Xuepeng Chen

Chinese Academy of Sciences, Peping, Beijing, China

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Publications (12)66.55 Total impact

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    ABSTRACT: We present high angular resolution 1.3 mm and 850 μm dust continuum data obtained with the Submillimeter Array toward 33 Class 0 protostars in nearby clouds (distance < 500 pc), which represents so far the largest survey toward protostellar binary/multiple systems. The median angular resolution in the survey is 2.''5, while the median linear resolution is approximately 600 AU. Compact dust continuum emission is observed from all sources in the sample. Twenty-one sources in the sample show signatures of binarity/multiplicity, with separations ranging from 50 AU to 5000 AU. The numbers of singles, binaries, triples, and quadruples in the sample are 12, 14, 5, and 2, respectively. The derived multiplicity frequency (MF) and companion star fraction (CSF) for Class 0 protostars are 0.64 ± 0.08 and 0.91 ± 0.05, respectively, with no correction for completeness. The derived MF and CSF in this survey are approximately two times higher than the values found in the binary surveys toward Class I young stellar objects, and approximately three (for MF) and four (for CSF) times larger than the values found among main-sequence stars, with a similar range of separations. Furthermore, the observed fraction of high-order multiple systems to binary systems in Class 0 protostars (0.50 ± 0.09) is also larger than the fractions found in Class I young stellar objects (0.31 ± 0.07) and main-sequence stars (≤0.2). These results suggest that binary properties evolve as protostars evolve, as predicted by numerical simulations. The distribution of separations for Class 0 protostellar binary/multiple systems shows a general trend in which CSF increases with decreasing companion separation. We find that 67% ± 8% of the protobinary systems have circumstellar mass ratios below 0.5, implying that unequal-mass systems are preferred in the process of binary star formation. We suggest an empirical sequential fragmentation picture for binary star formation, based on this work and existing lower resolution single-dish observations.
    The Astrophysical Journal 04/2013; 768(2):110. · 6.73 Impact Factor
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    ABSTRACT: We present 230 GHz Submillimeter Array continuum and molecular line observations of the newly discovered FUor candidate HBC722. We report the detection of seven 1.3 mm continuum sources in the vicinity of HBC722, none of which correspond to HBC722 itself. We compile infrared and submillimeter continuum photometry of each source from previous studies and conclude that three are Class 0 embedded protostars, one is a Class I embedded protostar, one is a Class I/II transition object, and two are either starless cores or very young, very low luminosity protostars or first hydrostatic cores. We detect a northwest-southeast outflow, consistent with the previous detection of such an outflow in low-resolution, single-dish observations, and note that its axis may be precessing. We show that this outflow is centered on and driven by one of the nearby Class 0 sources rather than HBC722, and find no conclusive evidence that HBC722 itself is driving an outflow. The non-detection of HBC722 in the 1.3 mm continuum observations suggests an upper limit of 0.02 solar masses for the mass of the circumstellar disk. This limit is consistent with typical T Tauri disks and with a disk that provides sufficient mass to power the burst.
    The Astrophysical Journal 06/2012; 755(2). · 6.73 Impact Factor
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    ABSTRACT: We present high angular resolution SMA and Spitzer observations toward the Bok globule CB17. SMA 1.3mm dust continuum images reveal within CB17 two sources with an angular separation of about 21" (about 5250 AU at a distance of 250 pc). The northwestern continuum source, referred to as CB17 IRS, dominates the infrared emission in the Spitzer images, drives a bipolar outflow extending in the northwest-southeast direction, and is classified as a low luminosity Class0/I transition object (L_bol ~ 0.5 L_sun). The southeastern continuum source, referred to as CB17 MMS, has faint dust continuum emission in the SMA 1.3mm observations (about 6 sigma detection; ~3.8 mJy), but is not detected in the deep Spitzer infrared images at wavelengths from 3.6 to 70 micron. Its bolometric luminosity and temperature, estimated from its spectral energy distribution, are less than 0.04 L_sun and 16 K, respectively. The SMA CO(2-1) observations suggest that CB17 MMS may drive a low-velocity molecular outflow (about 2.5 km/s), extending in the east-west direction. Comparisons with prestellar cores and Class0 protostars suggest that CB17 MMS is more evolved than prestellar cores but less evolved than Class0 protostars. The observed characteristics of CB17 MMS are consistent with the theoretical predictions from radiative/magneto hydrodynamical simulations of a first hydrostatic core, but there is also the possibility that CB17 MMS is an extremely low luminosity protostar deeply embedded in an edge-on circumstellar disk. Further observations are needed to study the properties of CB17 MMS and to address more precisely its evolutionary stage.
