G000.892+00.143. The lower panels of Figure A7 show a pronounced core elongated from the southeast to the northwest in the three studied molecular lines. Compared to the southeastern 870 μm emission peak, the northwestern one shows a higher temperature and coincides with the local HNCO 4₀₄–3₀₃, SiO 2–1, and HC₃N 10–9 peak positions. The SiO and HC₃N spectra exhibit asymmetric profiles. Abundance ratios N_HNCO/N_SiO and ${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$ decrease in the direction of Galactic longitude toward the central hot dust core, while ${N}_{{\mathrm{HC}}_{3}{\rm{N}}}/{N}_{\mathrm{SiO}}$ does the opposite.

G000.908+00.116. Toward G000.908+00.116, shown in the upper panels of Figure A8, HNCO 4₀₄–3₀₃ shows a dense clump in the central map, with emission peaking at a ∼29'' offset from the maximum of the dust continuum emission. However, the integrated intensity map of SiO 2–1 shows a more complex structure. HC₃N exhibits a simple morphology with a ridge elongated from the southeast to the northwest. The abundance ratios N_HNCO/N_SiO, ${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$, and ${N}_{{\mathrm{HC}}_{3}{\rm{N}}}/{N}_{\mathrm{SiO}}$ show no obvious trends.

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G001.226+00.095. The HC₃N emission peak has an offset from the continuum and SiO emission peak by about a beam (∼38'', Figure A8). In the dust temperature map the arc-like region of low dust temperature coincides with the HNCO emission. The SiO 2–1 line shows obvious redshifts of 8.2 and 4.1 km s⁻¹ relative to v_LSR of the HNCO 4₀₄–3₀₃ and HC₃N 10–9 lines. Abundance ratios ${N}_{{\mathrm{HC}}_{3}{\rm{N}}}/{N}_{\mathrm{SiO}}$ show an increasing trend and N_HNCO/N_SiO and ${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$ a decreasing trend toward the yellow cross position.

G001.344+00.258. A similarly centrally condensed structure is seen in HNCO, SiO, and HC₃N emission in the top panels of Figure A9. All molecular line emissions peak toward the low-temperature maximum of the 870 μm dust continuum emission. The spectra of HNCO 4₀₄–3₀₃, SiO 2–1, and HC₃N 10–9 show a single velocity component. The trend of ${N}_{{\mathrm{HC}}_{3}{\rm{N}}}/{N}_{\mathrm{SiO}}$ is opposite to that of the dust emission temperature.

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G001.381+00.201. From the lower panels of Figure A9, it can be seen that the integrated intensity maps of both HNCO and HC₃N show two compact cores. In addition, the upper compact cores of SiO, HNCO, and HC₃N do not coincide with each other. Moreover, compared to the southeastern core of HNCO and HC₃N, SiO emission shows more extended and complex structure. Abundance ratios and dust temperature appear not to be clearly correlated.

G001.510+00.155. In the top panels of Figure A10, the SiO, HNCO, and HC₃N emissions clearly show three, two, and one compact core(s), respectively. The HC₃N core is elongated from the southeast to the northwest, and coincides well with the region of low dust temperature. The SiO 2–1, HNCO 4₀₄–3₀₃, and HC₃N 10–9 lines can be fitted by a single Gaussian component. The curve of abundance ratio ${N}_{{\mathrm{HC}}_{3}{\rm{N}}}/{N}_{\mathrm{SiO}}$ shows an obvious plateau near the yellow cross position in the direction of Galactic latitude.

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G001.610−00.172 and G001.655−00.062. From Figures A10 and A11, it can be seen that both sources present condensed structures in the HNCO 4₀₄–3₀₃ and HC₃N 10–9 emissions in the region of low dust temperature. The peaks of HNCO 4₀₄–3₀₃ emission show offsets of ∼40'' and ∼55'' from the strongest 870 μm dust emission in G001.610−00.172 and G001.655−00.062, respectively. The SiO 2–1 emissions for these two sources all show complex structures. In addition, the SiO 2–1 line of G001.655−00.062 exhibits a double peak, while HNCO and HC₃N show a single velocity component at the dip of SiO 2–1. The curves of abundance ratios N_HNCO/N_SiO and ${N}_{{\mathrm{HC}}_{3}{\rm{N}}}/{N}_{\mathrm{SiO}}$ show the opposite trend to that of the dust emission temperature.

