Kai Yang

Context. Molecular outflows are important signposts of star formation since they provide critical feedback to molecular clouds. Aims. We systematically investigate the dynamical and energetic characteristics of dense molecular outflows in 22 massive star-forming regions with high-precision parallax distances. Methods. We present on-the-fly mapping observations of HCO ⁺ , HNC, and their optically thin isotopologs H ¹³ CO ⁺ and HN ¹³ C ( J = 1-0) using the IRAM 30 m telescope aimed toward our sample targets. Results. Dense molecular outflows were identified through line-wing analysis, which yielded high detection rates of 81.8% for HCO ⁺ and 77.3% for HNC; most sources exhibit distinct bipolar morphologies. A robust scaling relation is established between the dense outflow mass ( M _out ) and the core mass ( M _core ), M _out ∝ M _core ^α . The HCO ⁺ tracer yields a sublinear index of α = 0.68, indicating that the efficiency of generating dense outflows relative to the total mass reservoir decreases at the high-mass end. In contrast, HNC exhibits a steeper slope ( α = 1.13), likely reflecting temperature-dependent abundance increases in massive star-forming regions. Conclusions. These results highlight the complex, multiphase nature of stellar feedback and underscore the necessity of sourcespecific chemical modeling of high-mass star-forming environments. Excitation analysis indicates that HCO ⁺ J=1-0 traces extended, cooler outflow gas due to its lower critical density, while HCO ⁺ J=3-2 traces warmer, denser gas near the protostar, which is consistent with the derived spatial separations between the red and blue peaks of the two transitions in the same sources.

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.

We present high-resolution (∼1000 au) 3 mm observations with the NOrthern Extended Millimeter Array toward the DR21 South Filament (DR21SF), aiming to reveal its internal fragmentation and search for deeply embedded star-forming activities. Both the continuum and molecular line emissions align well with the filament axis traced by the low-resolution (∼18″) column density map. The 3 mm continuum, CS (2–1), and HCO ⁺ (1−0) emissions reveal continuous and diffuse structures with measured full width half maximum widths of 0.054, 0.029, and 0.030 pc, respectively. In contrast, the H ¹³ CO ⁺ (1−0) emission appears more clumpy and localized. The non-thermal motion in the filament is predominantly subsonic to transonic. We detect 13 dense cores in NH ₂ D (1 ₁₁ −1 ₀₁ ), three of which coincide with continuum peaks; virial analysis suggests most are gravitationally bound. Using a friend-of-friend algorithm, we identify 32, 34, and 22 velocity-coherent fibers from the CS, HCO ⁺ , and H ¹³ CO ⁺ data, respectively. Compared to fibers traced by CS and HCO ⁺ , H ¹³ CO ⁺ fibers are more frequently associated with NH ₂ D cold cores and exhibit higher average mass-per-unit-length values. Differences among CS, HCO ⁺ , and H ¹³ CO ⁺ emissions likely arise from variations in effective critical densities. These results are consistent with a hierarchical structure, in which the 3.6 pc DR21SF contains velocity-coherent fibers and gravitationally bound dense cores.

We present high-resolution ($\sim$1000 AU) 3 mm observations with the NOrthern Extended Millimeter Array toward the DR21 South Filament, aiming to reveal its internal fragmentation and search for deeply embedded star-forming activities. Both the continuum and molecular line emissions align well with the filament axis traced by the low-resolution ($\sim$18$^{\prime\prime}$) column density map. The 3 mm continuum, CS (2$-$1), and HCO$^+$ (1$-$0) emissions reveal continuous and diffuse structures with measured FWHM widths of 0.054, 0.029, and 0.030 pc, respectively. In contrast, the H$^{13}$CO$^+$ (1$-$0) emission appears more clumpy and localized. The non-thermal motion in the filament is predominantly subsonic to transonic. We detect 13 dense cores in NH$_2$D (1$_{11}-1_{01}$), three of which coincide with continuum peaks; virial analysis suggests most are gravitationally bound. Using a friend-of-friend algorithm, we identify 32, 34, and 22 velocity-coherent fibers from the CS, HCO$^+$, and H$^{13}$CO$^+$ data, respectively. Compared to fibers traced by CS and HCO$^+$, H$^{13}$CO$^+$ fibers are more frequently associated with NH$_2$D cold cores and exhibit higher average mass-per-unit-length values. Differences among CS, HCO$^+$, and H$^{13}$CO$^+$ emissions likely arise from variations in effective critical densities. These results are consistent with a hierarchical structure, in which the 3.6-pc DR21SF contains velocity-coherent fibers and gravitationally bound dense cores.