    03/2012;
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    ABSTRACT: We present high angular resolution observations of the Class 0 protostar IRAM04191+1522, using the Submillimeter Array (SMA). The SMA 1.3 mm continuum images reveal within IRAM04191+1522 two distinct sources with an angular separation of 7.8\,$\pm$\,0.2$"$. The two continuum sources are located in the southeast-northwest direction, with total gas masses of about 0.011 M_sun and about 0.005 M_sun, respectively. The southeastern source, associated with an infrared source seen in the Spitzer images, is the well-known Class 0 protostar with a bolometric luminosity of about 0.08 L_sun. The newly-discovered northwestern continuum source is not visible in the Spitzer images at wavelengths from 3.6 to 70 micron, and has an extremely low bolometric luminosity (< 0.03 L_sun). Complementary IRAM N2H+(1-0) data that probe the dense gas in the common envelope suggest that the two sources were formed through the rotational fragmentation of an elongated dense core. Furthermore, comparisons between IRAM04191+1522 and other protostars suggest that most cores with binary systems formed therein have ratios of rotational energy to gravitational energy $\beta_{\rm rot}$ > 1%. This is consistent with theoretical simulations and indicates that the level of rotational energy in a dense core plays an important role in the fragmentation process.
    The Astrophysical Journal Letters 02/2012; 747(2). · 6.35 Impact Factor
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    ABSTRACT: We present new 230 GHz Submillimeter Array observations of the candidate first hydrostatic core Per-Bolo 58. We report the detection of a 1.3 mm continuum source and a bipolar molecular outflow, both centered on the position of the candidate first hydrostatic core. The continuum detection has a total flux density of 26.6 +/- 4.0 mJy, from which we calculate a total (gas and dust) mass of 0.11 +/- 0.05 Msun and a mean number density of 2.0 +/- 1.6 X 10^7 cm-3. There is some evidence for the existence of an unresolved component in the continuum detection, but longer-baseline observations are required in order to confirm the presence of this component and determine whether its origin lies in a circumstellar disk or in the dense inner envelope. The bipolar molecular outflow is observed along a nearly due east-west axis. The outflow is slow (characteristic velocity of 2.9 km/s), shows a jet-like morphology (opening semi-angles ~8 degrees for both lobes), and extends to the edges of the primary beam. We calculate the kinematic and dynamic properties of the outflow in the standard manner and compare them to several other protostars and candidate first hydrostatic cores with similarly low luminosities. We discuss the evidence both in support of and against the possibility that Per-Bolo 58 is a first hydrostatic core, and we outline future work needed to further evaluate the evolutionary status of this object.
    The Astrophysical Journal 08/2011; 742(1). · 6.73 Impact Factor
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    Xuepeng Chen, Héctor G. Arce
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    ABSTRACT: We report the discovery of multiple condensations in the prestellar core candidate SMM 1A in the R Corona Australis cloud, which may represent the earliest phase of core fragmentation observed thus far. The separation between the condensations is between 1000 and 2100 AU, and their masses range from about 0.1 to 0.2 M ☉. We find that the three condensations have extremely low bolometric luminosities (<0.1 L ☉) and temperatures (<20 K), indicating that these are young sources that have yet to form protostars. We suggest that these sources were formed through the fragmentation of an elongated prestellar core. Our results, in concert with other observed protostellar binary systems with separations in the scale of 1000 AU, support the scenario that prompt fragmentation in the isothermal collapse phase is an efficient mechanism for wide binary star formation, while the fragmentation in the subsequent adiabatic phase may be an additional mechanism for close (≤100 AU) binary star formation.
    The Astrophysical Journal Letters 08/2010; 720(2):L169. · 6.35 Impact Factor
