Xi Chen

Chinese Academy of Sciences, Peping, Beijing, China

Are you Xi Chen?

Claim your profile

Publications (21)100.63 Total impact

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: In order to investigate the physical and chemical properties of massive star forming cores in early stages, we analyse the excitation and abundance of four organic species, CH3OH, CH3OCH3, HCOOCH3 and CH3CH2CN, toward 29 Extended Green Object (EGO) cloud cores that were observed by our previous single dish spectral line survey. The EGO cloud cores are found to have similar methanol J_3-J_2 rotation temperatures of ~44 K, a typical linear size of ~0.036 pc, and a typical beam averaged methanol abundance of several 10^(-9) (the beam corrected value could reach several 10^(-7)). The abundances of the latter three species, normalized by that of methanol, are found to be correlated also across a large variety of clouds such as EGO cloud cores, hot corinos, massive hot cores and Galactic Center clouds. The chemical properties of the EGO cloud cores lie between that of hot cores and hot corinos. However, the abundances and abundance ratios of the four species can not be satisfactorily explained by recent chemical models either among the EGO cloud cores or among the various types of cloud cores from literature.
    08/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Very-long-baseline interferometry (VLBI) monitoring of the 6.7 GHz methanol maser allows us to measure the internal proper motion of maser spots and therefore study the gas motion around high-mass young stellar objects. To this end, we have begun monitoring observations with the East-Asian VLBI Network. In this paper we present the results of the first epoch observation for 36 sources, including 35 VLBI images of the methanol maser. Since two independent sources were found in three images, images of 38 sources were obtained. In 34 sources, 10 or more spots were detected. The observed spatial scale of the maser distribution was from 9 to 4900 astronomical units, and the following morphological categories were observed: elliptical, arched, linear, paired, and complex. The position of the maser spot was determined with an accuracy of approximately 0.1 mas, which is sufficiently high to measure the internal proper motion from two years of monitoring observations. The VLBI observation, however, detected only approximately 20% of all maser emissions, suggesting that the remaining 80% of the total flux was spread into an undetectable extended distribution. Therefore, in addition to high-resolution observations, it is important to observe the whole structure of the maser emission including extended low-brightness structures, in order to reveal the associated site of the maser and gas motion.
    04/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Data for a number of OH maser lines have been collected from surveys. The posi- tions are compared to recent mid-infrared (MIR) surveys such as Spitzer-GLIMPSE and WISE, restricting the comparison to point sources. The colors and intensities of the IR sources are compared. There are many 18 cm OH masers, but far fewer in lines arising from higher energy levels. We also make a comparison with the 5 cm Class II methanol masers. We have divided the results into 3 subsamples: those associated with OH masers only, those associated with OH masers and Class II methanol masers, and those only associated with Class II methanol masers. There are no obvious dif- ferences in the color-color or color-magnitude results for the GLIMPSE point sources. However, according to the results from the WISE 22 {\mu}m survey, the sources associ- ated with OH masers are brighter than those associated with methanol masers. We interpret the presence of OH and methanol masers mark the locations of regions where stars are forming. The OH masers are located on the borders of sharp features found in the IR. These are referred to as bubbles. If the OH masers mark the positions of protostars, the result provides indirect evidence for triggered star formation caused by the expansion of the bubbles.
