Molecular environments of 51 Planck cold clumps in Orion complex

A mapping survey towards 51 Planck cold clumps projected on Orion complex was performed with J=1-0 lines of $^{12}$CO and $^{13}$CO at the 13.7 m telescope of Purple Mountain Observatory. The mean column densities of the Planck gas clumps range from 0.5 to 9.5$\times10^{21}$ cm$^{-2}$, with an average value of (2.9$\pm$1.9)$\times10^{21}$ cm$^{-2}$. While the mean excitation temperatures of these clumps range from 7.4 to 21.1 K, with an average value of 12.1$\pm$3.0 K. The averaged three-dimensional velocity dispersion $\sigma_{3D}$ in these molecular clumps is 0.66$\pm$0.24 km s$^{-1}$. Most of the clumps have $\sigma_{NT}$ larger than or comparable with $\sigma_{Therm}$. The H$_{2}$ column density of the molecular clumps calculated from molecular lines correlates with the aperture flux at 857 GHz of the dust emission. Through analyzing the distributions of the physical parameters, we suggest turbulent flows can shape the clump structure and dominate their density distribution in large scale, but not affect in small scale due to the local fluctuations. Eighty two dense cores are identified in the molecular clumps. The dense cores have an averaged radius and LTE mass of 0.34$\pm$0.14 pc and 38$_{-30}^{+5}$ M_{\sun}, respectively. And structures of low column density cores are more affected by turbulence, while those of high column density cores are more concerned by other factors, especially by gravity. The correlation of the velocity dispersion versus core size is very weak for the dense cores. The dense cores are found most likely gravitationally bounded rather than pressure confined. The relationship between $M_{vir}$ and $M_{LTE}$ can be well fitted with a power law. The core mass function here is much more flatten than the stellar initial mass function. The lognormal behavior of the core mass distribution is most likely determined by the internal turbulence.Comment: Accepted to The Astrophysical Journal Supplement Series (ApJS

Uniform Infall toward the Cometary H II Region in the G34.26+0.15 Complex?

Gas accretion is a key process in star formation. However, the gas infall detections in high-mass star forming regions with high-spatial resolution observations are rare. Here we report the detection of gas infall towards a cometary ultracompact H{\sc ii} region "C" in G34.26+0.15 complex. The hot core associated with "C" has a mass of $\sim$76 M_{\sun} and a volume density of 1.1$\times10^{8}$ cm$^{-3}$. The HCN (3--2), HCO$^{+}$ (1--0) lines observed by single-dishes and the CN (2--1) lines observed by the SMA show redshifted absorption features, indicating gas infall. We found a linear relationship between the line width and optical depth of the CN (2--1) lines. Those transitions with larger optical depth and line width have larger absorption area. However, the infall velocities measured from different lines seem to be constant, indicating the gas infall is uniform. We also investigated the evolution of gas infall in high-mass star forming regions. At stages prior to hot core phase, the typical infall velocity and mass infall rate are $\sim$ 1 km s$^{-1}$ and $\sim10^{-4}$ M_{\sun}\cdotyr$^{-1}$, respectively. While in more evolved regions, the infall velocity and mass infall rates can reach as high as serval km s$^{-1}$ and $\sim10^{-3}-10^{-2}$ M_{\sun}\cdotyr$^{-1}$, respectively. Accelerated infall has been detected towards some hypercompact H{\sc ii} and ultracompact H{\sc ii} regions. However, the acceleration phenomenon becomes inapparent in more evolved ultracompact H{\sc ii} regions (e.g. G34.26+0.15)

Molecular gas and triggered star formation surrounding Wolf-Rayet stars

The environments surrounding nine Wolf-Rayet stars were studied in molecular emission. Expanding shells were detected surrounding these WR stars (see left panels of Figure 1). The average masses and radii of the molecular cores surrounding these WR stars anti-correlate with the WR stellar wind velocities (middle panels of Figure 1), indicating the WR stars has great impact on their environments. The number density of Young Stellar Objects (YSOs) is enhanced in the molecular shells at $\sim$5 arcmin from the central WR star (lower-right panel of Figure 1). Through detailed studies of the molecular shells and YSOs, we find strong evidences of triggered star formation in the fragmented molecular shells (\cite[Liu et al. 2010]{liu_etal12}Comment: 1 page, IAUS29

