arXiv subjects

Qizhou Zhang

Publications and source records attributed to Qizhou Zhang.

At least 289 records · Page 16Linked to original sources

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

Outflows, Accretion, and Clustered Protostellar Cores around a Forming O Star

We present a Submillimeter Array study in the 1.3 mm waveband of the NGC 7538 IRS 1--3 massive star-forming region. The brightest core in the mm continuum map, MM1, harbors the IRS 1 young O star. The core has a gas temperature of about 245 K and shows spatially unresolved emission in complex organic molecules, all typical of a hot molecular core. Toward MM1, redshifted absorption is seen in molecular lines with different energies above the ground state. This absorption probes inward motion of the dense gas toward the central young O star, and the estimated mass accretion rate reaches 10^{-3} Msun/yr. Multiple outflows are seen in the CO and 13CO maps. The gas mass of 50 Msun and mass outflow rate of 2.5 by 10^{-3} Msun/yr measured in CO line wings are dominated by the MM1 outflow, which is most likely driven by a fast wide-angle wind. Apart from MM1, we discover eight new dusty cores, MM2--9, within a projected distance of 0.35 pc from MM1. These cores show no counterpart in infrared or radio continuum emission, while seven of them appear to be forming intermediate- to high-mass stars. This manifests a deeply embedded star-forming component of the parent cloud of IRS 1--3. Apparently we are observing a Trapezium system in formation, and the system is presumably surrounded by a cluster of lower mass stars.

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arXiv2010

Is protostellar heating sufficient to halt fragmentation? A case study of the massive protocluster G8.68-0.37

If star formation proceeds by thermal fragmentation and the subsequent gravitational collapse of the individual fragments, how is it possible to form fragments massive enough for O and B stars in a typical star-forming molecular cloud where the Jeans mass is about 1Msun at the typical densities (10^4 cm^-3) and temperatures (10K)? We test the hypothesis that a first generation of low-mass stars may heat the gas enough that subsequent thermal fragmentation results in fragments >=10Msun, sufficient to form B stars. We combine ATCA and SMA observations of the massive star-forming region G8.68-0.37 with radiative transfer modeling to derive the present-day conditions in the region and use this to infer the conditions in the past, at the time of core formation. Assuming the current mass/separation of the observed cores equals the fragmentation Jeans mass/length and the region's average density has not changed, requires the gas temperature to have been 100K at the time of fragmentation. The postulated first-generation of low-mass stars would still be around today, but the number required to heat the cloud exceeds the limits imposed by the observations. Several lines of evidence suggest the observed cores in the region should eventually form O stars yet none have sufficient raw material. Even if feedback may have suppressed fragmentation, it was not sufficient to halt it to this extent. To develop into O stars, the cores must obtain additional mass from outside their observationally defined boundaries. The observations suggest they are currently fed via infall from the very massive reservoir (~1500Msun) of gas in the larger pc scale cloud around the star-forming cores. This suggests that massive stars do not form in the collapse of individual massive fragments, but rather in smaller fragments that themselves continue to gain mass by accretion from larger scales.

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arXiv2010

High Velocity Molecular Outflows In Massive Cluster Forming Region G10.6-0.4

We report the arcsecond resolution SMA observations of the $^{12}$CO (2-1) transition in the massive cluster forming region G10.6-0.4. In these observations, the high velocity $^{12}$CO emission is resolved into individual outflow systems, which have a typical size scale of a few arcseconds. These molecular outflows are energetic, and are interacting with the ambient molecular gas. By inspecting the shock signatures traced by CH$_{3}$OH, SiO, and HCN emissions, we suggest that abundant star formation activities are distributed over the entire 0.5 pc scale dense molecular envelope. The star formation efficiency over one global free-fall timescale (of the 0.5 pc molecular envelope, $\sim10^{5}$ years) is about a few percent. The total energy feedback of these high velocity outflows is higher than 10$^{47}$ erg, which is comparable to the total kinetic energy in the rotational motion of the dense molecular envelope. From order-of-magnitude estimations, we suggest that the energy injected from the protostellar outflows is capable of balancing the turbulent energy dissipation. No high velocity bipolar molecular outflow associated with the central OB cluster is directly detected, which can be due to the photo-ionization.