    Monthly Notices of the Royal Astronomical Society 04/2014; 441(4). · 5.52 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present the results of a 6.7 GHz methanol maser survey from the Effelsberg 100 m radio telescope. A sample of 404 sources from the Bolocam Galactic Plane Survey (BGPS) 1.1 mm dust clump survey that met specific Galactic Legacy Infrared Mid-Plane Survey Extraodinaire (GLIMPSE) point-source color criteria was selected and 318 of these were observed. The new observations resulted in the detection of 29 methanol masers, including 12 new ones. Together with the additional 74 detections from the literature, this means that a total of 103 methanol masers are coincident with 1.1 mm dust clumps, yielding an overall detection rate of 26%. A comparison of the properties of a 1.1 mm dust clump and a 6.7 GHz methanol maser indicates that methanol masers with a higher flux density and/or luminosity are generally associated with more massive but less dense 1.1 mm dust clumps. The overall detection rate of 26% appears to vary as a function of the derived H2 column density of the associated 1.1 mm dust clump. The methanol masers were primarily detected toward the brighter and more massive 1.1 mm dust clumps. A subsample of 194 sources that overlapped sources with observations of the 95 GHz methanol line was investigated in more detail for the properties of 1.1 mm dust clumps. The statistical analysis reveals that 1.1 mm dust clumps with both class I and II counterparts have much higher mean and median values of mass, column density, and flux density than those with only class I or II counterparts. Based on our much larger sample, we slightly revise the boundary defined previously for selecting BGPS sources associated with a class II methanol maser, wherein ~80% of expected class II methanol masers will be detected with a detection rate in the range of 40-50%.
    02/2014;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Very long baseline interferometry (VLBI) monitoring of the 6.7 GHz methanol maser allows us to measure the internal proper motions of the maser spots and therefore study the gas motion around high-mass young stellar objects. To this end, we have begun monitoring observations with the East-Asian VLBI Network. In this paper we present the results of the first epoch observation for 36 sources, including 35 VLBI images of the methanol maser. Since two independent sources were found in three images, respectively, images of 38 sources were obtained. In 34 sources, more than or equal to 10 spots were detected. The observed spatial scale of the maser distribution was from 9 to 4900 astronomical units, and the following morphological categories were observed: elliptical, arched, linear, paired, and complex. The position of the maser spot was determined to an accuracy of approximately 0.1 mas, sufficiently high to measure the internal proper motion from two years of monitoring observations. The VLBI observation, however, detected only approximately 20% of all maser emission, suggesting that the remaining 80% of the total flux was spread into an undetectable extended distribution. Therefore, in addition to high-resolution observations, it is important to observe the whole structure of the maser emission including extended low-brightness structures, to reveal the associated site of the maser and gas motion.
    11/2013;
  • [Show abstract] [Hide abstract]
    ABSTRACT: We have started a VLBI monitor project of the 6.7 GHz methanol maser sources by using the Japanese VLBI Network (JVN) and the East-Asian VLBI Network (EAVN). The 6.7 GHz methanol maser emission can be one of the best probe to obtain 3-dimensional (3-D) velocity information, particularly on an accretion disk around forming high-mass protostars. The systematic VLBI survey monitor can provide us a chance to understand the evolution of the accretion disk, which is one of the basic issues in the high-mass star formation. Our VLBI monitor project for the 6.7 GHz methanol masers has been started since August 2010 toward 36 sources, which are at mainly southern hemisphere, brighter than 65 Jy in total flux densities, and no previous VLBI observations (including two previous VLBI sources for verification of imaging capabilities). In this presentation, we will show a result of the JVN/EAVN imaging survey for all of 36 target sources obtained in 2010-2012 as an initial result of this project. In this imaging survey, spatial distributions of the methanol maser spots in 35 sources were obtained, in which 33 sources provide new VLBI images. The new images increase the VLBI imaged sample of the 6.7 GHz methanol maser by 1.5 times compared to that so far. The spatial morphology was classified into five categories on the basis of the criteria used in the previous VLBI observations (Bartkiewicz et al. 2009), including elliptical, arched, and linear morphology which could be the best candidates associated with the disk (Fujisawa et al. to be submitted). The VLBI imaging survey result was compared to spatial distributions observed with the Australia Telescope Compact Array (ATCA). The ATCA observations provided us the spatial distribution without a missing flux, and solved ambiguities for some sources classified into the complex spatial morphology on the basis of the VLBI survey. We also present an initial result for a detection of relative proper motions and 3-D velocity information in the elliptical methanol maser sources, G006.79-00.25. The detected motions suggest that the methanol masers showing the elliptical morphology can trace the rotating disk.
