INFLOWS IN MASSIVE STAR FORMATION REGIONS

WU, YUEFANG,LIU, TIE,QIN, SHENGLI The Korean Astronomical Society 2015 天文學論叢 Vol.30 No.2

How high-mass stars form is currently unclear. Calculations suggest that the radiation pressure of a forming star can halt spherical infall, preventing further growth when it reaches $10M_{\odot}$. Two major theoretical models on the further growth of stellar mass have been proposed. One model suggests the merging of less massive stellar objects, and the other is through accretion, but with the help of a disk. Inflow motions are key evidence for how forming stars gain further mass to build up massive stars. Recent developments in technology have boosted the search for inflow motion. A number of high-mass collapse candidates were obtained with single dish observations, and mostly showed blue profiles. Infalling signatures seem to be more common in regions which have developed radiation pressure than in younger cores, which is the opposite of the theoretical prediction and is also very different from observations of low mass star formation. Interferometer studies so far confirm this tendency with more obvious blue profiles or inverse P Cygni profiles. Results seem to favor the accretion model. However, the evolution of the infall motion in massive star forming cores needs to be further explored. Direct evidence for monolithic or competitive collapse processes is still lacking. ALMA will enable us to probe more detail of the gravitional processes.

A STUDY OF DYNAMICAL PROCESSES IN THE ORION KL REGION USING ALMA-PROBING MOLECULAR OUTFLOW AND INFLOW

Wu, Yuefang,Liu, Tie,Qin, Sheng-Li IOP Publishing 2014 The Astrophysical journal Vol.791 No.2

<P>This work reports high spatial resolution observations toward the Orion KL region with high critical density lines of CH3CN (124-114) and CH3OH (8-1,8-70,7), as well as a continuum at similar to 1.3 mm band. The observations were made using the Atacama Large Millimeter/Submillimeter Array with a spatial resolution of similar to 1.'' 5 and sensitivity of about 0.07 K and similar to 0.18 K for continuum and line, respectively. The observational results showed that the gas in the Orion KL region consists of jet-propelled cores at the ridge and dense cores east and south of the region that are shaped like a wedge ring. The outflow has multiple lobes, which may originate from an explosive ejection, and is not driven by young stellar objects. Four infrared bubbles were found in the Spitzer/IRAC emissions. These bubbles, the distributions of the previously found H-2 jets, the young stellar objects, and molecular gas suggest that BN is the explosive center. The burst time was estimated to be similar to 1300 yr. At the same time, signatures of gravitational collapse toward Source I and the hot core were detected with material infall velocities of 1.5 km s(-1) and similar to 0.6 km s(-1), corresponding to mass accretion rates of 1.2 x 10-3M(circle dot)/yr and 8.0 x 10-5M(circle dot)/yr, respectively. These observations may support the belief that high-mass stars form via the accretion model, similar to their low-mass counterparts.</P>

High-mass Starless Clumps in the Inner Galactic Plane: The Sample and Dust Properties

Yuan, Jinghua,Wu, Yuefang,Ellingsen, Simon P.,II, Neal J. Evans,Henkel, Christian,Wang, Ke,Liu, Hong-Li,Liu, Tie,Li, Jin-Zeng,Zavagno, Annie American Astronomical Society 2017 The Astrophysical journal Supplement series Vol.231 No.1

<P>We report a sample of 463 high-mass starless clump (HMSC) candidates within -60 degrees < l < 60 degrees and -1 degrees < b < 1 degrees. This sample has been singled out from 10,861 ATLASGAL clumps. None of these sources are associated with any known star-forming activities collected in SIMBAD and young stellar objects identified using color-based criteria. We also make sure that the HMSC candidates have neither point sources at 24 and 70 mu m. nor strong extended emission at 24 mu m. Most of the identified HMSCs are infrared dark, and some are even dark at 70 mu m. Their distribution shows crowding in Galactic spiral arms and toward the Galactic center and some wellknown star-forming complexes. Many HMSCs are associated with large-scale filaments. Some basic parameters were attained from column density and dust temperature maps constructed via fitting far-infrared and submillimeter continuum data to modified blackbodies. The HMSC candidates have sizes, masses, and densities similar to clumps associated with Class II methanol masers and H. II. regions, suggesting that they will evolve into star-forming clumps. More than 90% of the HMSC candidates have densities above some proposed thresholds for forming high-mass stars. With dust temperatures and luminosity-to-mass ratios significantly lower than that for star-forming sources, the HMSC candidates are externally heated and genuinely at very early stages of high-mass star formation. Twenty sources with equivalent radii r(eq) < 0.15 pc and mass surface densities Sigma > 0.08 g cm(-2) could be possible high-mass starless cores. Further investigations toward these HMSCs would undoubtedly shed light on comprehensively understanding the birth of high-mass stars.</P>

