Chao Ou

Water masers provided powerful probes of shocked gas and protostellar activity during the early stages of star formation. Using Very Large Array K -band observations of the 22 GHz water line, we report the first detections of water masers toward dense cores in five 70 μ m infrared dark clumps, providing evidence for active star formation at an early evolutionary stage. We detected 10 unresolved maser spots comprising 13 velocity components, with peak flux densities of 13–499 mJy and deconvolved FWHMs of 0.51–3.54 km s ⁻¹ . Eleven components had FWHMs below 1 km s ⁻¹ , while their velocity offsets from the systemic velocities traced by the NH ₃ (1, 1) main line ranged from 0.83 to 50.73 km s ⁻¹ . Four water maser spots were associated with 1.3 mm continuum emission, three of which were also associated with molecular outflows traced by CO 2–1 emission. No CO outflow signature was detected toward the remaining spots within the observed field of view, suggesting that water masers could trace deeply embedded protostellar activity whose outflows were not yet detectable in CO emission. For AGAL031, the nondetection of Stokes V emission toward the strongest maser component yields an upper limit on the line-of-sight magnetic field strength on the order of a few hundred mG. Our results showed that star formation was already active in 70 μ m infrared dark clumps and that 22 GHz water masers provided insights into early protostellar environments that remained difficult to probe with common star formation tracers.

We present an investigation of the gas kinematics and an assessment of the evolutionary stages of dense cores embedded in four infrared dark clouds (IRDCs) using the NH_3(1,1) and NH_3(2,2) lines obtained from the VLA and GBT observations. There is no 1.3 cm continuum emission counterpart in 1.3 mm continuum emission revealed by the SMA toward these IRDCs. The low production rate of N_ ̊m uv ∼ 10^ 44 s -1 for the Lyman continuum photons indicates that the four IRDCs are in very early evolutionary stages, where free-free emission is still absent. We identified 61 dense cores at the size scale of ∼0.1 pc using the inner satellite groups of ) increases with an increasing radial distance from the center of dense core within the inner 0.1,pc toward eight dense cores, indicating that the turbulence is likely dissipated toward the center of these dense cores. In addition, two dense cores in AGAL 031.024+00.262 and one core in AGAL 024.314+00.086 exhibit nearly sonic motions on a small scale of 0.01 pc. The weakening of nonthermal support against gravity suggests that these dense cores are close to gravitational collapse or are already undergoing collapse. Conversely, there are seven dense cores in AGAL031.024+00.262 displaying a decrease in line. Among them, 38 dense cores exhibit a single velocity component, while 23 dense cores show multiple velocity components. We find that the nonthermal velocity dispersion (σ_ non non with increasing radial distance from the center. By comparing this trend with the outflow revealed by CO J=2-1 line, we find that these cores are likely associated with embedded star formation activities. Higher angular resolution observations on subparsec scales are essential to reveal the transition from thermal- to nonthermal-dominated regions, especially in dense cores associated with multiple velocity components.

