Qizhou Zhang

Search for CO Outflows toward a Sample of 69 High-Mass Protostellar Candidates. II. Outflow Properties

We present a study of molecular outflows toward a sample of 69 luminous IRAS point sources. The sample is associated with dense molecular gas and has far-infrared luminosities ranging from 102 to 105 L☉, indicating these objects as regions likely forming high-mass stars. Mapping in the CO J = 2-1 line shows that molecular outflows are ubiquitous in these regions. Most of the outflows have masses of tens of M☉. The typical dynamical timescale of the flow, without correcting for inclination of the flow axis, is a few times 104 yr. The typical energy in the outflows is 1046 ergs, comparable to the turbulent energy in the core. Nearly half of the outflows show spatially resolved bipolar lobes. This indicates that low-mass young stars that coexist in the region are not responsible for the bipolar outflows observed. It is the more massive stars that drive the outflow. The large detection rate of outflows in the region favors an accretion process in the formation of massive stars. The maximum mass-loss rate in the wind is about 10-4 M☉ yr-1. If these outflows are driven via accretion, the accretion rate should be as high as a few times 10-4 M☉ yr-1. We compare CO outflows with images at near-infrared wavelengths from the Two Micron All Sky Survey (2MASS) archive and find that some outflows are associated with extended emission in the K band, which may be partly due to vibrationally excited H2 emission at 2.12 μm.

Magnetic Fields in the Formation of Massive Stars

Stellar Hourglass Figure Star-forming clouds are thought to be supported against gravity by ordered interstellar magnetic fields, which are strong enough to slow gravitation collapse but too weak to prevent it. Girart et al. (p. 1408) measured polarized radio waves from dust particles around a forming massive star, which reveal an hourglass shape. The data imply that a magnetic field strength dominates over turbulence—the telltale signs of magnetically controlled star formation. These conditions mimic those found in low-mass star-forming regions, suggesting that the magnetic field plays an important role in star formation, irrespective of differences in mass. Observations of polarized dust emission show that the magnetic field controls the dynamical evolution of a massive star-forming region. Massive stars play a crucial role in the production of heavy elements and in the evolution of the interstellar medium, yet how they form is still a matter of debate. We report high-angular-resolution submillimeter observations toward the massive hot molecular core (HMC) in the high-mass star-forming region G31.41+0.31. We find that the evolution of the gravitational collapse of the HMC is controlled by the magnetic field. The HMC is simultaneously contracting and rotating, and the magnetic field lines threading the HMC are deformed along its major axis, acquiring an hourglass shape. The magnetic energy dominates over the centrifugal and turbulence energies, and there is evidence of magnetic braking in the contracting core.

A disk of dust and molecular gas around a high-mass protostar

The processes leading to the birth of low-mass stars such as our Sun have been well studied, but the formation of high-mass (over eight times the Suns mass, M[circdot]) stars remains poorly understood. Recent studies suggest that high-mass stars may form through accretion of material from a circumstellar disk, in essentially the same way as low-mass stars form, rather than through the merging of several low-mass stars. There is as yet, however, no conclusive evidence. Here we report the presence of a flattened disk-like structure around a massive 15M[circdot] protostar in the Cepheus A region, based on observations of continuum emission from the dust and line emission from the molecular gas. The disk has a radius of about 330 astronomical units (au) and a mass of 1 to 8 M[circdot]. It is oriented perpendicular to, and spatially coincident with, the central embedded powerful bipolar radio jet, just as is the case with low-mass stars, from which we conclude that high-mass stars can form through accretion.

Dynamical Collapse in W51 Massive Cores: CS (3-2) and CH3CN Observations

We present interferometric observations of the W51 region at 2 mm with the Nobeyama Millimeter Array. The 320 MHz band centered at 147 GHz covers transitions of the CS (3-2), CH3CN (8-7), H35α, CH3OCH3 (7-6) and (6-6), and HCOOCH3 (12-11), as well as the 2 mm continuum. Toward two dense cores, W51e2 and W51e8, spectroscopic signatures of cloud collapse are present in all the molecular lines observed. Line asymmetries increase systematically toward transitions of larger optical depths, consistent with expected signatures of a centrally condensed infalling cloud. Furthermore, the disparity between the blueshifted and the redshifted emission of the CS line is enhanced when the line is synthesized at higher angular resolution. Given that the continuum source is embedded in the core, we are able to locate the emitting gas of the blueshifted emission on the rear side and the gas of the redshifted emission on the front side of the central star. The new observations confirm our infall interpretations regarding the e2 and e8 cores based on the NH3 data. The compact CH3CN emission allows us to identify velocity gradients in the e2 and e8 cores. The gradient in the e2 core is consistent with the spin-up motions proposed by Zhang & Ho in 1997. CH3CN emission reveals hot components in the W51e2, W51e8, and W51-North:dust cores. The rotational temperature in each core is greater than 100 K. In W51-North, we detected a dust peak coincident with the peak of the dense molecular core. There is no 3.6 cm continuum detected at a level of less than 1 mJy. Although H2O and OH masers in the neighborhood indicate outflow activities, high-velocity gas is not apparent in the CS (3-2). This source may represent an extremely early evolutionary stage: the phase of massive protostars.