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    ABSTRACT: Intermediate between the prestellar and Class 0 protostellar phases, the first core is a quasi-equilibrium hydrostatic object with a short lifetime and an extremely low luminosity. Recent magnetohydrodynamic (MHD) simulations suggest that the first core can even drive a molecular outflow before the formation of the second core (i.e., protostar). Using the Submillimeter Array and the Spitzer Space Telescope, we present high angular resolution observations toward the embedded dense core IRS2E in L1448. We find that source L1448 IRS2E is not visible in the sensitive Spitzer infrared images (at wavelengths from 3.6 to 70 μm) and has weak (sub-) millimeter dust continuum emission. Consequently, this source has an extremely low bolometric luminosity (<0.1 L ☉). Infrared and (sub-) millimeter observations clearly show an outflow emanating from this source; L1448 IRS2E represents thus far the lowest luminosity source known to be driving a molecular outflow. Comparisons with prestellar cores and Class 0 protostars suggest that L1448 IRS2E is more evolved than prestellar cores but less evolved than Class 0 protostars, i.e., at a stage intermediate between prestellar cores and Class 0 protostars. All these results are consistent with the theoretical predictions of the radiative/MHD simulations, making L1448 IRS2E the most promising candidate of the first hydrostatic core revealed so far.
    The Astrophysical Journal 05/2010; 715(2):1344. · 6.73 Impact Factor
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    ABSTRACT: We present millimeter (mm) interferometric observations of the young stellar object SVS 13 in NCG 1333 in the N2H+ (1-0) line and at 1.4 mm and 3 mm dust continuum, using the IRAM Plateau de Bure interferometer. The results are complemented by infrared data from the Spitzer Space Telescope. The mm dust continuum images resolve four sources (A, B, C, and VLA 3) in SVS 13. With the dust continuum images, we derive gas masses of 0.2-1.1 M sun for the sources. N2H+ (1-0) line emission is detected and spatially associated with the dust continuum sources B and VLA 3. The observed mean line width is ~ 0.48 km s-1 and the estimated virial mass is ~ 0.7 M sun. By simultaneously fitting the seven hyperfine line components of N2H+, we derive the velocity field and find a symmetric velocity gradient of ~ 28 km s-1 pc-1 across sources B and VLA 3, which could be explained by core rotation. The velocity field suggests that sources B and VLA 3 are forming a physically bound protobinary system embedded in a common N2H+ core. Spitzer images show mid-infrared emission from sources A and C, which is spatially associated with the mm dust continuum emission. No infrared emission is detected from source B, implying that the source is deeply embedded. Based on the morphologies and velocity structure, we propose a hierarchical fragmentation picture for SVS 13 where the three sources (A, B, and C) were formed by initial fragmentation of a filamentary prestellar core, while the protobinary system (sources B and VLA 3) was formed by rotational fragmentation of a single collapsing subcore.
    The Astrophysical Journal 01/2009; · 6.73 Impact Factor
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    ABSTRACT: Binary and higher-order multiple stellar systems represent the preferred outcome of the star formation process, but at present we do not understand well how this occurs. Our current knowledge on binary star formation mostly relies on observations of main sequence and pre-main sequence stars and the constraints they put on the theoretical models. Direct observations of the earliest, embedded phase of binary star formation were long limited by the low angular resolution of single-dish telescopes. Only the recent advance of large (sub-)millimeter interferometers has enabled us to directly observe the formation phase of binary stars, although the number of known and well-studied systems is still very small. To search for protostellar binary systems and to derive their kinematic properties, we have started a systematic program to observe, at high angular resolution, a number of low-mass protostellar cores. The initial survey was conducted at the OVRO mm array, and is now continued with the ATCA, IRAM-PdBI, and SMA arrays. The observations were mainly carried out at mm dust continuum and in the N2H+ and CO molecular lines. With the mm dust continuum images, we identified several protobinary candidates and derived their circumstellar mass distribution, density profiles, and mass ratios. Based on the molecular lines data, we could, for the first time, derive the velocity fields and the distribution of specific angular momentum of the protobinary systems, and kinematically study the fragmentation process. In this talk we present the main results of this program.
    01/2009;