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arXiv2010

The magnetic field in the NGC 2024 FIR 5 dense core

We used the Submillimeter Array (SMA) to observe the thermal polarized dust emission from the protostellar source NGC 2024 FIR 5. The polarized emission outlines a partial hourglass morphology for the plane-of-sky component of the core magnetic field. Our data are consistent with previous BIMA maps, and the overall magnetic field geometries obtained with both instruments are similar. We resolve the main core into two components, FIR 5A and FIR 5B. A possible explanation for the asymmetrical field lies in depolarization effects due to the lack of internal heating from FIR 5B source, which may be in a prestellar evolutionary state. The field strength was estimated to be 2.2 mG, in agreement with previous BIMA data. We discuss the influence of a nearby H{\sc ii} region over the field lines at scales of $\sim 0.01$ pc. Although the hot component is probably compressing the molecular gas where the dust core is embedded, it is unlikely that the radiation pressure exceeds the magnetic tension. Finally, a complex outflow morphology is observed in CO (3 $\rightarrow$ 2) maps. Unlike previous maps, several features associated with dust condensations other than FIR 5 are detected.

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arXiv2010

From the Convergence of Filaments to Disk-Outflow Accretion: Massive-Star Formation in W33A

Interferometric observations of the W33A massive star-formation region, performed with the Submillimeter Array (SMA) and the Very Large Array (VLA) at resolutions from 5 arcsec (0.1 pc) to 0.5 arcsec (0.01 pc) are presented. Our three main findings are: (1) parsec-scale, filamentary structures of cold molecular gas are detected. Two filaments at different velocities intersect in the zone where the star formation is occurring. This is consistent with triggering of the star-formation activity by the convergence of such filaments, as predicted by numerical simulations of star formation initiated by converging flows. (2) The two dusty cores (MM1 and MM2) at the intersection of the filaments are found to be at different evolutionary stages, and each of them is resolved into multiple condensations. MM1 and MM2 have markedly different temperatures, continuum spectral indices, molecular-line spectra, and masses of both stars and gas. (3) The dynamics of the "hot-core" MM1 indicates the presence of a rotating disk in its center (MM1-Main) around a faint free-free source. The stellar mass is estimated to be approximately 10 Msun. A massive molecular outflow is observed along the rotation axis of the disk.

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arXiv2010

The Decrease of Specific Angular Momentum and the Hot Toroid Formation: The Massive Clump G10.6-0.4

This is the first paper of our series of high resolution (1") studies of the massive star forming region G10.6--0.4. We present the emission line observations of the hot core type tracers (O$^{13}$CS, OCS, SO$_{2}$) with $\sim$0$"$.5 resolution. By comparing the results to the high--resolution NH$_{3}$ absorption line observation, we confirm for the first time the rotationally flattened hot toroid in the central $<$0.1 pc region, which has a rotational axis perpendicular to its geometrical major axis. In addition, we present the observations of NH$_{3}$, $^{13}$CS, and CH$_{3}$CN with $\sim$1$"$ resolution, and follow the dynamics of the molecular accretion flow from the 0.3 pc radius to the inner 0.03 pc radius. With reference to the rotational axis of the hot toroid, we measure the rotational velocity from the molecular emission in the region. The results are consistent with an envelope with a rapid decrease of the specific angular momentum from the outer to the inner region. These new results improve the current understanding of the molecular accretion flow in an ultracompact (UC) H\textsc{ii} region created by the embedded O-type cluster.

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arXiv2010

N2H+ depletion in the massive protostellar cluster AFGL 5142

We aim at investigating with high angular resolution the NH3/N2H+ ratio toward the high-mass star-forming region AFGL 5142 in order to study whether this ratio behaves similarly to the low-mass case, for which the ratio decreases from starless cores to cores associated with YSOs. CARMA was used to observe the 3.2 mm continuum and N2H+(1-0) emission. We used NH3(1,1) and (2,2), HCO+(1-0) and H13CO+(1-0) data from the literature and we performed a time-dependent chemical modeling of the region. The 3.2 mm continuum emission reveals a dust condensation of ~23 Msun associated with the massive YSOs, deeply embedded in the strongest NH3 core (hereafter central core). The N2H+ emission reveals two main cores, the western and eastern core, located to the west and to the east of the mm condensation, and surrounded by a more extended and complex structure of ~0.5 pc. Toward the central core the N2H+ emission drops significantly, indicating a clear chemical differentiation in the region. We found low values of the NH3/N2H+ ratio ~50-100 toward the western/eastern cores, and high values up to 1000 in the central core. The chemical model indicates that density, and in particular temperature, are key parameters in determining the NH3/N2H+ ratio. The high density and temperature reached in the central core allow molecules like CO to evaporate from grain mantles. The CO desorption causes a significant destruction of N2H+, favoring the formation of HCO+. This result is supported by our observations, which show that N2H+ and HCO+ are anticorrelated in the central core. The observed values of the NH3/N2H+ ratio in the central core can be reproduced by our model for times t~4.5-5.3x10^5 yr (central) and t~10^4-3x10^6 yr (western/eastern). The NH3/N2H+ ratio in AFGL 5142 does not follow the same trend as in regions of low-mass star formation mainly due to the high temperature reached in hot cores.

