arXiv subjects

Yuefang Wu

Publications and source records attributed to Yuefang Wu.

At least 91 records · Page 5Linked to original sources

arXiv2012

Follow-up observations of Planck cold clumps in $^{12}$CO/$^{13}$CO/C$^{18}$O (1--0) transitions

The Planck Early Cold Cores Catalog (ECC) provides an unbiased list of Galactic cold clumps, which form an ideal sample for studying the early phases of star formation (\cite[Planck Collabrators et al. 2011]{Planck_etal11}). To study their properties, we have carried out a molecular line ($^{12}$CO/$^{13}$CO/C$^{18}$O) survey towards 674 Planck cold clumps in the ECC with the PMO 13.7 m telescope.

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arXiv2012

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}

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arXiv2012

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 $σ_{3D}$ in these molecular clumps is 0.66$\pm$0.24 km s$^{-1}$. Most of the clumps have $σ_{NT}$ larger than or comparable with $σ_{Therm}$. 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.

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arXiv2012

Gas emissions in Planck cold dust clumps---A Survey of the J=1-0 Transitions of $^{12}$CO, $^{13}$CO, and C$^{18}$O

A survey toward 674 Planck cold clumps of the Early Cold Core Catalogue (ECC) in the J=1-0 transitions of $^{12}$CO, $^{13}$CO and C$^{18}$O has been carried out using the PMO 13.7 m telescope. 673 clumps were detected with the $^{12}$CO and $^{13}$CO, and 68% of the samples have C$^{18}$O emission. Additional velocity components were also identified.A close consistency of the three line peak velocities was revealed for the first time. Kinematic distances are given out for all the velocity components and half of the clumps are located within 0.5 and 1.5 kpc. Excitation temperatures range from 4 to 27 K, slightly larger than those of $T_d$. Line width analysis shows that the majority of ECC clumps are low mass clumps. Column densities N$_{H_{2}}$ span from 10$^{20}$ to 4.5$\times10^{22}$ cm$^{-2}$ with an average value of (4.4$\pm$3.6)$\times10^{21}$ cm$^{-2}$. N$_{H_{2}}$ cumulative fraction distribution deviates from the lognormal distribution, which is attributed to optical depth. The average abundance ratio of the $^{13}$CO to C$^{18}$O in these clumps is 7.0$\pm$3.8, higher than the terrestrial value. Dust and gas are well coupled in 95% of the clumps. Blue profile, red profile and line asymmetry in total was found in less than 10% of the clumps, generally indicating star formation is not developed yet. Ten clumps were mapped. Twelve velocity components and 22 cores were obtained. Their morphologies include extended diffuse, dense isolated, cometary and filament, of which the last is the majority. 20 cores are starless.Only 7 cores seem to be in gravitationally bound state. Planck cold clumps are the most quiescent among the samples of weak-red IRAS, infrared dark clouds, UC H{\sc ii} region candidates, EGOs and methanol maser sources, suggesting that Planck cold clumps have expanded the horizon of cold Astronomy.

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arXiv2012

Triggered star formation surrounding Wolf-Rayet star HD 211853

The environment surrounding Wolf-Rayet star HD 211853 is studied in molecular emission, infrared emission, as well as radio and HI emission. The molecular ring consists of well-separated cores, which have a volume density of 10$^{3}$ cm$^{-3}$ and kinematic temperature $\sim$20 K. Most of the cores are under gravitational collapse due to external pressure from the surrounding ionized gas. From SED modeling towards the young stellar objects (YSOs), sequential star formation is revealed on a large scale in space spreading from the Wolf-Rayet star to the molecular ring. A small scale sequential star formation is revealed towards core A, which harbors a very young star cluster. Triggered star formations is thus suggested. The presence of PDR, the fragmentation of the molecular ring, the collapse of the cores, the large scale sequential star formation indicate the "Collect and Collapse" process functions in this region. The star forming activities in core A seem to be affected by the "Radiation-Driven Implosion" (RDI) process.