    07/2013;
  • [Show abstract] [Hide abstract]
    ABSTRACT: We have undertaken a survey of molecular lines in the 3 mm band toward 57 young stellar objects using the Australia Telescope National Facility Mopra 22 m radio telescope. The target sources were young stellar objects with active outflows (extended green objects (EGOs)) newly identified from the GLIMPSE II survey. We observe a high detection rate (50%) of broad line wing emission in the HNC and CS thermal lines, which combined with the high detection rate of class I methanol masers toward these sources (reported in Paper I) further demonstrates that the GLIMPSE II EGOs are associated with outflows. The physical and kinematic characteristics derived from the 3 mm molecular lines for these newly identified EGOs are consistent with these sources being massive young stellar objects with ongoing outflow activity and rapid accretion. These findings support our previous investigations of the mid-infrared properties of these sources and their association with other star formation tracers (e.g., infrared dark clouds, methanol masers and millimeter dust sources) presented in Paper I. The high detection rate (64%) of the hot core tracer CH3CN reveals that the majority of these new EGOs have evolved to the hot molecular core stage. Comparison of the observed molecular column densities with predictions from hot core chemistry models reveals that the newly identified EGOs from the GLIMPSE II survey are members of the youngest hot core population, with an evolutionary time scale of the order of 103 yr.
    The Astrophysical Journal Supplement Series 06/2013; 206(2):22. · 16.24 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We have produced a catalog containing 98 newly identified massive young stellar object (MYSO) candidates associated with ongoing outflows (known as extended green objects, or EGOs). These have been identified from the Spitzer Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) II data set and our new identifications increase the number of known EGOs to ~400 in our Galaxy, adding to the ~300 previously identified EGOs reported by Cyganowski et al. from the GLIMPSE I survey. The high detection rate (~70%) of 95 GHz class I methanol masers achieved in a survey toward 57 of these new EGOs with the Mopra 22 m radio telescope demonstrates that the new EGOs are associated with outflows. Investigations of the mid-infrared properties and physical associations with other star formation tracers (e.g., infrared dark clouds, class I and II methanol masers, and millimeter Bolocam Galactic Plane Survey sources) reveal that the newly identified EGOs are very similar in nature to those in the sample of Cyganowski et al. All of the observational evidence supports the hypothesis that EGOs correspond to MYSOs at the earliest evolutionary stage, with ongoing outflow activity, and active rapid accretion.
    The Astrophysical Journal Supplement Series 05/2013; 206(1):9. · 16.24 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We performed polarization sensitive VLBI observations of 6.7 GHz methanol masers toward high-mass young stellar objects with clear outflow seen from Spitzer IRAC images in the 4.5 μm band (i.e. EGOs, see Cyganowski et al. 2008) with the EVN to investigate the birthplace of the masers. By comparing direction of the major axis of methanol maser distributions with directions of higher resolution outflow and magnetic field vector, we suggest that the methanol masers toward source G28.83-0.25 may arise from surrounding disk.
    Proceedings of the International Astronomical Union 03/2013; 8(S292):42-42.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We have observed a sample of 288 molecular outflow sources including 123 high-mass and 165 low-mass sources to search for class I methanol masers at 95 GHz transition and to investigate relationship between outflow characteristics and class I methanol maser emission with the PMO-13.7m radio telescope. Our survey detected 62 sources with 95 GHz methanol masers above 3$\sigma$ detection limit, which include 47 high-mass sources and 15 low-mass sources. Therefore the detection rate is 38% for high-mass outflow sources and 9% for low-mass outflow sources, suggesting that class I methanol maser is relatively easily excited in high-mass sources. There are 37 newly detected 95 GHz methanol masers (including 27 high-mass and 10 low-mass sources), 19 of which are newly identified (i.e. first identification) class I methanol masers (including 13 high-mass and 6 low-mass sources). Statistical analysis for the distributions of maser detections with the outflow parameters reveals that the maser detection efficiency increases with outflow properties (e.g. mass, momentum, kinetic energy and mechanical luminosity of outflows etc.). Systematic investigations of relationships between the intrinsic luminosity of methanol maser and the outflow properties (including mass, momentum, kinetic energy, bolometric luminosity and mass loss rate of central stellar sources) indicate a positive correlations. This further supports that class I methanol masers are collisionally pumped and associated with shocks, where outflows interact with the surrounding ambient medium.