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    ABSTRACT: We present interferometric observations in the 12CO (2-1) line and at 1.3 mm dust continuum of the low-mass protostellar binary system in the cometary globule CG 30, using the Submillimeter Array. The dust continuum images resolve two compact sources (CG 30N and CG 30S), with a linear separation of ~8700 AU and total gas masses of ~1.4 and ~0.6 Msolar, respectively. With the CO images, we discover two high-velocity bipolar molecular outflows, driven by the two sources. The two outflows are nearly perpendicular to each other, showing a quadrupolar morphology. The northern bipolar outflow extends along the southeast (redshifted, with a velocity up to ~23 km s-1) and northwest (blueshifted, velocity up to ~30 km s-1) directions, while the southern pair has an orientation from southwest (blueshifted, velocity up to ~13 km s-1) to northeast (redshifted, velocity up to ~41 km s-1). The outflow mass of the northern pair, driven by the higher mass source CG 30N, is ~9 times larger than that of the southern pair. The discovery of the quadrupolar molecular outflow in the CG 30 protobinary system, as well as the presence of other quadrupolar outflows associated with binary systems, demonstrate that the disks in (wide) binary systems are not necessarily co-aligned after fragmentation.
    The Astrophysical Journal 09/2008; · 6.73 Impact Factor
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    ABSTRACT: We present interferometric observations of the isolated, low-mass protostellar double cores CG 30 and BHR 71 in the N2H+ (1-0) line and at 3 mm dust continuum, using the Australian Telescope Compact Array (ATCA). The results are complemented by infrared data from the Spitzer Space Telescope. In CG 30, the 3 mm dust-continuum images resolve two compact sources with a separation of ~21.7" (~8700 AU). In BHR 71, strong dust-continuum emission is detected at the position of the mid-infrared source IRS 1, while only weak emission is detected from the secondary mid-infrared source IRS 2. Assuming the standard gas-to-dust ratio and optically thin 3 mm dust-continuum emission, we derive hydrogen gas masses of 0.05-2.1 Msolar for the four subcores. N2H+ (1-0) line emission is detected in both CG 30 and BHR 71. By simultaneously fitting the seven hyperfine line components of N2H+, we derive the velocity fields and find symmetric velocity gradients in both sources. Estimated virial masses of the subcores range from 0.1 to 0.6 Msolar. Spitzer images show the mid-infrared emission from all four subcores, which is spatially associated with the 3 mm dust-continuum emission. All four sources appear to drive their own outflows, as seen in the Spitzer 4.5 mum images. Based on the ATCA and Spitzer observations, we construct spectral energy distributions (SEDs) and derive temperatures and luminosities for all cores. We suggest that the subcores in CG 30 were formed by initial fragmentation of a filamentary prestellar core, while those in BHR 71 could originate from rotational fragmentation of a single collapsing protostellar core.
    The Astrophysical Journal 01/2008; · 6.73 Impact Factor
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    ABSTRACT: We present the results of an interferometric study of the N2H+(1--0) emission from nine nearby, isolated, low-mass protostellar cores, using the OVRO millimeter array. The main goal of this study is the kinematic characterization of the cores in terms of rotation, turbulence, and fragmentation. Eight of the nine objects have compact N2H+ cores with FWHM radii of 1200 -- 3500 AU, spatially coinciding with the thermal dust continuum emission. The only more evolved (Class I) object in the sample (CB 188) shows only faint and extended N2H+ emission. The mean N2H+ line width was found to be 0.37 km/s. Estimated virial masses range from 0.3 to 1.2 M_sun. We find that thermal and turbulent energy support are about equally important in these cores, while rotational support is negligible. The measured velocity gradients across the cores range from 6 to 24 km/s/pc. Assuming these gradients are produced by bulk rotation, we find that the specific angular momenta of the observed Class 0 protostellar cores are intermediate between those of dense (prestellar) molecular cloud cores and the orbital angular momenta of wide PMS binary systems. There appears to be no evolution (decrease) of angular momentum from the smallest prestellar cores via protostellar cores to wide PMS binary systems. In the context that most protostellar cores are assumed to fragment and form binary stars, this means that most of the angular momentum contained in the collapse region is transformed into orbital angular momentum of the resulting stellar binary systems. Comment: 35 pages, 9 figures (one in color), 6 tables. Accepted by ApJ (to appear in Nov. 2007)
    The Astrophysical Journal 07/2007; · 6.73 Impact Factor

Publication Stats

98 Citations
66.55 Total Impact Points

Institutions

  • 2013
    • Chinese Academy of Sciences
      • Purple Mountain Observatory
      Peping, Beijing, China
  • 2010–2012
    • Yale University
      • Department of Astronomy
      New Haven, Connecticut, United States
  • 2007–2009
    • Max Planck Institute for Astronomy
      Heidelburg, Baden-Württemberg, Germany