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G001.694−00.385. A north–south elongated and compact structure is shown in the HNCO 4₀₄–3₀₃ and HC₃N 10–9 emissions, which contains three cores (Figure A11). These coincide with the region of low-temperature 870 μm dust emission. The SiO emission shows an extended complex structure also associated with the region of higher dust temperature in the southeast. The spectrum of SiO 2–1 is noisy. Near the yellow cross position, ${N}_{{\mathrm{HC}}_{3}{\rm{N}}}/{N}_{\mathrm{SiO}}$ reaches maxima, while N_HNCO/N_SiO and ${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$ and the dust temperature are low.

G001.699−00.366. Two compact cores can be seen in the integrated intensity maps of HNCO 4₀₄–3₀₃ and HC₃N 10–9 in the top panels of Figure A12. This is consistent with the distributions of low-temperature dust emission. There are also two cores in the SiO 2–1 line map, but here the emission peaks are offset from the dust continuum emission peaks (∼35''). Both of the HNCO and HC₃N lines show a single Gaussian velocity component, but SiO shows two velocity components on either side of the systemic velocity of the source. ${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$ follows the trend of the dust temperature.

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G001.734−00.410 and G001.883−00.062. In Figures A12 and A13, our three main molecular tracers show similar morphologies and a centrally condensed structure. Moreover, obvious offsets of ∼15'' and ∼30'' between the molecular line emission peaks and the lower-temperature 870 μm dust continuum emission peaks are found in G001.734−00.410 and G001.883−00.062, respectively. All spectra show one Gaussian component. Near the yellow cross position, ${N}_{{\mathrm{HC}}_{3}{\rm{N}}}/{N}_{\mathrm{SiO}}$ reaches maxima, while N_HNCO/N_SiO and ${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$ and the dust temperature are low.

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G003.240+00.635. Extended emission can be seen in the integrated intensity maps of the SiO and HC₃N lines in the lower panels of Figure A13. In addition, HNCO has the most compact emission morphology, and coincides with the 870 μm emission peak, which shows the lowest dust temperature. The HNCO and SiO lines are Gaussian toward the yellow cross position. All three plotted abundance ratios follow the trend determined by the dust temperature.

G003.338+00.419. The top panels of Figure A14 show a compact core, which is seen in the 870 μm continuum and the HNCO and HC₃N emission. SiO shows an overall similar morphology to HC₃N but its distribution on smaller scales appears to be more complex. The SiO 2–1 line shows a skewed profile with a blueshifted peak that is weaker than the redshifted one. Near the yellow cross position, ${N}_{{\mathrm{HC}}_{3}{\rm{N}}}/{N}_{\mathrm{SiO}}$ reaches maxima, while N_HNCO/N_SiO and ${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$ and the dust temperature are low.

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G008.671−00.357. Figure A15 shows a bubble, CN129, nearby, in the northwest of the presented maps of integrated line emission. This corresponds to a hot dust core, which is close to a class II CH₃OH maser (red diamond shown in the lower left panel of Figure A15) (Caswell et al. 1993). The H₂O maser (red asterisk shown in the same panel) lies in between the two dust emission cores, which indicates a possible interaction. The three molecular lines all show a morphology extending from northwest to southeast. Two peaks of HC₃N emission are seen toward the dust continuum emission. The trend of N_HNCO/N_SiO is opposite to that of the dust temperature in the direction of Galactic latitude. Inflow and outflow activities are simultaneously detected in this source from analyzing molecular line profiles of HCO⁺ 1–0 (see Section 5.4 and Figure 11 for details).

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G010.473+00.028. From the lower panels of Figure A15, it can be seen that the molecular line emissions exhibit centrally condensed structure. SiO and HNCO emissions peak toward the faint dust continuum emission core, which contains OH (Caswell 1998) and class II CH₃OH masers (Caswell et al. 1993; Pestalozzi et al. 2005), while HC₃N emission peaks toward the maximum of hot dust continuum emission. There is a bubble, CN151, close to the peak of the HC₃N emission, and an OH maser between them. Because of the low signal-to-noise ratio of the SiO 2–1 data, the integrated intensity is calculated in the wide velocity range between 55.54 and 84.44 km s⁻¹. ${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$ increases with increasing Galactic longitude, while ${N}_{{{\rm{HC}}}_{3}{\rm{N}}}/{N}_{{\rm{SiO}}}$ follows the opposite trend.

G322.159+00.635. It is identified as an IRDC of SDC 322.149+0.639 by Peretto & Fuller (2009). As Figure A16 shows, the map consists of two hot molecular cores embedded in bubble MWP1G322154+006351 identified by Simpson et al. (2012). The northwestern molecular core contains OH (Caswell & Haynes 1987) and CH₃OH (Caswell et al. 1993) masers, while the southeastern molecular core is close to another OH maser, which lies in between two dust cores. All three molecular lines show a single velocity component. Abundance ratios N_HNCO/N_SiO and ${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$ show similarly varying tendencies.