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arXiv2012

The molecular emissions and the infall motion in the high-mass young stellar object G8.68-0.37

We present a multi-wavelength observational study towards the high-mass young stellar object G8.68-0.37. A single massive gas-and-dust core is observed in the (sub)millimeter continuum and molecular line emissions. We fitted the spectral energy distribution (SED) from the dust continuum emission. The best-fit SED suggests the presence of two components with temperature of $T_{\rm d}=20$ K and 120 K, respectively. The core has a total mass of up to $1.5\times10^3$ $M_{\odot}$ and bolometric luminosity of $2.3\times10^4 L_{\odot}$. Both the mass and luminosity are dominated by the cold component ($T_{\rm d}=20$ K). The molecular lines of C$^{18}$O, C$^{34}$S, DCN, and thermally excited CH$_3$OH are detected in this core. Prominent infall signatures are observed in the $^{12}$CO $(1-0)$ and $(2-1)$. We estimated an infall velocity of 0.45 km s$^{-1}$ and mass infall rate of $7\times10^{-4} M_{\odot}$ year$^{-1}$. From the molecular lines, we have found a high DCN abundance and relative abundance ratio to HCN. The overabundant DCN may originate from a significant deuteration in the previous cold pre-protostellar phase. And the DCN should now be rapidly sublimated from the grain mantles to maintain the overabundance in the gas phase.

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arXiv2011

Protostellar Outflow Heating in a Growing Massive Protocluster

The dense molecular clump P1 in the infrared dark cloud (IRDC) complex G28.34+0.06 harbors a massive protostellar cluster at its extreme youth. Our previous Submillimeter Array (SMA) observations revealed several jet-like CO outflows emanating from the protostars, indicative of intense accretion and potential interaction with ambient natal materials. Here we present the Expanded Very Large Array (EVLA) spectral line observations toward P1 in the NH3 (J,K) = (1,1), (2,2), (3,3) lines, as well as H2O and class I CH3OH masers. Multiple NH3 transitions reveal the heated gas widely spread in the 1 pc clump. The temperature distribution is highly structured; the heated gas is offset from the protostars, and morphologically matches the outflows very well. Hot spots of spatially compact, spectrally broad NH3 (3,3) emission are also found coincident with the outflows. A weak NH3 (3,3) maser is discovered at the interface between an outflow jet and the ambient gas. These findings suggest that protostellar heating may not be effective in suppressing fragmentation during the formation of massive cores.

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arXiv2011

Outflow activities in the young high-mass stellar object G23.44-0.18

We present an observational study towards the young high-mass star forming region G23.44-0.18 using the Submillimeter Array. Two massive, radio-quiet dusty cores MM1 and MM2 are observed in 1.3 mm continuum emission and dense molecular gas tracers including thermal CH$_3$OH, CH$_3$CN, HNCO, SO, and OCS lines. The $^{12}$CO (2--1) line reveals a strong bipolar outflow originated from MM2. The outflow consists of a low-velocity component with wide-angle quasi-parabolic shape and a more compact and collimated high-velocity component. The overall geometry resembles the outflow system observed in the low-mass protostar which has a jet-driven fast flow and entrained gas shell. The outflow has a dynamical age of $6\times10^3$ years and a mass ejection rate $\sim10^{-3} M_{\odot}$ year$^{-1}$. A prominent shock emission in the outflow is observed in SO and OCS, and also detected in CH$_3$OH and HNCO. We investigated the chemistry of MM1, MM2 and the shocked region. The dense core MM2 have molecular abundances of 3 to 4 times higher than those in MM1. The abundance excess, we suggest, can be a net effect of the stellar evolution and embedded shocks in MM2 that calls for further inspection.