Context . Linear C ₄ H and cyclic c -C ₃ H ₂ , as small unsaturated hydrocarbons, are the key precursors to complex organic molecules and are critical components of the interstellar medium. However, observational constraints on the evolution of these molecules in late-stage massive star-forming regions remain scarce. Aims . We present on-the-fly mapping observations of C ₄ H 9–8 lines, c -C ₃ H ₂ 2–1, H ¹³ CO ⁺ 1–0, and H42 α toward a sample of 22 massive star-forming regions using the IRAM 30 m telescope. Our aim is to further explore the evolution of these carbon-chain molecules by combining observational results obtained in cold cores. Methods . We employed H ¹³ CO ⁺ 1–0 and H42 α as tracers to probe the positions of molecular cloud cores and ionised hydrogen regions (H II regions), respectively. One chemical model in particular, which includes gas, dust grain surface, and icy mantle phases for C ₄ H and c -C ₃ H ₂ molecules, was used to make comparisons with observed abundances. Results . From mapping observations targeting 31 regions across 22 sources, C ₄ H 9–8 ( J = 19/2–17/2) and C ₄ H 9–8 ( J = 17/2–15/2) were detected in only 17 regions, while H ¹³ CO ⁺ 1–0 and c -C ₃ H ₂ 2–1 were successfully detected in all 31 regions. We find that the emission of C ₄ H 9–8 and c -C ₃ H ₂ 2–1 is concentrated at the edges of H42 α emission regions. The C ₄ H/H ¹³ CO ⁺ and c -C ₃ H ₂ /H ¹³ CO ⁺ relative abundance ratios range from 0.17 to 1.77 (median ~0.57) and 1.42 to 6.69 (median ~4.19), respectively, with a median C ₄ H/ c -C ₃ H ₂ ratio of 0.13. By combining the observational results of cold cores, we find that C ₄ H/H ¹³ CO ⁺ and c -C ₃ H ₂ /H ¹³ CO ⁺ ratios show a strong decreasing trend as molecular cores evolve. Conclusions . The decreasing trends in C ₄ H/H ¹³ CO ⁺ and c -C ₃ H ₂ /H ¹³ CO ⁺ ratios imply that small unsaturated hydrocarbons can be consumed and converted into other organic molecules during the evolution of molecular cores. The spatial concentration of C ₄ H and c -C ₃ H ₂ emission at the edges of H42 α regions further supports their role as precursors in the chemical pathways that lead to complex organic molecules in the interstellar medium.

Kinetic temperature is a fundamental parameter in molecular clouds. Symmetric top molecules, such as NH ₃ and CH ₃ CCH, are often used as thermometers. However, at high temperatures, NH ₃ (2,2) can be collisionally excited to NH ₃ (2,1) and rapidly decay to NH ₃ (1,1), which can lead to an underestimation of the kinetic temperature when using rotation temperatures derived from NH ₃ (1,1) and NH ₃ (2,2). In contrast, CH ₃ CCH is a symmetric top molecule with lower critical densities of its rotational levels than those of NH ₃ , which can be thermalized close to the kinetic temperature at relatively low densities of about 10 ⁴ cm ⁻³ . To compare the rotation temperatures derived from NH ₃ (1,1) and (2,2) and CH ₃ CCH rotational levels in warm molecular gas, we used observational data toward 55 massive star-forming regions obtained with Yebes 40 m and TMRT 65 m. Our results show that rotation temperatures derived from NH ₃ (1,1) and (2,2) are systematically lower than those from CH ₃ CCH 5–4. This suggests that CH ₃ CCH rotational lines with the same J +1 → J quantum number may be a more reliable thermometer than NH ₃ (1,1) and (2,2) in warm molecular gas located in the surroundings of massive young stellar objects or, more generally, in massive star-forming regions.

The abundance ratios of $^{28}$Si, $^{29}$Si, and $^{30}$Si provide valuable constraints for Galactic Chemical Evolution models. We report simultaneous observations of the $^{28}$SiO, $^{29}$SiO, and $^{30}$SiO 4–3 lines toward 51 massive star-forming regions, conducted with the IRAM 30 m telescope. Detections were achieved in 46, 25, and 24 sources for $^{28}$SiO, $^{29}$SiO, and $^{30}$SiO, respectively. Based on the largest sample of silicon isotope ratios to date, the relationships between $^{28}$Si/$^{30}$Si, $^{28}$Si/$^{29}$Si, and $^{29}$Si/$^{30}$Si ratios, and Galactocentric distance ($D_{\mathrm{GC}}$), are presented. No clear trend between these ratios and ($D_{\mathrm{GC}}$) is found. On the other hand, the abundance ratios of $^{28}$Si/$^{30}$Si and $^{29}$Si/$^{30}$Si, show a positive correlation, which implies that nucleosynthesis of $^{29}$Si and $^{30}$Si is closely related.