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G326.653+00.618. In the lower panels of Figure A16, there is an obvious dust temperature gradient decreasing from the southeast to the northwest. Furthermore, there is a bubble, S79, to the southeast. The peaks of molecular line emission have a significant offset of ∼30'' from the peak of the southeastern hot dust clump, which might be caused by the shock between the bubble and the molecular clouds. The SiO spectrum from the maximum of dust continuum emission presents an additional velocity component at ∼−30 km s⁻¹. All these lines present two cores located on either side of the brightest dust peak, which also hosts an H₂O maser (Batchelor et al. 1980). Abundance ratios and dust temperature appear not to be clearly correlated.

G327.293−00.579. G327.293−00.579 is identified as an IRDC, named SDC 327.306−0.566 by Peretto & Fuller (2009). As Figure A17 shows, the spectral line maps reveal an isolated core. The peak molecular line emissions are slightly offset by ∼20'' from the maximum of the dust continuum emission, which shows a 12 GHz CH₃OH maser (Caswell et al. 1995a) close to the bubble, MWP1G327281−005629. Because of many saturated pixels in the dust temperature map, we just show integrated intensity ratios ${I}_{\mathrm{HNCO}}/{I}_{{\mathrm{HC}}_{3}{\rm{N}}}$, I_HNCO/I_SiO, and ${I}_{{\mathrm{HC}}_{3}{\rm{N}}}/{I}_{\mathrm{SiO}}$ and find that they exhibit similar trends.

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G345.004−00.224. G345.004−00.224 is identified as an IRDC, named SDC 345.000−0.232 by Peretto & Fuller (2009). As shown in the lower panels of Figure A17, molecular line emissions show a similar, centrally condensed structure. The separation between the bubble centroid and dust emission peak is ∼160'', corresponding to a projected distance of ∼1 pc at a kinematic distance of 1.4 kpc (Urquhart et al. 2018). SiO and HC₃N emissions peak toward the hot dust clump, which contains H₂O (Caswell et al. 1983), 12 GHz CH₃OH (Caswell et al. 1995a), and OH (Caswell & Haynes 1983) masers, while the HNCO emission peak has an offset of ∼20'' from the maximum of the dust continuum emission. Moreover, the SiO line shows non-Gaussian line wing emission, which indicates outflow activities. Abundance ratios N_HNCO/N_SiO and ${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$ increase away from the central hot clump. Inflow and outflow activities are simultaneously detected in this source from analyzing the molecular line profile of HCO⁺ 1–0 and SiO 2–1, respectively (see Section 5.4 and Figure 11 for details).

G350.101+00.083. G350.101+00.083 is identified as an IRDC, named SDC 350.117+0.096 by Peretto & Fuller (2009). In the top panels of Figure A18, the SiO, HNCO, and HC₃N molecular emissions show similar morphologies, with extended structure associated well with the hot 870 μm, OH maser (Caswell & Haynes 1983) and water vapor maser emission (Caswell et al. 1983). There are three bubbles—MWP1G350105+000838, MWP1G350107+000852, which is a smaller bubble inside the area of MWP1G350105+000838, and CS112 (Simpson et al. 2012)—located southwest and south of the molecular emission peak. The SiO 2–1 spectrum shows a low signal-to-noise ratio. The curves of abundance ratios N_HNCO/N_SiO and ${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$ show the opposite trend to that of the dust emission temperature.

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G351.443+00.659. In the lower panels of Figure A18, the integrated intensity maps of the SiO 2–1, HNCO 4₀₄–3₀₃, and HC₃N 10–9 lines all show a similar, centrally condensed structure. The peak molecular line emissions are obviously offset from the maximum of dust continuum emission by ∼15'', ∼30'', and ∼15'' for SiO, HNCO, and HC₃N, respectively, which might be caused by the shocks from bubbles MWP1G351426+006596 and MWP1G351418+006431. The SiO 2–1 line shows broad wings. Abundance ratios ${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$ and N_HNCO/N_SiO show the opposite trend with respect to the dust temperature in the direction of Galactic latitude. Inflow and outflow activities are simultaneously detected in this source from analyzing the molecular line profiles of HCO⁺ 1–0 (see Section 5.4 and Figure 11 for details).

G351.582−00.352. In the top panels of Figure A19, the three molecular emissions show similar morphologies with a single emission peak toward the hot dust clump, which contains H₂O (Caswell et al. 1983), class II CH₃OH (Caswell et al. 2010), and OH (Caswell & Haynes 1983) masers. A bubble, MWP1G351585−003560, identified by Simpson et al. (2012) is close to the maximum of dust continuum emission. The velocity range of the SiO line is the largest. ${I}_{{\mathrm{HC}}_{3}{\rm{N}}}/{I}_{\mathrm{SiO}}$ has an obvious maximum toward the northeast of the molecular emission peak.