    The Astrophysical Journal 11/2012; 763(1). · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report a survey with the Purple Mountain Observatory (PMO) 13.7-m radio telescope for class I methanol masers from the 95 GHz (8_0 - 7_1 A^+) transition. The 214 target sources were selected by combining information from both the Spitzer GLIMPSE and 1.1 mm BGPS survey catalogs. The observed sources satisfy both the GLIMPSE mid-IR criteria of [3.6]-[4.5]>1.3, [3.6]-[5.8]>2.5, [3.6]-[8.0]>2.5 and 8.0 um magnitude less than 10, and also have an associated 1.1 mm BGPS source. Class I methanol maser emission was detected in 63 sources, corresponding to a detection rate of 29% for this survey. For the majority of detections (43), this is the first identification of a class I methanol maser associated with these sources. We show that the intensity of the class I methanol maser emission is not closely related to mid-IR intensity or the colors of the GLIMPSE point sources, however, it is closely correlated with properties (mass and beam-averaged column density) of the BGPS sources. Comparison of measures of star formation activity for the BGPS sources with and without class I methanol masers indicate that the sources with class I methanol masers usually have higher column density and larger flux density than those without them. Our results predict that the criteria log(S_{int})<-38.0+1.72log(N_{H_{2}}^{beam}) and log(N_{H_{2}}^{beam})>22.1, which utilizes both the integrated flux density (S_{int}) and beam-averaged column density (N_{H_{2}}^{beam}) of the BGPS sources, are very efficient for selecting sources likely to have an associated class I methanol maser. Our expectation is that searches using these criteria will detect 90% of the predicted number of class I methanol masers from the full BGPS catalog (~ 1000), and do so with a high detection efficiency (~75%).
    The Astrophysical Journal Supplement Series 02/2012; 200(1). · 16.24 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report the results of a systematic survey for 95 GHz class I methanol masers towards a new sample of 192 massive young stellar object (MYSO) candidates associated with ongoing outflows (known as extended green objects or EGOs) identified from the Spitzer GLIMPSE survey. The observations were made with the Australia Telescope National Facility (ATNF) Mopra 22-m radio telescope and resulted in the detection of 105 new 95 GHz class I methanol masers. For 92 of the sources our observations provide the first identification of a class I maser transition associated with these objects (i.e. they are new class I methanol maser sources). Our survey proves that there is indeed a high detection rate (55%) of class I methanol masers towards EGOs. Comparison of the GLIMPSE point sources associated with EGOs with and without class I methanol maser detections shows they have similar mid-IR colors, with the majority meeting the color selection criteria -0.6<[5.8]-[8.0]<1.4 and 0.5<[3.6]-[4.5]<4.0. Investigations of the IRAC and MIPS 24 um colors and the associated millimeter dust clump properties (mass and density) of the EGOs for the sub-samples based on which class of methanol masers they are associated with suggests that the stellar mass range associated with class I methanol masers extends to lower masses than for class II methanol masers, or alternatively class I methanol masers may be associated with more than one evolutionary phase during the formation of a high-mass star.
    The Astrophysical Journal Supplement Series 07/2011; 196(1). · 16.24 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The primary aim of this project is to use VLBI observations to measure accurate distances to and proper motions of 6.7 GHz methanol masers in the third and fourth quadrants of the Galaxy. These observations will allow us to improve our knowledge of the structure of the Galaxy and complements a similar study being done in the northern hemisphere looking at 12.2 GHz methanol masers in the first and second quadrants. The northern observations suggest high-mass star formation regions on average orbit the Galaxy 15 km/s more slowly than expected from circular orbit models and measure a circular rotation speed for the Sun of 254 km/s. These results are however, derived from limited data and remain controversial. More measurements of parallax and proper motion for high-mass star forming regions in the southern sky of the Milky Way are critical to better constrain and confirm these results.