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arXiv2010

IRAS 22198+6336: Discovery of an Intermediate-Mass Hot Core

We present new SMA and PdBI observations of the intermediate-mass object IRAS 22198+6336 in the millimeter continuum and in several molecular line transitions. The millimeter continuum emission reveals a strong and compact source with a mass of around 5 Msun and with properties of Class 0 objects. CO emission shows an outflow with a quadrupolar morphology centered on the position of the dust condensation. The CO outflow emission seems to come from two distinct outflows, one of them associated with SiO outflow emission. A large set of molecular lines has been detected toward a compact dense core clearly coincident with the compact millimeter source, and showing a velocity gradient perpendicular to the outflow traced by CO and SiO. The chemically rich spectrum and the rotational temperatures derived from CH$_3$CN and CH$_3$OH (100-150 K) indicate that IRAS 22198+6336 is harbouring one the few intermediate-mass hot cores known at present.

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arXiv2010

A large, massive, rotating disk around an isolated young stellar object

We present multi-wavelengths observations and a radiative transfer model of a newly discovered massive circumstellar disk of gas and dust which is one of the largest disks known today. Seen almost edge-on, the disk is resolved in high-resolution near-infrared (NIR) images and appears as a dark lane of high opacity intersecting a bipolar reflection nebula. Based on molecular line observations we estimate the distance to the object to be 3.5 kpc. This leads to a size for the dark lane of ~10500 AU but due to shadowing effects the true disk size could be smaller. In Spitzer/IRAC 3.6 micron images the elongated shape of the bipolar reflection nebula is still preserved and the bulk of the flux seems to come from disk regions that can be detected due to the slight inclination of the disk. At longer IRAC wavelengths, the flux is mainly coming from the central regions penetrating directly through the dust lane. Interferometric observations of the dust continuum emission at millimeter wavelengths with the SMA confirm this finding as the peak of the unresolved mm-emission coincides perfectly with the peak of the Spitzer/IRAC 5.8 micron flux and the center of the dark lane seen in the NIR images. Simultaneously acquired CO data reveal a molecular outflow along the northern part of the reflection nebula which seems to be the outflow cavity. An elongated gaseous disk component is also detected and shows signs of rotation. The emission is perpendicular to the molecular outflow and thus parallel to but even more extended than the dark lane in the NIR images. Based on the dust continuum and the CO observations we estimate a disk mass of up to a few solar masses depending on the underlying assumptions. Whether the disk-like structure is an actual accretion disk or rather a larger-scale flattened envelope or pseudodisk is difficult to discriminate with the current dataset (abridged).

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arXiv2010

L1448 IRS2E: A candidate first hydrostatic core

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 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 towards 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 um), and has weak (sub-)millimeter dust continuum emission. Consequently, this source has an extremely low bolometric luminosity (< 0.1 L_sun). 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/magneto hydrodynamical simulations, making L1448 IRS2E the most promising candidate of the first hydrostatic core revealed so far.

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arXiv2010

The standard model of star formation applied to massive stars: accretion disks and envelopes in molecular lines

We address the question of whether the formation of high-mass stars is similar to or differs from that of solar-mass stars through new molecular line observations and modeling of the accretion flow around the massive protostar IRAS20126+4104. We combine new observations of NH3(1,1) and (2,2) made at the Very Large Array, new observations of CHCN(13-12) made at the Submillimeter Array, previous VLA observations of NH(3,3), NH(4,4), and previous Plateau de Bure observations of C34S(2-1), C34S(5-4), and CHCN(12-11) to obtain a data set of molecular lines covering 15 to 419 K in excitation energy. We compare these observations against simulated molecular line spectra predicted from a model for high-mass star formation based on a scaled-up version of the standard disk-envelope paradigm developed for accretion flows around low-mass stars. We find that in accord with the standard paradigm, the observations require both a warm, dense, rapidly-rotating disk and a cold, diffuse infalling envelope. This study suggests that accretion processes around 10 M stars are similar to those of solar mass stars.