Context. Deuterated molecules can be used to study the physical conditions and astro-chemical evolution of molecular clouds. Aims. Large-sample surveys for deuterated molecules are needed to understand the enhancement of deuterated molecules from diffuse molecular gas to cold cores. Methods. A single-pointing survey toward the 559 Planck cold clumps of the Early Cold Core Catalogue (ECC) was conducted using the Arizona Radio Observatory 12-meter telescope, focusing on the J = 1–0 transitions of DCO⁺ and DCN. The survey included observations of 309 cores for DCO⁺ and DCN 1–0 simultaneously, followed by 71 of these cores where DCO⁺ 1–0 was detected for H¹³CO⁺ and H¹³CN 1–0 simultaneously, aiming to determine the deuterated fraction (D_frac). Additionally, 250 cores were observed for DCO⁺, DCN, H¹³CO⁺, and H¹³CN 1–0 simultaneously. Results. DCO⁺ and DCN 1–0 were detected in 79 and 11 of the 309 sources, with detection rates of 25.6% and 3.6%, respectively. In the 250 sources that were observed for all four species, DCO⁺, DCN, H¹³CO⁺, and H¹³CN 1–0 were detected in 58, 9, 57, and 13 sources, with a detection rate of 23.2%, 3.6%, 22.8%, and 5.2%, respectively. The D_frac(HCO⁺) values in 112 sources range from 0.89% to 7.4%, with a median value of 3.1%, while D_frac(HCN) values in 11 sources range from 1.5% to 5.5%, with a median value of 2.3%. The line widths of the DCO⁺ and H¹³CO⁺ 1–0 detections are mostly within 1 km s⁻¹. Conclusions. The similarity in D_frac values between HCO+ and HCN indicates that the higher detection rate of DCO⁺ 1–0 compared with DCN 1–0 is due to the lower critical density of DCO⁺ 1–0. We suggest that the enhancement of DCO⁺ and DCN likely begins in the early diffuse stage of the molecular cloud and not during the cold-core formation stage.

Context. The abundance ratio of ¹⁴N/¹⁵N is, in principle, a powerful tool for tracing stellar nucleosynthesis. Aims. This work aims to measure and analyze (¹⁴N/¹⁵N) × (¹³C/¹²C) and ¹⁴N/¹⁵N abundance ratios in massive star-forming regions across a range of galactocentric distances to provide constraints on galactic chemical evolution (GCE) models. Methods. We present H¹³CN and HC¹⁵N J = 2–1 results toward 51massive star-forming regions obtained with the Institut de Radioastronomie Millimétrique (IRAM) 30 meter telescope. We used these results to derive (¹⁴N/¹⁵N) × (¹³C/¹²C) abundance ratios as well as ¹⁴N/¹⁵N ratios using the double isotope method. Results. We find an overall decreasing trend in the (¹⁴N/¹⁵N) × (¹³C/¹²C) abundance ratio and an increasing trend in the ¹⁴N/¹⁵N ratio with increasing galactocentric distance (D_GC), which provides a good constraint for the GCE model based on high signal to noise ratio measurements. While the predicted (¹⁴N/¹⁵N) × (¹³C/¹²C) ratios between 6 and 12 kpc determined using current GCE models are consistent with our observational results, the ratios from models for D_GC less than 6 kpc are significantly higher than the observational results, which indicates GCE models for ¹⁴N/¹⁵N and/or ¹³C/¹²C ratios need to be updated for at least this range.