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arXiv2011

Evolution of the dust/gas environment around Herbig Ae/Be stars

With the KOSMA 3-m telescope, 54 Herbig Ae/Be stars were surveyed in CO and $^{13}$CO emission lines. The properties of the stars and their circumstellar environments are studied by fitting the SEDs. The mean line width of $^{13}$CO (2-1) lines of this sample is 1.87 km s$^{-1}$. The average column density of H$_{2}$ is found to be $4.9\times10^{21}$ cm$^{-2}$ for the stars younger than $10^{6}$ yr, while drops to $2.5\times10^{21}$ cm$^{-2}$ for those older than $10^{6}$ yr. No significant difference is found among the SEDs of Herbig Ae stars and Herbig Be stars at the same age. The infrared excess decreases with age. The envelope masses and the envelope accretion rates decease with age after $10^{5}$ yr. The average disk mass of the sample is $3.3\times10^{-2} M_{\sun}$. The disk accretion rate decreases more slowly than the envelope accretion rate. A strong correlation between the CO line intensity and the envelope mass is found.

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arXiv2011

Hierarchical Fragmentation and Jet-like Outflows in IRDC G28.34+0.06, a Growing Massive Protostar Cluster

We present Submillimeter Array (SMA) λ= 0.88mm observations of an infrared dark cloud (IRDC) G28.34+0.06. Located in the quiescent southern part of the G28.34 cloud, the region of interest is a massive ($>10^3$\,\msun) molecular clump P1 with a luminosity of $\sim 10^3$ \lsun, where our previous SMA observations at 1.3mm have revealed a string of five dust cores of 22-64 \msun\ along the 1 pc IR-dark filament. The cores are well aligned at a position angle of 48 degrees and regularly spaced at an average projected separation of 0.16 pc. The new high-resolution, high-sensitivity 0.88\,mm image further resolves the five cores into ten compact condensations of 1.4-10.6 \msun, with sizes a few thousands AU. The spatial structure at clump ($\sim 1$ pc) and core ($\sim 0.1$ pc) scales indicates a hierarchical fragmentation. While the clump fragmentation is consistent with a cylindrical collapse, the observed fragment masses are much larger than the expected thermal Jeans masses. All the cores are driving CO(3-2) outflows up to 38 km/s, majority of which are bipolar, jet-like outflows. The moderate luminosity of the P1 clump sets a limit on the mass of protostars of 3-7 \msun. Because of the large reservoir of dense molecular gas in the immediate medium and ongoing accretion as evident by the jet-like outflows, we speculate that P1 will grow and eventually form a massive star cluster. This study provides a first glimpse of massive, clustered star formation that currently undergoes through an intermediate-mass stage.

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arXiv2011

Infall and outflow motions in the high-mass star forming complex G9.62+0.19

We present the results of a high resolution study with the Submillimeter Array towards the massive star forming complex G9.62+0.19. Three sub-mm cores are detected in this region. The masses are 13, 30 and 165 M$_{\sun}$ for the northern, middle and southern dust cores, respectively. Infall motions are found with HCN (4-3) and CS (7-6) lines at the middle core (G9.62+0.19 E). The infall rate is $4.3\times10^{-3}~M_{\odot}\cdot$yr$^{-1}$. In the southern core, a bipolar-outflow with a total mass about 26 M$_{\sun}$ and a mass-loss rate of $3.6\times10^{-5}~M_{\odot}\cdot$yr$^{-1}$ is revealed in SO ($8_{7}-7_{7}$) line wing emission. CS (7-6) and HCN (4-3) lines trace higher velocity gas than SO ($8_{7}-7_{7}$). G9.62+0.19 F is confirmed to be the driving source of the outflow. We also analyze the abundances of CS, SO and HCN along the redshifted outflow lobes. The mass-velocity diagrams of the outflow lobes can be well fitted by a single power law. The evolutionary sequence of the cm/mm cores in this region are also analyzed. The results support that UC~H{\sc ii} regions have a higher blue excess than their precursors.