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G351.775−00.537. In the lower panels of Figure A19, the three molecular lines show a similar morphology with emission peaks near the bubble MWP1G351780−005500S. The peak molecular line emission of HC₃N is offset by ∼30'' from the maximum of hot continuum emission. The SiO 2–1 line profile shows a particularly wide velocity range. The trend of the ${I}_{\mathrm{HNCO}}/{I}_{{\mathrm{HC}}_{3}{\rm{N}}}$ ratio is opposite to that of ${I}_{{\mathrm{HC}}_{3}{\rm{N}}}/{I}_{\mathrm{SiO}}$. Inflow and outflow activities are simultaneously detected in this source from analyzing molecular line profiles of HCO⁺ 1–0 (see Section 5.4 and Figure 11 for details).

G329.030−00.202. G329.030−00.202 is identified as an IRDC, named SDC 329.028−0.202 by Peretto & Fuller (2009). As seen in Figure A20, the morphologies of our three molecular lines are consistent with that of the 870 μm hot dust emission. The source contains 12 GHz CH₃OH (Caswell et al. 1995a) and H₂O (Scalise et al. 1989) masers. The SiO 2–1 line may show a red-skewed profile with the redshifted peak being stronger than the blueshifted peak. HNCO and HC₃N lines show a single velocity component peaking near the dip of the SiO 2–1 line. In addition, we note that the SiO line profile shows wing emission. The abundance ratios N_HNCO/N_SiO and ${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$ show an obvious decrease toward the hot dense core, while ${N}_{{\mathrm{HC}}_{3}{\rm{N}}}/{N}_{\mathrm{SiO}}$ does the opposite. Moreover, inflow and outflow activities are simultaneously detected in this source from analyzing molecular line profiles of HCO⁺ 1–0 (see Section 5.4 and Figure 12 for details).

G331.709+00.602 & G331.708+00.583. From the lower panels of Figure A20, it can be seen that the three molecular line emissions show similar morphologies, and are consistent with two hot dust clumps, G331.709+00.602 and G331.708+00.583, which are indicated by a pink cross and a yellow cross, respectively. N_HNCO/N_SiO and ${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$ have a tendency to decrease with dust temperature toward the inner hot and dense region. Inflow and outflow activities are simultaneously detected in these two sources from analyzing molecular line profiles of HCO⁺ 1–0 (see Section 5.4 and Figure 12 for details).

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G335.586−00.289. G335.586−00.289 is identified as an IRDC, named SDC 335.579−0.292 by Peretto & Fuller (2009). As seen in the top panels of Figure A21, the emissions of the HNCO, SiO, and HC₃N lines display similar morphologies with a single core peaking at the hot 870 μm dust emission clump, which contains CH₃OH (Caswell et al. 1995b) and OH (Caswell 1998) masers. The SiO line profile shows a peak and a blue shoulder, but the HNCO and HC₃N profiles exhibit single Gaussian components. Similar to sources G331.709+00.602 and G331.708+00.583, N_HNCO/N_SiO and ${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$ show a decreasing trend toward the warmer inner region. Inflow and outflow activities are simultaneously detected in this source from analyzing molecular line profiles of HCO⁺ 1–0 (see Section 5.4 and Figure 12 for details).

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G348.754−00.941. G348.754−00.941 is identified as an IRDC, named SDC 348.765−0.924 by Peretto & Fuller (2009). The peaks of the HNCO, SiO, and HC₃N emission, showing a cometary distribution, are slightly offset from the maximum of low-temperature filamentary dust continuum emission (Figure A21). SiO emission is relatively weak in G348.754−00.941. Moreover, the trends of N_HNCO/N_SiO and ${N}_{\mathrm{HNCO}}/{N}_{{\mathrm{HC}}_{3}{\rm{N}}}$ are opposite to that of the dust temperature.

G351.157+00.701. In Figure A22, a northwest–southeast elongated structure is shown in the three molecular emissions, which contain one core, with emission peaks obviously offset from the maximum of dust continuum emission by ∼30'', ∼45'', and ∼25'' for SiO, HNCO, and HC₃N, respectively. SiO and HC₃N emission peaks toward H₂O (Caswell et al. 1983) and OH (Caswell 1998) masers, where the dust temperature is high. I_HNCO/I_SiO and ${I}_{{\mathrm{HC}}_{3}{\rm{N}}}/{I}_{\mathrm{SiO}}$ show a decreasing trend toward the yellow cross site.

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