GAS OF 96 PLANCK COLD CLUMPS IN THE SECOND QUADRANT

Zhang, Tianwei,Wu, Yuefang,Liu, Tie,Meng, Fanyi American Astronomical Society 2016 The Astrophysical journal Supplement series Vol.224 No.2

<P>Ninety-six Planck cold dust clumps in the second quadrant were mapped with (CO)-C-12 (1-0), (CO)-C-13 (1-0), and (CO)-O-18 (1-0) lines at the 13.7 m telescope of Purple Mountain Observatory. (CO)-C-12 (1-0) and (CO)-C-13 (1-0) emissions were detected for all 96 clumps, while (CO)-O-18. (1-0) emissions were detected in 81 of them. Fifteen clumps have more than one velocity component. In the 115 mapped velocity components, 225 cores were obtained. We found that 23.1% of the cores have non-Gaussian profiles. We acquired the V-lsr, FWHM, and T-A of the lines. Distances, T-ex, velocity dispersions, N-H2, and masses were also derived. Generally, turbulence may dominant the cores because sigma(NT)/sigma(Therm) > 1 in almost all of the cores and Larson's relationship is not apparent in our massive cores. Virial parameters are adopted to test the gravitational stability of cores and 51% of the cores are likely collapsing. The core mass function of the cores in the range 0-1 kpc suggests a low core-to-star conversional efficiency (0.62%). Only 14 of 225 cores (6.2%) have associated stellar objects at their centers, while the others are starless. The morphologies of clumps are mainly filamentary structures. Seven clumps may be located on an extension of the new spiral arm in the second quadrant while three are on the known outer arm.</P>

FOLLOW-UP OBSERVATIONS TOWARD PLANCK COLD CLUMPS WITH GROUND-BASED RADIO TELESCOPES

LIU, TIE,WU, YUEFANG,MARDONES, DIEGO,KIM, KEE-TAE,MENTEN, KARL M.,TATEMATSU, KEN,CUNNINGHAM, MARIA,JUVELA, MIKA,ZHANG, QIZHOU,GOLDSMITH, PAUL F,LIU, SHENG-YUAN,ZHANG, HUA-WEI,MENG, FANYI,LI, DI,LO, NA The Korean Astronomical Society 2015 天文學論叢 Vol.30 No.2

The physical and chemical properties of prestellar cores, especially massive ones, are still far from being well understood due to the lack of a large sample. The low dust temperature (< 14 K) of Planck cold clumps makes them promising candidates for prestellar objects or for sources at the very initial stages of protostellar collapse. We have been conducting a series of observations toward Planck cold clumps (PCCs) with ground-based radio telescopes. In general, when compared with other star forming samples (e.g. infrared dark clouds), PCCs are more quiescent, suggesting that most of them may be in the earliest phase of star formation. However, some PCCs are associated with protostars and molecular outflows, indicating that not all PCCs are in a prestellar phase. We have identified hundreds of starless dense clumps from a mapping survey with the Purple Mountain Observatory (PMO) 13.7-m telescope. Follow-up observations suggest that these dense clumps are ideal targets to search for prestellar objects.

