The Identification Of Infall Candidates In Molecular Clouds And The Study Of Their Physical And Chemical Properties

The study of star formation shows that stars form through inside-out gravitational collapse in dense molecular clumps.Gravitational infall mainly occurs in the beginning stage of star formation.In the subsequent evolutionary processes,accretion flow is maintained to feed the forming stars.Therefore,it is an important part of the starforming process.The study of this process helps us to understand the initial stage of the star formation.Generally,a pair of optically thick and thin lines are used as a tracer to identify gas infall motion.The optically thick line shows a blue profile,while optically thin line shows a single-peaked profile,and indicates the central radial velocity.The Milky Way Imaging Scroll Painting（MWISP）project which provides ¹²CO（1-0）、¹³CO（1-0）and C¹⁸O（1-0）simultaneously can be used to search the infall candidate preliminarily.The combinations of ¹²CO/¹³CO and ¹³CO/C¹⁸0 can be used as two pairs of optically thick and thin lines.Used these tracers we identified more than 3200 infall candidates,and selected 133 with significant blue profiles in ¹²CO（1-0）,and C¹⁸O（1-0）intensities greater than 1 K as a subsample for further study.We used the PMO 13.7-m radio telescope to observe 133 infall candidates.The observed lines are HCO⁺（1-0）and HCN（1-0）.We used HCO⁺（1-0）and HCN（1-0）as optically thick lines,and C¹⁸O（1-0）as optically thin line,to further identify these infall candidates.And the dimensionless parameters δV_HCO⁺and δV_HCN to quantify the asymmetry of the optically thick line profiles are obtained.We identify that clumps with infall profile in at least one optically thick line,and no expansion evidence are infall sources with higher confidence.Based on this,we identified 56 infall sources,and the detection rate is about 42%.It suggests that using CO data to restrict the sample can effectively improve the infall detection rate.Among these confirmed infall sources,there are 43 sources associated with Class 0/I young stellar objects,and 13 are not.These 13 sources are probably associated with the sources in earlier evolutionary stage.The infall sources which are associated with Class 0/I young stellar objects have higher excitation temperatures and column densities,while the other sources are colder and have lower column densities.The infall velocities of the confirmed infall sources are between 10^-1 to 10⁰ km s^-1,which is consistent with previous studies.For these confirmed infall sources,we carry out mapping observations.We used the IRAM 30-m radio telescope to mapping the infall candidates identified through the HCO⁺（1-0）and HCN（1-0）single-point observations.The observed frequency range are 83.7-91.5 GHz（L band）and 99.4-107.2 GHz（U band）,including the H¹³CO+（1-0）and HCO⁺（1-0）lines.we have completed the observations for 24 sources and analyzed the observation results.All 24 sources show HCO⁺ emissions,while 18 sources show H¹³CO+ emissions.The HCO⁺ integrated intensity maps of 17 sources show clear clumpy structures;for the H¹³CO+ line,15 sources show clumpy structures.We estimated the column densities of HCO⁺and H¹³CO+ using the RADEX radiation transfer code,and the obtained[HCO⁺]/[H₂]and[H¹³CO+]/[HCO⁺]of these sources are about 10^-11-10^-7 and 10^-3-1,respectively.In addition,based on the asymmetry of the HCO⁺ line profile,we distinguish these sources:19 sources show blue asymmetric profiles,and the other sources show red profiles,symmetric profiles or multiple peaked profiles.For eight sources that have double-peaked blue profiles and signal-to-noise ratios greater than 10,the RATRAN model is used to fit their HCO⁺（1-0）lines,and to estimate their infall parameters.The mean Vin of these sources are 0.3-1.3 km s^-1,and the Min are about 10^-3-10^-4 M⊙ yr^-1,which are consistent with the results of intermediate or massive star formation in previous studies.The Vinestimated from the Myers model are 0.1-1.6 km s^-1,and the Min are 10^-3-10^-5 M⊙ yr^-1.In addition,some infall sources also show other star-forming activities,such as outflows and maser emissions.Especially for those sources with a double-peaked blue profile,most of them have molecular outflow.In addition,we have also obtained many other molecular lines.Using the XCLASS software,we identified a total of 7 to 27 different molecules and isotopic transition lines in nine infall sources with double-peaked blue profile,including carbon chain molecules such as CCH,c-C₃H₂ and HC₃N.According to the radiation transfer model,we estimated the rotation temperatures and column densities of these molecules.A chemical evolution model established by a physical model including the core collapse phase was used to compare with the observed molecular abundances.The results show that the evolutionary timescale of most sources is about 3.3 × 10⁵ yr after the start of the gravitational collapse.