A study of dynamical processes in the Orion KL region using ALMA-- Probing molecular outflow and inflow

This work reports a high spatial resolution observations toward Orion KL region with high critical density lines of CH$_{3}$CN (12$_{4}$-11$_{4}$) and CH$_{3}$OH (8$_{-1, 8}$-7$_{0, 7}$) as well as continuum at $\sim$1.3 mm band. The observations were made using the Atacama Large Millimeter/Submillimeter Array with a spatial resolution of $\sim$1.5$^{\prime\prime}$ and sensitives about 0.07 K and $\sim$0.18 K for continuum and line, respectively. The observational results showed that the gas in the Orion KL region consists of jet-propelled cores at the ridge and dense cores at east and south of the region, shaped like a wedge ring. The outflow has multiple lobes, which may originate from an explosive ejection and is not driven by young stellar objects. Four infrared bubbles were found in the Spitzer/IRAC emissions. These bubbles, the distributions of the previously found H$_2$ jets, the young stellar objects and molecular gas suggested that BN is the explosive center. The burst time was estimated to be $\leq$ 1300 years. In the mean time, signatures of gravitational collapse toward Source I and hot core were detected with material infall velocities of 1.5 km~s$^{-1}$ and $\sim$ 0.6 km~s$^{-1}$, corresponding to mass accretion rates of 1.2$\times$10$^{-3}$M_{\sun}/Yr and 8.0$\times$10$^{-5}$M_{\sun}/Yr, respectively. These observations may support that high-mass stars form via accretion model, like their low-mass counterparts.Comment: Accepted to Ap

Competitive accretion in the protocluster G10.6-0.4?

We present the results of high spatial resolution observations at 1.1 mm waveband, with the Submillimetre Array (SMA), towards the protocluster G10.6-0.4. The 1.1 mm continuum emission reveals seven dense cores, towards which infall motions are all detected with the red-shifted absorption dips in HCN (3--2) line. This is the first time that infall is seen towards multiple sources in a protocluster. We also identified four infrared point sources in this region, which are most likely Class 0/I protostars. Two jet-like structures are also identified from Spitzer/IRAC image. The dense core located in the centre has much larger mass than the off-centre cores. The clump is in overall collapse and the infall motion is supersonic. The standard deviation of core velocities and the velocity differences between the cores and the cloud/clump are all larger than the thermal velocity dispersion. The picture of G10.6-0.4 seems to favor the "competitive accretion" model but needs to be tested by further observations.Comment: 9 pages, 9 figures, 2 tables, Submitted to MNRA

A feedback-driven bubble G24.136+00.436: a possible site of triggered star formation

We present a multi-wavelength study of the IR bubble G24.136+00.436. The J=1-0 observations of $^{12}$CO, $^{13}$CO and C$^{18}$O 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, shaping as a shell with a total mass of $\sim2\times10^{4}$ $M_{\odot}$. It is likely assembled during the expansion of the bubble. The expanding shell consists of six dense cores. Their dense (a few of $10^{3}$ cm$^{-3}$) and massive (a few of $10^{3}$ $M_{\odot}$) characteristics coupled with the broad linewidths ($>$ 2.5 km s$^{-1}$) suggest they are promising sites of forming high-mass stars or clusters. This could be further consolidated by the detection of compact HII regions in Cores A and E. We tentatively identified and classified 63 candidate YSOs based on the \emph{Spitzer} and UKIDSS data. They are found to be dominantly distributed in regions with strong emission of molecular gas, indicative of active star formation especially in the shell. The HII region inside the bubble is mainly ionized by a $\sim$O8V star(s), of the dynamical age $\sim$1.6 Myr. The enhanced number of candidate YSOs and secondary star formation in the shell as well as time scales involved, indicate a possible scenario of triggering star formation, signified by the "collect and collapse" process.Comment: 13 pages, 10 figures, 4 tables, accepted by Ap