    ATNF Proposal. 04/2010;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report the first systematic survey of molecular lines (including HCO+ (1-0) and 12CO, 13CO, C18O (1-0) lines at 3 mm band) towards a new sample of 88 massive young stellar object (MYSO) candidates associated with ongoing outflows (known as extended green objects or EGOs) identified from the Spitzer GLIMPSE survey in the northern hemisphere with the PMO-13.7 m radio telescope. By analyzing the asymmetries of the optically thick line HCO+ for 69 of 72 EGOs with HCO+ detection, we found 29 sources with blue asymmetric profiles and 19 sources with red asymmetric profiles. This results in a blue excess of 0.14, seen as a signature of collapsing cores in the observed EGO sample. The relatively small blue excess measured in our full sample due to that the observed EGOs are mostly dominated by outflows and at an earlier evolutionary phase associated with IRDCs and 6.7 GHz methanol masers. The physical properties of clouds surrounding EGOs derived from CO lines are similar to those of massive clumps wherein the massive star forming cores associated with EGOs possibly embedded. The infall velocities and mass infall rates derived for 20 infall candidates are also consistent with the typical values found in MYSOs. Thus our observations further support the speculation of Cyganowski et al. (2008) that EGOs trace a population with ongoing outflow activity and active rapid accretion stage of massive protostellar evolution from a statistical view, although there maybe have limitations due to single-pointing survey with a large beam. Comment: 44 pages, 4 figures, accepted for publication in ApJ
    The Astrophysical Journal 12/2009; · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We have compared the results of a number of published class I methanol maser surveys with the catalogue of high-mass outflow candidates identified from the GLIMPSE survey (known as extended green objects or EGOs). We find class I methanol masers associated with approximately two-thirds of EGOs. Although the association between outflows and class I methanol masers has long been postulated on the basis of detailed studies of a small number of sources, this result demonstrates the relationship for the first time on a statistical basis. Despite the publication of a number of searches for class I methanol masers, a close physical association with another astrophysical object which could be targeted for the search is still lacking. The close association between class I methanol masers and EGOs therefore provides a large catalogue of candidate sources, most of which have not previously been searched for class I methanol masers. Interstellar masers and outflows have both been proposed to trace an evolutionary sequence for high-mass star formation, therefore a better understanding of the relationship between class I methanol masers and outflow offers the potential for comparison and amalgamation of these two evolutionary sequences.
    04/2009;
  • [Show abstract] [Hide abstract]
    ABSTRACT: This project will use the ATNF Mopra 22-m antenna to perform a systemic 3 mm spectral line survey toward a new sample (total 269 sources) of massive young stellar objects. These objects, called Extended Green Objects (EGOs) have been identified from the Spitzer GLIMPSE survey as being associated with ongoing outflows. These observations will mainly focus on: 1) A search for class I 95 GHz methanol masers in outflow environments associated with EGOs, and determine whether the detection rate of class I methanol masers are higher in outflow environment as is predicted by theory; 2) to compare with the results from our ongoing project searching for class II 6.7 GHz methanol masers with the University of Tasmania radio telescopes, and investigate the relationship between class I and class II methanol masers; 3) search for evidence of outflow and infall in EGOs through observations of optically thick lines (CS and HNC) and estimate the cloud parameters (e.g. temperature and column density) through a series of CH3CN lines. The combination of these three will help us understand the massive star formation process and dynamics.