Search for CO Outflows toward a Sample of 69 High-Mass Protostellar Candidates: Frequency of Occurrence

A survey for molecular outflows was carried out by mapping the CO J = 2-1 line toward a sample of 69 luminous IRAS point sources. Sixty objects have IRAS luminosities from 103 to 105 L☉ and are associated with dense gas traced by NH3, identifying them as high-mass star-forming regions. Among 69 sources, 65 sources have data that are suitable for outflow identification. Thirty-nine regions show spatially confined high-velocity wing emission in CO, indicative of molecular outflows. Most objects without identifiable outflows lie within 0° < l < 50° where outflow signatures are confused by multiple cloud components along the line of sight. Excluding 26 sources with 0° < l < 50°, we found 35 outflows out of 39 sources, which yields an outflow detection rate of 90%. Many of the outflows contain masses of more than 10 M☉ and have momenta of a few hundred M☉ km s-1, at least 2 orders of magnitude larger than those in typical low-mass outflows. This class of massive and energetic outflows is most likely driven by high-mass young stellar objects. The high detection rate indicates that molecular outflows are common toward high-mass young stars. Given the connection between outflows and accretion disks in low-mass stars, we suggest that high-mass stars may form via an accretion-outflow process, similar to their low-mass counterparts.

Organic Molecules in Low-Mass Protostellar Hot Cores: Submillimeter Imaging of IRAS 16293?2422

Arcsecond-resolution spectral observations toward the protobinary system IRAS 16293-2422 at 344 and 354 GHz were conducted using the Submillimeter Array. Several complex organic molecules, such as CH3OH and HCOOCH3, were detected and mapped. Together with the rich organic inventory revealed, it clearly indicates the existence of two, rather than one, compact hot molecular cores (400 AU in radius) associated with each of the protobinary components identified by their dust continuum emission in the inner star-forming core.

Hierarchical fragmentation and differential star formation in the Galactic ‘Snake’: infrared dark cloud G11.11−0.12

We present Submillimeter Array (SMA) λ = 0.88 and 1.3 mm broad-band observations, and Very Large Array (VLA) observations in NH_3 (J, K) = (1,1) up to (5,5), H_2O and CH_3OH maser lines towards the two most massive molecular clumps in infrared dark cloud (IRDC) G11.11−0.12. Sensitive high-resolution images reveal hierarchical fragmentation in dense molecular gas from the ∼1 pc clump scale down to ∼0.01 pc condensation scale. At each scale, the mass of the fragments is orders of magnitude larger than the Jeans mass. This is common to all four IRDC clumps we studied, suggesting that turbulence plays an important role in the early stages of clustered star formation. Masers, shock heated NH_3 gas, and outflows indicate intense ongoing star formation in some cores while no such signatures are found in others. Furthermore, chemical differentiation may reflect the difference in evolutionary stages among these star formation seeds. We find NH_3 ortho/para ratios of 1.1 ± 0.4, 2.0 ± 0.4, and 3.0 ± 0.7 associated with three outflows, and the ratio tends to increase along the outflows downstream. Our combined SMA and VLA observations of several IRDC clumps present the most in-depth view so far of the early stages prior to the hot core phase, revealing snapshots of physical and chemical properties at various stages along an apparent evolutionary sequence.

