Thirty massive clumps associated with bright infrared sources were observed to detect the infall signatures and characterize infall properties in the envelope of the massive clumps by APEX telescope in CO(4-3) and C$^{17}$O(3-2) lines. Eighteen objec
ts have blue profile in CO(4-3) line with virial parameters less than 2, suggesting that global collapse is taking place in these massive clumps. The CO(4-3) lines were fitted by the two-layer model in order to obtain infall velocities and mass infall rates. Derived mass infall rates are from 10$^{-3}$ to 10$^{-1}$ M$_{odot}$yr$^{-1}$. A positive relationship between clump mass and infall rate appears to indicate that gravity plays a dominant role in the collapsing process. Higher luminosity clump has larger mass infall rate, implying that the clump with higher mass infall rate has higher star formation rate.
To deepen our understanding of the chemical properties of the Planck Galactic Cold Clump (PGCC) G168.72-15.48, we performed observations of nine molecular species, namely, ce{c-C3H}, ce{H2CO}, ce{HC5N}, ce{HC7N}, ce{SO}, ce{CCH}, ce{N2H+}, ce{CH3OH},
and ce{CH3CCH}, toward two dense cores in PGCC G168.72-15.48 using the Tianma Radio Telescope and Purple Mountain Observatory Telescope. We detected ce{c-C3H}, ce{H2CO}, ce{HC5N}, ce{N2H+}, ce{CCH}, and ce{CH3OH} in both G168-H1 and G168-H2 cores, whereas ce{HC7N} and ce{CH3CCH} were detected only in G168-H1 and SO was detected only in G168-H2. Mapping observations reveal that the ce{CCH}, ce{N2H+}, ce{CH3OH}, and ce{CH3CCH} emissions are well coupled with the dust emission in G168-H1. Additionally, ce{N2H+} exhibits an exceptionally weak emission in the denser and more evolved G168-H2 core, which may be attributed to the ce{N2H+} depletion. We suggest that the ce{N2H+} depletion in G168-H2 is dominated by ce{N2} depletion, rather than the destruction by CO. The local thermodynamic equilibrium calculations indicate that the carbon-chain molecules of ce{CCH}, ce{HC5N}, ce{HC7N}, and ce{CH3CCH} are more abundant in the younger G168-H1 core. We found that starless core G168-H1 may have the properties of cold dark clouds based on its abundances of carbon-chain molecules. While, the prestellar core G168-H2 exhibits lower carbon-chain molecular abundances than the general cold dark clouds. With our gas-grain astrochemical model calculations, we attribute the observed chemical differences between G168-H1 and G168-H2 to their different gas densities and different evolutionary stages.
Previous observations have revealed an accretion disk and outflow motion in high-mass star-forming region G192.16-3.84. While collapse have not been reported before. We present here molecular line and continuum observations toward massive core G192.1
6-3.84 with the Submillimeter Array. C$^{18}$O(2-1) and HCO$^{+}$(3-2) lines show pronounced blue profiles, indicating gas infalling in this region. This is the first time that the infall motion has been reported in G192.16-3.84 core. Two-layer model fitting gave infall velocities of 2.0$pm$0.2 and 2.8$pm$0.1 km s$^{-1}$. Assuming that the cloud core follows a power-law density profile ($rho$$propto$$r^{1.5}$), the corresponding mass infall rates are (4.7$pm$1.7)$times10^{-3}$ and (6.6$pm$2.1)$times10^{-3}$ M$_{sun}$ yr$^{-1}$ for C$^{18}$O(2-1) and HCO$^{+}$(3-2), respectively. The derived infall rates are in agreement with the turbulent core model and those in other high-mass star-forming regions, suggesting that high accretion rate is a general requirement to form a massive star.
This paper was withdrawed from the ApJ after the comments from the referee, please be carefully.
Planck Galactic Cold Clumps (PGCCs) possibly represent the early stages of star formation. To understand better the properties of PGCCs, we studied 16 PGCCs in the L1495 cloud with molecular lines and continuum data from Herschel, JCMT/SCUBA-2 and th
e PMO 13.7 m telescope. Thirty dense cores were identified in 16 PGCCs from 2-D Gaussian fitting. The dense cores have dust temperatures of $T_{rm d}$ = 11-14 K, and H$_{2}$ column densities of $N_{rm H_{2}}$ = 0.36-2.5$times10^{22}$ cm$^{-2}$. We found that not all PGCCs contain prestellar objects. In general, the dense cores in PGCCs are usually at their earliest evolutionary stages. All the dense cores have non-thermal velocity dispersions larger than the thermal velocity dispersions from molecular line data, suggesting that the dense cores may be turbulence-dominated. We have calculated the virial parameter $alpha$ and found that 14 of the dense cores have $alpha$ $<$ 2, while 16 of the dense cores have $alpha$ $>$ 2. This suggests that some of the dense cores are not bound in the absence of external pressure and magnetic fields. The column density profiles of dense cores were fitted. The sizes of the flat regions and core radii decrease with the evolution of dense cores. CO depletion was found to occur in all the dense cores, but is more significant in prestellar core candidates than in protostellar or starless cores. The protostellar cores inside the PGCCs are still at a very early evolutionary stage, sharing similar physical and chemical properties with the prestellar core candidates.