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arXiv2010

The reflection-symmetric wiggle of the young protostellar jet HH 211

HH 211 is a highly collimated jet originating from a nearby young Class 0 protostar. Here is a follow-up study of the jet with our previous observations at unprecedented resolution up to ~ 0.3" in SiO (J=8-7), CO (J=3-2), and SO (N_J=8_9-7_8). SiO, CO, and SO can all be a good tracer of the HH 211 jet, tracing the internal shocks in the jet. Although the emissions of these molecules show roughly the same morphology of the jet, there are detailed differences. In particular, the CO emission traces the jet closer to the source than the SiO and SO emissions. In addition, in the better resolved internal shocks, both the CO and SO emission are seen slightly ahead of the SiO emission. The jet is clearly seen on both sides of the source with more than one cycle of wiggle. The wiggle is reflection-symmetric about the source and can be reasonably fitted by an orbiting source jet model. The best-fit parameters suggest that the source itself could be a very low-mass protobinary with a total mass of ~ 60 M_Jup and a binary separation of ~ 4.6 AU. The abundances of SiO and SO in the gas phase are found to be highly enhanced in the jet as compared to the quiescent molecular clouds, even close to within 300 AU from the source where the dynamical time scale is <10 yrs. The abundance enhancements of these molecules are closely related to the internal shocks. The detected SiO is either the consequence of the release of Si-bearing material from dust grains or of its formation via gas chemistry in the shocks. The SO, on the other hand, seems to form via gas chemistry in the shocks.

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arXiv2010

Deuterium Fractionation as an Evolutionary Probe in the Infrared Dark Cloud G28.34+0.06

We have observed the J=3-2 transition of N2H+ and N2D+ to investigate the trend of deuterium fractionation with evolutionary stage in three selected regions in the Infrared Dark Cloud (IRDC) G28.34+0.06 with the Submillimeter Telescope (SMT) and the Submillimeter Array (SMA). A comprehensible enhancement of roughly 3 orders of magnitude in deuterium fractionation over the local interstellar D/H ratio is observed in all sources. In particular, our sample of massive star-forming cores in G28.34+0.06 shows a moderate decreasing trend over a factor of 3 in the N(N2D+)/N(N2H+) ratio with evolutionary stage, a behavior resembling what previously found in low-mass protostellar cores. This suggests a possible extension for the use of the N(N2D+)/N(N2H+) ratio as an evolutionary tracer to high-mass protostellar candidates. In the most evolved core, MM1, the N2H+ (3-2) emission appears to avoid the warm region traced by dust continuum emission and emission of 13CO sublimated from grain mantles, indicating an instant release of gas-phase CO. The majority of the N2H+ and N2D+ emission is associated with extended structures larger than 8" (~ 0.2 pc).

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arXiv2009

Formation of an O-Star Cluster by Hierarchical Accretion in G20.08-0.14 N

Spectral line and continuum observations of the ionized and molecular gas in G20.08-0.14 N explore the dynamics of accretion over a range of spatial scales in this massive star-forming region. Very Large Array observations of NH_3 at 4'' angular resolution show a large-scale (0.5 pc) molecular accretion flow around and into a star cluster with three small, bright HII regions. Higher resolution (0.4'') observations with the Submillimeter Array in hot core molecules (CH_3CN, OCS, and SO_2) and the VLA in NH_3, show that the two brightest and smallest HII regions are themselves surrounded by smaller scale (0.05 pc) accretion flows. The axes of rotation of the large- and small-scale flows are aligned, and the timescale for the contraction of the cloud is short enough, 0.1 Myr, for the large-scale accretion flow to deliver significant mass to the smaller scales within the star formation timescale. The flow structure appears to be continuous and hierarchical from larger to smaller scales. Millimeter radio recombination line (RRL) observations at 0.4" angular resolution indicate rotation and outflow of the ionized gas within the brightest HII region (A). The broad recombination lines and a continuum spectral energy distribution (SED) that rises continuously from cm to mm wavelengths, are both characteristic of the class of HII regions known as "broad recombination line objects". The SED indicates a density gradient inside this HII region, and the RRLs suggest supersonic flows. These observations are consistent with photoevaporation of the inner part of the rotationally flattened molecular accretion flow. We also report the serendipitous detection of a new NH_3 (3,3) maser.