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arXiv2010

Infall and outflow detections in a massive core JCMT 18354-0649S

We present a high-resolution study of a massive dense core JCMT 18354-0649S with the Submillimeter Array. The core is mapped with continuum emission at 1.3 mm, and molecular lines including CH$_{3}$OH ($5_{23}$-$4_{13}$) and HCN (3-2). The dust core detected in the compact configuration has a mass of $47 M_{\odot}$ and a diameter of $2\arcsec$ (0.06 pc), which is further resolved into three condensations with a total mass of $42 M_{\odot}$ under higher spatial resolution. The HCN (3-2) line exhibits asymmetric profile consistent with infall signature. The infall rate is estimated to be $2.0\times10^{-3} M_{\odot}\cdot$yr$^{-1}$. The high velocity HCN (3-2) line wings present an outflow with three lobes. Their total mass is $12 M_{\odot}$ and total momentum is $121 M_{\odot}\cdot$km s$^{-1}$, respectively. Analysis shows that the N-bearing molecules especially HCN can trace both inflow and outflow.

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arXiv2010

Inclination-angle of the outflow in IRAS 05553+1631: A method to correct the projection effect

A mapping study of IRAS 05553+1631 was performed with 12CO J=3-2 and 13CO J=2-1 lines observed by the KOSMA 3 m-telescope. A core with a size of 0.65 pc and with a LTE mass of 120 M\odot was defined by the mapping with 13CO J=2-1 line. We have identified a bipolar outflow with 12CO J=3-2. For accuracy in the calculation of outflow parameters, overcoming the projection effect is important. We propose a new method to directly calculate the inclination-angle θ. We establish two basic equations with the help of outflow contour diagram and finally obtain the "angle function" and the "angle equation" to derive θ. We apply our method to the outflow of IRAS 05553+1631, finding that θblue is 73\circ and θred is 78\circ. Compared to the parameters initially estimated under an assumption of 45\circ inclination-angle, the newly derived parameters are changed with different factors. For instance, the timescales for the blue and the red lobes are reduced by 0.31 and 0.21, respectively. Larger influences apply to mechanical luminosity, driving force, and mass-loss rate. The comparisons between parameters before and after the correction show that the effect of the inclination-angle cannot be neglected.

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arXiv2009

Dust and HCO+ Gas in the Star Forming Core W3-SE

We report new results from CARMA observations of both continuum and HCO+(1-0) emission at 3.4 mm from W3-SE, a molecular core of intermediate mass, together with the continuum observations at 1.1 and 0.85/0.45 mm with the SMA and JCMT. A continuum emission core elongated from SE to NW (~10"), has been observed at the and further resolved into a double source with the SMA at 1.1 mm, with a separation of ~4". Together with the measurements from the Spitzer and MSX at mid-IR, we determined the SED of W3-SE and fit it with a thermal dust emission model, suggesting the presence of two dust components with different temperatures. The emission at mm/submm wavelengths is dominated by a major cold (~41 K) with a mass of ~65 Msun. In addition, there is a weaker hot component (~400 K) which accounts for emission in the mid-IR, suggesting that a small fraction of dust has been heated by newly formed stars. We also imaged the molecular core in the HCO+(1-0) line using CARMA at an angular resolution ~6". With the CARMA observations, we have verified the presence of a blue-dominated double peak profile toward this core. The line profile cannot be explained by infall alone. The broad velocity wings of the line profile suggest that other kinematics such as outflows within the central 6" of the core likely dominate the resulting spectrum. The kinematics of the sub-structures of this core suggest that the molecular gas outside the main component appears to be dominated by the bipolar outflow originated from the dust core with a dynamical age of >30000 yr. Our analysis, based on the observations at wavelengths from mm/submm to mid-IR suggests that the molecular core W3-SE hosts a group of newly formed young stars and protostars.