A FEEDBACK-DRIVEN BUBBLE G24.136+00.436: A POSSIBLE SITE OF TRIGGERED STAR FORMATION

Liu, Hong-Li,Wu, Yuefang,Li, JinZeng,Yuan, Jing-Hua,Liu, Tie,Dong, Xiaoyi IOP Publishing 2015 The Astrophysical journal Vol.798 No.1

<P>We present a multi-wavelength study of the IR bubble G24.136+00.436. The J = 1-0 observations of (CO)-C-12, (CO)-C-13, and (CO)-O-18 were carried out with the Purple Mountain Observatory 13.7 m telescope. Molecular gas with a velocity of 94.8 km s(-1) is found prominently in the southeast of the bubble, shaped as a shell with a total mass of similar to 2 x 10(4) M-circle dot. It was likely assembled during the expansion of the bubble. The expanding shell consists of six dense cores, whose dense (a few of 10(3) cm(-3)) and massive (a few of 10(3) M-circle dot) characteristics coupled with the broad linewidths (>2.5 km s(-1)) suggest that they are promising sites for forming high-mass stars or clusters. This could be further consolidated by the detection of compact H II regions in Cores A and E. We tentatively identified and classified 63 candidate young stellar objects (YSOs) based on the Spitzer and UKIDSS data. They are found to be dominantly distributed in regions with strong molecular gas emission, indicative of active star formation, especially in the shell. The H II region inside the bubble is mainly ionized by a similar to O8V star(s), of the dynamical age of similar to 1.6 Myr. The enhanced number of candidate YSOs and secondary star formation in the shell as well as the timescales involved, indicate a possible scenario for triggering star formation, signified by the 'collect and collapse' process.</P>

DENSE GAS IN MOLECULAR CORES ASSOCIATED WITH<i>PLANCK</i>GALACTIC COLD CLUMPS

Yuan (袁敬华,), Jinghua,Wu, Yuefang,Liu, Tie,Zhang, Tianwei,Li, Jin Zeng,Liu, Hong-Li,Meng, Fanyi,Chen, Ping,Hu, Runjie,Wang, Ke American Astronomical Society 2016 The Astrophysical journal Vol.820 No.1

<P>We present the first survey of dense gas toward Planck Galactic Cold Clumps (PGCCs). Observations in the J = 1-0 transitions of HCO+ and HCN toward 621 molecular cores associated with PGCCs were performed using the Purple Mountain Observatory's 13.7 m telescope. Among them, 250 sources were detected, including 230 cores detected in HCO+. and 158 in HCN. Spectra of the J = 1-0 transitions from (CO)-C-12, (CO)-C-13, and (CO)-O-18 at the centers of the 250 cores were extracted from previous mapping observations to construct a multi-line data set. The significantly low detection rate of asymmetric double-peaked profiles, together with the good consistency among central velocities of CO, HCO+, and HCN spectra, suggests that the CO-selected Planck cores are more quiescent than classical star-forming regions. The small difference between line widths of (CO)-O-18. and HCN indicates that the inner regions of CO-selected Planck cores are no more turbulent than the exterior. The velocity-integrated intensities and abundances of HCO+ are positively correlated with those of HCN, suggesting that these two species are well coupled and chemically connected. The detected abundances of both HCO+ and HCN are significantly lower than values in other low- to high-mass star-forming regions. The low abundances may be due to beam dilution. On the basis of an inspection of the parameters given in the PGCC catalog, we suggest that there may be about 1000 PGCC objects that have a sufficient reservoir of dense gas to form stars.</P>

SMA OBSERVATIONS OF THE W3(OH) COMPLEX: PHYSICAL AND CHEMICAL DIFFERENTIATION BETWEEN W3(H₂O) AND W3(OH)

Qin, Sheng-Li,Schilke, Peter,Wu, Jingwen,Wu, Yuefang,Liu, Tie,Liu, Ying,Sá,nchez-Monge, Á,lvaro IOP Publishing 2015 The Astrophysical journal Vol.803 No.1

<P>We report on the Submillimeter Array (SMA) observations of molecular lines at 270 GHz toward the W3(OH) and W3(H2O) complex. Although previous observations already resolved the W3(H2O) into two or three subcomponents, the physical and chemical properties of the two sources are not well constrained. Our SMA observations clearly resolved the W3(OH) and W3(H2O) continuum cores. Taking advantage of the line fitting tool XCLASS, we identified and modeled a rich molecular spectrum in this complex, including multiple CH3CN and CH3OH transitions in both cores. HDO, C2H5CN, (OCS)-C-13, and vibrationally excited lines of HCN, CH3CN, and CH3OCHO were only detected in W3(H2O). We calculate gas temperatures and column densities for both cores. The results show that W3(H2O) has higher gas temperatures and larger column densities than W3(OH) as previously observed, suggesting physical and chemical differences between the two cores. We compare the molecular abundances in W3(H2O) to those in the Sgr B2(N) hot core, the Orion KL hot core, and the Orion Compact Ridge, and discuss the chemical origin of specific species. An east-west velocity gradient is seen in W3 (H2O), and the extension is consistent with the bipolar outflow orientation traced by water masers and radio jets. A north-south velocity gradient across W3(OH) is also observed. However, with current observations we cannot be assured whether the velocity gradients are caused by rotation, outflow, or radial velocity differences of the subcomponents of W3(OH).</P>