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arXiv2009

Discovery of Extremely High Velocity "Molecular Bullets" in the HH 80-81 High-Mass Star-Forming Region

We present Submillimter Array 1.3 mm waveband continuum and molecular line observations of the HH 80-81 high-mass star-forming region. The dust continuum emission reveals two dominant peaks MM1 and MM2, and line emission from high-density tracers suggests the presence of another core MC. Molecular line emission from MM1, which harbors the exciting source of the HH 80-81 radio jet, yields a hot molecular core at a gas temperature of 110 K. The two younger cores MM2 and MC both appear to power collimated CO outflows. In particular, the outflow arising from MM2 exhibits a jet-like morphology and a broad velocity range of 190 km/s. The outflow contains compact and fast moving molecular clumps, known as "molecular bullets" first discovered in low-mass class 0 protostellar outflows. These "bullets" cannot be locally entrained or swept up from the ambient gas, but are more likely ejected from the close vicinity of the central protostar. The discovery of this remarkable outflow manifests an episodic, disk-mediated accretion for massive star formation.

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arXiv2009

Rotation and Outflow motions in the very low-mass Class 0 protostellar system HH 211 at subarcsecond resolution

HH 211 is a nearby young protostellar system with a highly collimated jet. We have mapped it in 352 GHz continuum, SiO (J=8-7), and HCO+ (J=4-3) emission at up to ~ 0.2" resolution with the Submillimeter Array (SMA). The continuum source is now resolved into two sources, SMM1 and SMM2, with a separation of ~ 84 AU. SMM1 is seen at the center of the jet, probably tracing a (inner) dusty disk around the protostar driving the jet. SMM2 is seen to the southwest of SMM1 and may trace an envelope-disk around a small binary companion. A flattened envelope-disk is seen in HCO+ around SMM1 with a radius of ~ 80 AU perpendicular to the jet axis. Its velocity structure is consistent with a rotation motion and can be fitted with a Keplerian law that yields a mass of ~ 50+-15 Jupiter mass (a mass of a brown dwarf) for the protostar. Thus, the protostar could be the lowest mass source known to have a collimated jet and a rotating flattened envelope-disk. A small-scale (~ 200 AU) low-speed (~ 2 km/s) outflow is seen in HCOP+ around the jet axis extending from the envelope-disk. It seems to rotate in the same direction as the envelope-disk and may carry away part of the angular momentum from the envelope-disk. The jet is seen in SiO close to ~ 100 AU from SMM1. It is seen with a "C-shaped" bending. It has a transverse width of <~ 40 AU and a velocity of ~ 170+-60 km/s. A possible velocity gradient is seen consistently across its innermost pair of knots, with ~ 0.5 km/s at ~ 10 AU, consistent with the sense of rotation of the envelope-disk. If this gradient is an upper limit of the true rotational gradient of the jet, then the jet carries away a very small amount of angular momentum of ~ 5 AU km/s and thus must be launched from the very inner edge of the disk near the corotation radius.

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arXiv2009

Fragmentation at the Earliest Phase of Massive Star Formation

We present 1.3mm continuum and spectral line images of two massive molecular clumps P1 and P2 in the G28.34+0.06 region with the Submillimeter Array. While the two clumps contain masses of 1000 and 880 \msun, respectively, P1 has a luminosity $< 10^2$ \lsun, and a lower gas temperature and smaller line width than P2. Thus, P1 appears to be at a much earlier stage of massive star formation than P2. The high resolution SMA observations reveal two distinctive cores in P2 with masses of 97 and 49 \msun, respectively. The 4 GHz spectral bandpass captures line emission from CO isotopologues, SO, CH$_3$OH, and CH$_3$CN, similar to hot molecular cores harboring massive young stars. The P1 clump, on the other hand, is resolved into five cores along the filament with masses from 22 to 64 \msun and an average projected separation of 0.19 pc. Except $^{12}$CO, no molecular line emission is detected toward the P1 cores at a 1$σ$ rms of 0.1 K. Since strong $^{12}$CO and C$^{18}$O emissions are seen with the single dish telescope at a resolution of 11$''$, the non-detection of these lines with the SMA indicates a depletion factor upto $10^3$. While the spatial resolution of the SMA is better than the expected Jeans length, the masses in P1 cores are much larger than the thermal Jeans mass, indicating the importance of turbulence and/or magnetic fields in cloud fragmentation. The hierarchical structures in the P1 region provide a glimpse of the initial phase of massive star and cluster formation.

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