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arXiv2009

The relation between 13CO(2-1) line width in molecular clouds and bolometric luminosity of associated IRAS sources

We search for evidence of a relation between properties of young stellar objects (YSOs) and their parent molecular clouds to understand the initial conditions of high-mass star formation. A sample of 135 sources was selected from the Infrared Astronomical Satellite (IRAS) Point Source Catalog, on the basis of their red color to enhance the possibility of discovering young sources. Using the Kolner Observatorium fur SubMillimeter Astronomie (KOSMA) 3-m telescope, a single-point survey in 13CO(2-1) was carried out for the entire sample, and 14 sources were mapped further. Archival mid-infrared (MIR) data were compared with the 13CO emissions to identify evolutionary stages of the sources. A 13CO observed sample was assembled to investigate the correlation between 13CO line width of the clouds and the luminosity of the associated YSOs. We identified 98 sources suitable for star formation analyses for which relevant parameters were calculated. We detected 18 cores from 14 mapped sources, which were identified with eight pre-UC HII regions and one UC HII region, two high-mass cores earlier than pre-UC HII phase, four possible star forming clusters, and three sourceless cores. By compiling a large (360 sources) 13CO observed sample, a good correlation was found between the 13CO line width of the clouds and the bolometric luminosity of the associated YSOs, which can be fitted as a power law: lg(dV13/km/s)=-0.023+0.135lg(Lbol/Lsolar). Results show that luminous (>10^3Lsolar) YSOs tend to be associated with both more massive and more turbulent (dV13>2km/s) molecular cloud structures.

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arXiv2008

High-Resolution Observations of Molecular Lines toward the Hot Core G28.20-0.04N

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$α$. The observations and analysis indicate a hot core associated with G28.20-0.04N. The outflow and possible rotation are detected in this region.

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arXiv2008

A Multi-wavelength Study of the Massive Star-forming Region S87

This article presents a multi-wavelength study towards S87, based on a dataset of submillimeter/far-/mid-infrared (sub-mm/FIR/MIR) images and molecular line maps. The sub-mm continuum emission measured with JCMT/SCUBA reveals three individual clumps, namely, SMM1, SMM2, and SMM3. The MIR/FIR images obtained by the Spitzer Space Telescope indicate that both SMM1 and SMM3 harbor point sources. The J=1-0 transitions of CO, 13CO, C18O, and HCO+, measured with the 13.7m telescope of the Purple Mountain Observatory, exhibit asymmetric line profiles. Our analysis of spectral energy distributions (SEDs) shows that all of the three sub-mm clumps are massive (110--210 $M_{\odot}$), with average dust temperatures in the range ~20--40K. A multi-wavelength comparison convinces us that the asymmetric profiles of molecular lines should result from two clouds at slightly different velocities, and it further confirms that the star-forming activity in SMM1 is stimulated by a cloud-cloud collision. The stellar contents and SEDs suggest that SMM1 and SMM3 are high-mass and intermediate-mass star-forming sites respectively. However, SMM2 has no counterpart downwards 70 micron, which is likely to be a cold high-mass starless core. These results, as mentioned above, expose multiple phases of star formation in S87.

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arXiv2007

Signatures of inflow motion in cores of massive star formation: Potential collapse candidates

Using the IRAM 30 m telescope, a mapping survey in optically thick and thin lines was performed towards 46 high mass star-forming regions. The sample includes UC H{\sc ii} precursors and UC H{\sc ii} regions. Seventeen sources are found to show "blue profiles", the expected signature of collapsing cores. The excess of sources with blue over red profiles ([$N_{\rm blue}$ -- $N_{\rm red}$]/$N_{\rm total}$) is 29% in the HCO$^+$ $J$=1--0 line, with a probability of 0.6% that this is caused by random fluctuations. UC H{\sc ii} regions show a higher excess (58%) than UC H{\sc ii} precursors (17%), indicating that material is still accreted after the onset of the UC H{\sc ii} phase. Similar differences in the excess of blue profiles as a function of evolutionary state are not observed in low mass star-forming regions. Thus, if confirmed for high mass star-forming sites, this would point at a fundamental difference between low- and high-mass star formation. Possible explanations are inadequate thermalization, stronger influence of outflows in massive early cores, larger gas reserves around massive stellar objects or different trigger mechanisms between low- and high- mass star formation.

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