The central molecular zone (CMZ) of our Galaxy exhibits widespread emission from SiO and various complex organic molecules (COMs), yet the exact origin of such emission is uncertain. Here we report the discovery of a unique class of long ($>$0.5 pc) and narrow ($<$0.03 pc) filaments in the emission of SiO 5$-$4 and eight additional molecular lines, including several COMs, in our ALMA 1.3 mm spectral line observations toward two massive molecular clouds in the CMZ, which we name as slim filaments. However, these filaments are not detected in the 1.3 mm continuum at the 5$σ$ level. Their line-of-sight velocities are coherent and inconsistent with being outflows. The column densities and relative abundances of the detected molecules are statistically similar to those in protostellar outflows but different from those in dense cores within the same clouds. Turbulent pressure in these filaments dominates over self gravity and leads to hydrostatic inequilibrium, indicating that they are a different class of objects than the dense gas filaments in dynamical equilibrium ubiquitously found in nearby molecular clouds. We argue that these newly detected slim filaments are associated with parsec-scale shocks, likely arising from dynamic interactions between shock waves and molecular clouds. The dissipation of the slim filaments may replenish SiO and COMs in the interstellar medium and lead to their widespread emission in the CMZ.

The central molecular zone (CMZ) of our Galaxy exhibits widespread emission from SiO and various complex organic molecules (COMs), yet the exact origin of such emission is uncertain. Here we report the discovery of a unique class of long (>0.5 pc) and narrow (σ level. Their line-of-sight velocities are coherent and inconsistent with being outflows. The column densities and relative abundances of the detected molecules are statistically similar to those in protostellar outflows but different from those in dense cores within the same clouds. Turbulent pressure in these filaments dominates over self gravity and leads to hydrostatic inequilibrium, indicating that they are a different class of objects than the dense gas filaments in dynamical equilibrium ubiquitously found in nearby molecular clouds. We argue that these newly detected slim filaments are associated with parsec-scale shocks, likely arising from dynamic interactions between shock waves and molecular clouds. The dissipation of the slim filaments may replenish SiO and COMs in the interstellar medium and lead to their widespread emission in the CMZ.

We present a 6.0 GHz excited-state OH maser survey toward 155 northern star formation regions utilizing the Shanghai Tianma Radio Telescope. In total, we detect 44 6.0 GHz OH masers, 8 of which are new detections (one at 6016 MHz, two at 6030 MHz, and five at 6035 MHz). Of these 44 detected 6.0 GHz OH masers, 13 sites exhibit 6030 MHz OH masers, all 44 sites show the 6035 MHz transition, one site (G009.620+0.194) has the only 6016 MHz OH maser, and one site (W3(OH)) shows the only 6049 MHz OH maser. The 6016 MHz OH maser is the first OH maser of this transition detected in star formation regions. Investigations of the association between 6030/6035 MHz OH masers with 1665 MHz ground-state OH, 6.7 GHz methanol, and 22 GHz water masers show that 1665 MHz OH masers are good indicators of 6.0 GHz OH masers (29% detection rate), which is consistent with previous results. The majority (more than 77%) of 6030 and 6035 MHz OH masers are associated with 6.7 GHz methanol and/or 22 GHz water masers. Comparison between our spectra with previous spectra, we find that only two sources remain fairly stable since their discoveries. We identify 80 Zeeman pairs in 32 6.0 GHz OH maser sites with typical magnetic field strengths of less than 10 mG. The magnetic field directions derived from these 32 maser sites with Zeeman pairs are consistent with previous work, which indicates that the 6.0 GHz OH masers may be potential tracers of the large-scale Galactic magnetic field.