    ATNF Proposal. 04/2009;
  • Source
    Xi Chen, Zhi-Qiang Shen
    [Show abstract] [Hide abstract]
    ABSTRACT: We report the first Very Long Baseline Array (VLBA) observations of 43 GHz v=1, J=1-0 SiO masers in the circumstellar envelope of the M-type semi-regular supergiant variable star AH Sco at 2 epochs separated by 12 days in March 2004. These high-resolution VLBA images reveal that the distribution of SiO masers is roughly on a persistent elliptical ring with the lengths of the major and minor axes of about 18.5 and 15.8 mas, respectively, along a position angle of 150^{\circ}. And the red-shifted masers are found to be slightly closer to the central star than the blue-shifted masers. The line-of-sight velocity structure of the SiO masers shows that with respect to the systemic velocity of -6.8 km/s the higher velocity features are closer to the star, which can be well explained by the simple outflow or infall without rotation kinematics of SiO masers around AH Sco. Study of proper motions of 59 matched features between two epochs clearly indicates that the SiO maser shell around AH Sco was undergoing an overall contraction to the star at a velocity of ~13 km/s at a distance of 2.26 kpc to AH Sco. Our 3-dimensional maser kinematics model further suggests that such an inward motion is very likely due to the gravitation of the central star. The distance to AH Sco of 2.26$\pm$0.19 kpc obtained from the 3-dimensional kinematics model fitting is consistent with its kinematic distance of 2.0 kpc. Comment: 23 pages, 4 figures, 4 tables, accepted for publication in ApJ
    The Astrophysical Journal 03/2008; · 6.73 Impact Factor
  • Source
    Xi Chen, Zhi-Qiang Shen, Ye Xu
    [Show abstract] [Hide abstract]
    ABSTRACT: We report on 43 GHz v=1, J=1–0 SiO maser proper motions in the circumstellar envelope of the M-type semi-regular variable star VX Sgr, observed by Very Long Baseline Array (VLBA) at 3 epochs during 1999 April–May. Applying the statistical parallax analysis to these proper motions, we estimated a distance of VX Sgr of 1.57±0.27 kpc, which is consistent with that based on the proper motions of H 2 O masers, or on the assumption that VX Sgr belongs to the Sgr OB1 association. At this distance, VX Sgr can be classified as a red supergiant. Comparing the statistical parallax method with those of model fitting and annual parallax, we think that the statistical parallax method may be a good way of estimating SiO maser distances at present.
    Chinese Journal of Astronomy and Astrophysics 01/2007; 7:531-538. · 0.89 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The 12CO (J=1-0), 13CO (J=1-0) and C18O (J=1-0) emissions in 9 massive star forming regions, which are believed to be at different stages of massive star formation, were mapped with the 13.7 m millimeter wave telescope at Qinghai Station of Purple Mountain Observatory. Of the observed 9 sources, 13CO cores were detected in seven of them, and C18O cores in five of them. And only two sources associated with C18O cores and H2O masers showed the extended structures and strong outflows. This is the first detection of outflow associated with IRAS 22566+5828 in the observing field of S152/S153. The physical parameters of cores and outflows for these sources, derived from Local Thermal Equilibrium (LTE) analysis, are presented. These observing results suggest that the C18O cores will only appear when the gas density is high enough and the probability to have an outflow is very high when the clumps show the C18O and H2O maser simultaneously.
    Science in China Series G Physics Mechanics and Astronomy 09/2006; 49(5):626-640. · 1.41 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Mapping observations were made towards IRAS 20110+3321 using the Nobeyama 45 m and the Delingha 13.7 m radio telescopes. The high angular resolution (~21'') image with the 45 m telescope shows that there is a high-velocity bipolar molecular outflow in this region, which is in the NW SE direction with a collimation factor of ~ 2.2. The outflow has significantly higher mass loss rate and mechanical luminosity than those from low mass YSOs, indicating that the outflow is driven by the IRAS source. A dense massive core was detected by mapping C18O (J = 1-0) line in the area with the 13.7 m telescope. The IRAS source lies within the core but slightly offsets from its emission peak.
    Chinese Physics Letters 01/2006; 23(7):1958-1960. · 0.92 Impact Factor

Publication Stats

35 Citations
100.63 Total Impact Points

Institutions

  • 2006–2014
    • Chinese Academy of Sciences
      • • Shanghai Astronomical Observatory
      • • Galaxies, Cosmology Research Department
      Peping, Beijing, China
  • 2009–2013
    • Northeast Institute of Geography and Agroecology
      • Galaxies, Cosmology Research Department
      Peping, Beijing, China
  • 2007
    • National Taiwan Normal University
      T’ai-pei, Taipei, Taiwan