The Formation of Massive Stars. I. High-Resolution Millimeter and Radio Studies of High-Mass Protostellar Candidates

We used the Owens Valley Millimeter Array and the Very Large Array to obtain interferometric maps at millimeter and centimeter wavelengths in both the continuum and various lines [HCO+ (1-0), H13CO+ (1-0), SiO (v = 0, J = 2-1) and H13CN (1-0)] toward a sample of 11 high-mass protostellar candidates. These sources are known from a previous study to be associated with dense gas and dust and to not be associated with H II regions. All 11 sources were detected in HCO+ (1-0), nine in the millimeter continuum and five (of eight observed) in the centimeter continuum. The derived physical parameters confirm the high-mass nature of these molecular clumps and suggest they are gravitationally bound. Molecular outflows were detected toward six sources, with flow masses and momenta much higher than in low-mass young stellar objects. In many of the sources the molecular emission is organized in substructures, resolved both spatially and in velocity. We find that the sources may be characterized by their degree of fragmentation, turbulence, and outflow activity, with the sample dividing into two groups: group 1 cores have multiple peaks but with a clearly dominant component and larger line widths and are systematically associated with outflows, while group 2 cores have several comparable subentities, smaller line widths, and no association with outflows. We speculate that more massive cores may form from smaller cores via coalescence or competitive accretion. Even conservative estimates of outflow mass-loss rates, however, indicate that accretion is the dominant process in the later formation of massive protostars from such cores. We find a flattening of the outflow mass spectra with increasing flow velocities, at variance with previous studies that suggest a steepening with increasing flow velocities. In the light of this result we suggest a reevaluation of the wide-angle wind momentum-driven flow models to describe the acceleration of outflows in the earliest stages of massive star formation.

Multiple jets from the high-mass (proto)stellar cluster AFGL 5142

We present studies of the massive protocluster AFGL 5142 in the J ¼ 2Y1 transition of the CO isotopologues, SO, CH3OH, and CH3CN lines, as well as in the continuum at 225 GHz and 8.4 GHz. The 225 GHz continuum emission reveals at least five dust continuum peaks. The strongest peaks, MM-1 and MM-2, are associated with hot cores with temperatures of 90 � 20 and 250 � 40 K, respectively. With similar core mass, the higher temperature and CH3CN abundance in the MM-2 core suggest that it might be at a more evolved stage than the MM-1 core. A total of 22 lines fromninemoleculesaredetected.Thelinestrengthvariesremarkablyintheregion.StrongSOemissionisfoundboth in molecular outflows and cloud cores. CH3OH emission, onthe contrary, is much weaker in molecular outflows, and isdetectedtowardhotcoresMM-1andMM-2,butisabsentinthelessmassiveandperhapslessevolvedcoresMM-3, MM-4, and MM-5. The CO and SO emission reveals at least three molecular outflows originating from the center of thedustcore.Theoutflowsarewellcollimated,withterminalvelocitiesupto50kms � 1 fromthecloudvelocity.Since jetlike outflows and disk-mediated accretion process are physically connected, the well-collimated outflows indicate that even in this cluster environment, accretion is responsible for the formation of individual stars in the cluster. Subject headingg s: H ii regions — ISM: clouds — ISM: individual (AFGL 5142) — ISM: kinematics and dynamics — masers — stars: formation

SPITZER IRAC AND MIPS IMAGING OF CLUSTERS AND OUTFLOWS IN NINE HIGH-MASS STAR FORMING REGIONS

WepresentSpitzerSpaceTelescopeIRACand MIPS observations towardasample of ninehigh-mass star forming regions at a distance of around 2 kpc. Based on IRAC and MIPS 24 � m photometric results and 2MASS JHKs data, we carry out a census of young stellar objects (YSOs) in a 5 0 ; 5 0 field toward each region. Toward seven out of the nine regions, we detect parsec-sized clusters with around 20 YSOs surrounded by a more extended and sparse distribution of young stars and protostars. For the other two regions, IRAS 20126+4104 and IRAS 22172+5549, the formerhasthelowestnumberof YSOsinthesampleandshowsnoobviouscluster,andthelatterappearstobepartof alarger, potentially more evolved cluster. Thedeep IRAC imaging reveals at least 12 outflows in eight out of the nine regions, with nine outflows prominent in the 4.5 � m band most probably attributed to shocked H 2 emission, two outflowsdominatedbyscatteredlightinthe3.6and4.5 � mbands,andoneoutflowstandingoutfromitshydrocarbon emission in the 8.0 � m band. In comparison with previous ground-based observations, our IRAC observations reveal newoutflowstructuresinfiveregions.Thedramaticallydifferentmorphologiesof detectedoutflowscanbetentatively interpreted in terms of possible evolution of massive outflows. The driving sources of these outflows are deeply embedded in dense dusty cores revealed by previous millimeter interferometric observations. We detect infrared counterparts of these dusty cores in the IRAC or MIPS 24 � m bands. Reflection nebulae dominated by the emission from UV-heated hydrocarbons in the 8 � m band can be found in most regions and they may imply the presence of young B stars. Subject headingg infrared: stars — ISM: jets and outflows — reflection nebulae — stars: formation — stars: preYmain-sequence