Context. S-bearing molecules are powerful tools for determining the physical conditions inside a massive star-forming region. The abundances of S-bearing molecules, including H₂S, H₂CS, and HCS⁺, are highly dependent on physical and chemical changes, which means that they are good tracers of the evolutionary stage of massive star formation. Aims. We present observational results of H₂S 1₁₀-1₀₁, H₂³⁴S 1₁₀-1₀₁, H₂CS 5₁₄-4₁₄, HCS⁺ 4-3, SiO 4-3, HC₃N 19-18, and C¹⁸O 1-0 toward a sample of 51 late-stage massive star-forming regions, and study the relationships between H₂S, H₂ CS, HCS⁺, and SiO in hot cores. We discuss the chemical connections of these S-bearing molecules based on the relations between the relative abundances in our sources. Methods. H₂³⁴S 1₁₀-1₀₁, as the isotopic line of H₂S 1₁₀-1₀₁, was used to correct the optical depths ofH₂S 1₁₀-1₀₁. Beam-averaged column densities of all molecules were calculated, as were the abundances of H₂S, H₂CS, and HCS⁺ relative to H₂, which were derived from C¹⁸ O. Results from a chemical model that included gas, dust grain surface, and icy mantle phases, were compared with the observed abundances of H₂S, H₂CS, and HCS⁺ molecules. Results. H₂S 1₁₀-1₀₁, H₂³⁴S 1₁₀-1₀₁, H₂CS 5₁₄-4₁₄, HCS⁺ 4-3, andHC₃N 19-18 were detected in 50 of the 51 sources, SiO 4-3 was detected in 46 sources, and C¹⁸O 1-0 was detected in all sources. The Pearson correlation coefficients between H₂CS and HCS⁺ normalized by H₂ and H₂S are 0.94 and 0.87, respectively, and a tight linear relationship with a slope of 1.00 and 1.09 is found; this relationship is 0.77 and 0.98 between H₂S and H₂CS and 0.76 and 0.97 between H₂S and HCS⁺. The full widths at half maxima of H₂³⁴S 1₁₀-1₀₁, H₂CS 5₁₄-4₁₄, HCS⁺ 4-3, and HC₃N 19-18 in each source are similar to each other, which indicates that they may trace similar regions. By comparing the observed abundance with model results, we see that there is one possible time (2−3 × 10⁵ yr) a which each source in the model matches the measured abundances of H₂S, H₂CS, and HCS⁺. The abundances of HCS⁺, H₂CS, and H₂S increase with the SiO abundance in these sources, which implies that shock chemistry may be playing a large role. Conclusions. The close abundance relation of H₂S, H₂CS, and HCS⁺ and the similar line widths in observational results indicate that these three molecules could be chemically linked, with HCS⁺ and H₂CS the most correlated. The comparison of the observational results with chemical models shows that the abundances can be reproduced for almost all the sources at a specific time. The observational results, including the abundances in these sources need to be considered in further modeling of H₂S, H₂CS, and HCS⁺ in hot cores with shock chemistry.

Dense outflowing gas, traced by transitions of molecules with large dipole moment, is important for understanding mass loss and feedback of massive star formation. HCN 3-2 and HCO⁺ 3-2 are good tracers of dense outflowing molecular gas, which are closely related to active star formation. In this study, we present on-the-fly mapping observations of HCN 3-2 and HCO⁺ 3-2 toward a sample of 33 massive star-forming regions using the 10-m Submillimeter Telescope. With the spatial distribution of line wings of HCO⁺ 3-2 and HCN 3-2, outflows are detected in 25 sources, resulting in a detection rate of 76%. The optically thin H¹³CN and H¹³CO⁺ 3-2 lines are used to identify line wings as outflows and estimate core mass. The mass M _out, momentum P _out, kinetic energy E _K , force F _out, and mass-loss rate M ̇ out of outflow and core mass, are obtained for each source. A sublinear tight correlation is found between the mass of dense molecular outflow and core mass, with an index of ∼0.8 and a correlation coefficient of 0.88.

The ¹⁸O/¹⁷O abundance ratio is, in principle, a powerful tool to estimate the relative contributions of massive stars and low- to intermediate-mass stars to the chemical enrichment of galaxies. We present ¹⁸O/¹⁷O ratios derived from simultaneous observations of C¹⁸O and C¹⁷O 1-0 toward fifty-one massive star forming regions with the Institut de Radioastronomie Millimétrique (IRAM) 30 meter telescope. Simultaneous observations of HC¹⁸O⁺ 1-0 and HC¹⁷O⁺ 1-0 with the Yebes 40m telescope toward five sources from this sample were also done to test the consistency of ¹⁸O/¹⁷O ratios derived from different isotopic pairs. From our improved measurements, resulting in smaller errors than previous work in the literature, we obtain a clear trend of increasing ¹⁸O/¹⁷O ratio with increasing galactocentric distance (D_GC), which provides a significant constraint on Galactic chemical evolution (GCE) models. Current GCE models have to be improved in order to explain the observed C¹⁸O/C¹⁷O 1-0 gradient.