We present Submillimeter Array (SMA) molecular line observations in two 2 GHz-wide bands centered at 217.5 and 227.5 GHz, toward the massive star forming region W51 North. We identified 84 molecular line transitions from 17 species and their isotopol
ogues. The molecular gas distribution of these lines mainly peaks in the continuum position of W51 North, and has a small tail extending to the west, probably associated with W51 d2. In addition to the commonly detected nitrogen and oxygen-bearing species, we detected a large amount of transitions of the Acetone (CH$_3$COCH$_3$) and Methyl Formate (CH$_3$OCHO), which may suggest that these molecules are present in an early evolutionary stage of the massive stars. We also found that W51 North is an ethanol-rich source. There is no obvious difference in the molecular gas distributions between the oxygen-bearing and nitrogen-bearing molecules. Under the assumption of Local Thermodynamic Equilibrium (LTE), with the XCLASS tool, the molecular column densities, and rotation temperatures are estimated. We found that the oxygen-bearing molecules have considerable higher column densities and fractional abundances than the nitrogen-bearing molecules. The rotation temperatures range from 100 to 200 K, suggesting that the molecular emission could be originated from a warm environment. Finally, based on the gas distributions, fractional abundances and the rotation temperatures, we conclude that CH$_3$OH, C$_2$H$_5$OH, CH$_3$COCH$_3$ and CH$_3$CH$_2$CN might be synthesized on the grain surface, while gas phase chemistry is responsible for the production of CH$_3$OCH$_3$, CH$_3$OCHO and CH$_2$CHCN.
We present the results from arcsecond resolution observations of various line transitions at 1.3 mm toward hypercompact HII region G28.20-0.04N. With the SMA data, we have detected and mapped the transitions in the CH$_{3}$CN, CO, $^{13}$CO, SO$_{2}$
, OCS, and CH$_{3}$OH molecular lines as well as the radio recombination line H30$alpha$. The observations and analysis indicate a hot core associated with G28.20-0.04N. The outflow and possible rotation are detected in this region.
Observations of two H$_2$CO ($3_{03}-2_{02}$ and $3_{21}-2_{20}$) lines and continuum emission at 1.3 mm towards Sgr B2(N) and Sgr B2(M) have been carried out with the SMA. The mosaic maps of Sgr B2(N) and Sgr B2(M) in both continuum and lines show a
complex distribution of dust and molecular gas in both clumps and filaments surrounding the compact star formation cores. We have observed a decelerating outflow originated from the Sgr B2(M) core, showing that both the red-shifted and blue-shifted outflow components have a common terminal velocity. This terminal velocity is 58$pm$2 km s$^{-1}$. It provides an excellent method in determination of the systematic velocity of the molecular cloud. The SMA observations have also shown that a large fraction of absorption against the two continuum cores is red-shifted with respect to the systematic velocities of Sgr B2(N) and Sgr B2(M), respectively, suggesting that the majority of the dense molecular gas is flowing into the two major cores where massive stars have been formed. We have solved the radiative transfer in a multi-level system with LVG approximation. The observed H$_2$CO line intensities and their ratios can be adequately fitted with this model for the most of the gas components. However, the line intensities between the higher energy level transition H$_2$CO ($3_{21}-2_{20}$) and the lower energy level transition H$_2$CO ($3_{03}-2_{02}$) is reversed in the red-shifted outflow region of Sgr B2(M), suggesting the presence of inversion in population between the ground levels in the two K ladders (K$_{-1}$= 0 and 2). The possibility of weak maser processes for the H$_2$CO emission in Sgr B2(M) is discussed.
Observations of H$_{2}$CO lines and continuum at 1.3 mm towards Sgr B2(N) and Sgr B2(M) cores were carried out with the SMA. We imaged H$_{2}$CO line absorption against the continuum cores and the surrounding line emission clumps. The results show th
at the majority of the dense gas is falling into the major cores where massive stars have been formed. The filaments and clumps of the continuum and gas are detected outside of Sgr B2(N) and Sgr B2(M) cores. Both the spectra and moment analysis show the presence of outflows from Sgr B2(M) cores. The H$_{2}$CO gas in the red-shifted outflow of Sgr B2(M) appears to be excited by a non-LTE process which might be related to the shocks in the outflow.