Context.Theories and models have suggested that circumstellar disks could channel material to the central protostar, and resist star formation feedback. Our current knowledge of the picture and role of disks around massive protostars is unclear because the observational evidence of these circumstellar disks is limited.Aims.To investigate whether disk-mediated accretion is the primary mechanism in high-mass star formation, we have established a survey of a large sample of massive dense cores within a giant molecular cloud.Methods.We used high angular resolution (~1.8″) observations with SMA to study the dust emission and molecular line emission of about 50 massive dense cores in Cygnus-X. At a typical distance of 1.4 kpc for Cygnus-X, these massive dense cores are resolved into ~2000 au condensations. We combined the CO outflow emission and gas kinematics traced by several high-density tracers to search for disk candidates.Results.We extracted hundreds of dust condensations from the SMA 1.3 mm dust continuum emission. The CO data show bipolar or unipolar outflow signatures toward 49 dust condensations. Among them, only 27 sources are detected in dense gas tracers, which reveals the gas kinematics, and nine sources show evidence of rotating envelopes, suggesting the existence of embedded accretion disks. The position-velocity diagrams along the velocity gradient of all rotating condensations suggest that four condensations are possible to host Keplerian-like disks.Conclusions.A detailed investigation of the 27 sources detected in dense gas tracers suggests that the nine disk candidates are at earlier evolutionary stages compared to the remaining 18 sources. Non-detection of rotating disks in our sample may be due to several factors, including an unknown inclination angle of the rotation axis and an early evolutionary stage of the central source, and the latter could be important, considering that young and powerful outflows could confuse the observational evidence for rotation. The detection rate of disk candidates in our sample is 1/3, which confirms that disk accretion is a viable mechanism for high-mass star formation, although it may not be the only one.

Context. The SiO emissions are usually used to trace high-velocity outflow shocks in star-forming regions. However, several studies have found low-velocity and widespread SiO emissions not associated with outflows in molecular clouds.Aims. We aim to detect and characterize the SiO emissions in massive dense cores (MDCs), and explore the properties of the central sources of SiO emission.Methods. We present high-angular-resolution (~1.5″) observations of the SiO (5−4) line made with the Submillimeter Array towards a sample of 48 MDCs in the Cygnus-X star-forming complex. We studied the SiO emission structures, including their morphologies, kinematics, and energetics, and investigated their relationship with the evolution of the central sources.Results. The SiO (5−4) emission is detected in 16 out of 48 MDCs. We identify 14 bipolar and 18 unipolar SiO (5−4) outflows associated with 29 dust condensations. Most outflows (24 out of 32) are associated with excessSpitzer4.5 μm emissions. We also find diffuse low-velocity (Δv≤ 1.2 km s⁻¹) SiO (5−4) emission closely surrounding the dust condensations in two MDCs, and suggest that it may originate from decelerated outflow shocks or large-scale shocks from global cloud collapse.Conclusions. We find that the SMA SiO (5−4) emission in MDCs is mostly associated with outflows. Probably due to the relatively high excitation of SiO (5−4) compared to SiO (2−1) and due to the spatial filtering effect, we do not detect large-scale low-velocity SiO (5−4) emission, but detect more compact low-velocity emission in close proximity to the dust condensations. We group the sources into different evolutionary stages based on the infrared emission, radio continuum emission, and gas temperature properties of the outflow central sources, and find that the 24 μm luminosity tends to increase with evolution.