INTERACTIONS OF THE INFRARED BUBBLE N4 WITH ITS SURROUNDINGS

Liu, Hong-Li,Li, Jin-Zeng,Wu, Yuefang,Yuan, Jing-Hua,Liu, Tie,Dubner, G.,Paron, S.,Ortega, M. E.,Molinari, Sergio,Huang, Maohai,Zavagno, Annie,Samal, Manash R.,Huang, Ya-Fang,Zhang, Si-Ju American Astronomical Society 2016 The Astrophysical journal Vol.818 No.1

<P>The physical mechanisms that induce the transformation of a certain mass of gas in new stars are far from being well understood. Infrared bubbles associated with H II regions have been considered to be good samples for investigating triggered star formation. In this paper we report on the investigation of the dust properties of the infrared bubble N4 around the H II. region G11.898+0.747, analyzing its interaction with its surroundings and star formation histories therein, with the aim of determining the possibility of star formation triggered by the expansion of the bubble. Using Herschel PACS and SPIRE images with a wide wavelength coverage, we reveal the dust properties over the entire bubble. Meanwhile, we are able to identify six dust clumps surrounding the bubble, with a mean size of 0.50 pc, temperature of about 22 K, mean column density of 1.7 x 10(22) cm(-2), mean volume density of about 4.4 x 10(4) cm(-3), and a mean mass of 320M(circle dot). In addition, from PAH emission seen at 8 mu m, free-free emission detected at 20 cm, and a probability density function in special regions, we could identify clear signatures of the influence of the H II region on the surroundings. There are hints of star formation, though further investigation is required to demonstrate that N4 is the triggering source.</P>

High-mass Star Formation through Filamentary Collapse and Clump-fed Accretion in G22

Yuan, Jinghua,Li, Jin-Zeng,Wu, Yuefang,Ellingsen, Simon P.,Henkel, Christian,Wang, Ke,Liu, Tie,Liu, Hong-Li,Zavagno, Annie,Ren, Zhiyuan,Huang, Ya-Fang American Astronomical Society 2018 The Astrophysical journal Vol.852 No.1

<P>How mass is accumulated from cloud-scale down to individual stars is a key open question in understanding highmass star formation. Here, we present the mass accumulation process in a hub-filament cloud G22 that is composed of four supercritical filaments. Velocity gradients detected along three filaments indicate that they are collapsing with a total mass infall rate of about 440M(circle dot) Myr(-1), suggesting the hub mass would be doubled in six free-fall times, adding up to similar to 2 Myr. A fraction of the masses in the central clumps C1 and C2 can be accounted for through large-scale filamentary collapse. Ubiquitous blue profiles in HCO+. (3-2) and (CO)-C-13. (3-2) spectra suggest a clump-scale collapse scenario in the most massive and densest clump C1. The estimated infall velocity and mass infall rate are 0.31 km s(-1) and 7.2 x. 10(-4)M(circle dot) yr(-1), respectively. In clump C1, a hot molecular core (SMA1) is revealed by the Submillimeter Array observations and an outflow-driving high-mass protostar is located at the center of SMA1. The mass of the protostar is estimated to be 11-15M(circle dot) and it is still growing with an accretion rate of 7 x. 10(-5)M(circle dot) yr(-1). The coexistent infall in filaments, clump C1, and the central hot core in G22 suggests that pre-assembled mass reservoirs (i.e., high-mass starless cores) may not be required to form high-mass stars. In the course of high-mass star formation, the central protostar, the core, and the clump can simultaneously grow in mass via core-fed/disk accretion, clump-fed accretion, and filamentary/cloud collapse.</P>