Massive Star Formation
 

  • Sub-arcsec Observations of NGC 7538 IRS 1: Continuum Distribution and Dynamics of Molecular Gas

Abstract We report new results based on the analysis of the SMA and CARMA observations of NGC 7538 IRS 1 at 1.3 and 3.4 mm with sub-arcsec resolutions. With angular resolutions ~0.7", the SMA and CARMA observations show that the continuum emission at 1.3 and 3.4 mm from the hyper-compact HII region IRS 1 is dominated by a compact source with a tail-like extended structure to the southwest of IRS 1. With a CARMA B-array image at 1.3 mm convolved to 0.1", we resolve the hyper-compact HII region into two components: an unresolved hyper-compact core, and a north-south extension with linear sizes of <270 AU and ~2000 AU, respectively. The fine structure observed with CARMA is in good agreement with the previous VLA results at centimeter wavelengths, suggesting that the hyper-compact HII region at the center of IRS 1 is associated with an ionized bipolar outflow. We image the molecular lines OCS(19-18) and CH3CN(12-11) as well as 13CO(2-1) surrounding IRS 1, showing a velocity gradient along the southwest-northeast direction. The spectral line profiles in 13CO(2-1), CO(2-1), and HCN(1-0) observed toward IRS 1 show broad redshifted absorption, providing evidence for gas infall with rates in the range of 3-10x10-3 Mo yr-1 inferred from our observations.


Lei Zhu, Jun-Hui Zhao, Melvyn Wright, Göran Sandell, Hui Shi, Yue-Fang Wu, Crystal Brogan, Stuartt Corder, ApJ accepted
  • Observations of a high-mass protostar in NGC 7538S

Abstract We present high angular resolution continuum observations of the high-mass protostar NGC 7538S with BIMA and CARMA at 3 and 1.4 mm, VLA observations at 1.3, 2, 3.5 and 6 cm, and archive IRAC observations from the Spitzer Space Observatory, which detect the star at 4.5, 5.8, and 8 $\mu$m. The star looks rather unremarkable in the mid-IR. The excellent positional agreement of the IRAC source with the VLA free-free emission, the OH, CH3OH, H2O masers, and the dust continuum confirms that this is the most luminous object in the NGC 7538S core. The continuum emission at millimeter wavelengths is dominated by dust emission from the dense cold cloud core surrounding the protostar. Including all array configurations, the emission is dominated by an elliptical source with a size of ~ 8" x 3". If we filter out the extended emission we find three compact mm-sources inside the elliptical core. The strongest one, SA, coincides with the VLA/IRAC source and resolves into a double source at 1.4 mm, where we have sub-arcsecond resolution. The measured spectral index, $\alpha$, between 3 and 1.4 mm is ~ 2.3, and steeper at longer wavelengths, suggesting a low dust emissivity or that the dust is optically thick. We argue that the dust in these accretion disks is optically thick and estimate a mass of an accretion disk or infalling envelope surrounding SA to be ~ 60 solar masses.


Melvyn Wright, Jun-Hui Zhao, Goran Sandell, Stuartt Corder, W. M. Goss, Lei Zhu, 2012, ApJ, 746, 187
  • Outflow, Infall and Protostars in the Star-Forming Core W3-SE

Abstract We report new results on outflow and infall in the star forming cores W3-SE SMA-1 and SMA-2 based on analysis of ~2.5" resolution observations of the molecular lines HCN(3-2), HCO+(3-2), N2H+ (3-2) and CH 3 OH(5 2,3 -4 1,3 ) with the Submillimeter Array. A high-velocity bipolar outflow originating from the proto-stellar core SMA-1 was observed in the HCN(3-2) line, with a projected outflow axis in position angle 48o. The detection of the outflow is confirmed from other molecular lines. An inverse P-Cygni profile in the HCN(3-2) line toward SMA-1 suggests that at least one of the double cores accretes matters from the molecular core. A flamentary structure in the molecular gas surrounds SMA-1 and SMA-2. Based on the SMA observations, our analysis suggests that the double pre-stellar cores SMA-1 and SMA-2 result from fragmentation in the collapsing massive molecular core W3-SE, and it is likely that they are forming intermediate to high-mass stars which will be new members of a star cluster in the W3-SE region.


Zhu, Lei, Zhao, Jun-Hui, and Wright, M. C. H., 2011, ApJ, 740, 114
  • Dust and HCO+ Gas in the Star-Forming Core W3-SE

Abstract We report new results from recent Combined Array for Research in Millimeter-wave Astronomy (CARMA) observations of both continuum and HCO+(1-0) line emission at 3.4 mm from W3-SE, a molecular core of intermediate mass, together with the observations of continuum emission at 1.1 and 0.85/0.45 mm with the Submillimeter Array (SMA) and the James Clerk Maxwell Telescope, respectively. A continuum emission core elongated from SE to NW, with a size of ~10", has been observed at the millimeter and submillimeter wavelengths. The dust core has been resolved into a double source with the SMA at 1.1 mm. The angular separation between the two components is ~4". Together with the measurements from the Spitzer Space Telescope and the Midcourse Space Experiment at mid-IR wavelengths, we determined the spectral energy distribution (SED) of the continuum emission from W3-SE and fit it with a thermal dust emission model. Our best fitting of the SED suggests the presence of two dust components with different temperatures. The emission at millimeter/submillimeter wavelengths is dominated by a major component that is characterized by a temperature of Td = 41 +/- 6 K with a mass of 65 +/- 10 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". In the central region of ~50", the integrated HCO+(1-0) line emission shows a main component A that coincides with the dust core, as well as two substructures B and C which are located N and SE of the dust core, respectively. 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 substructures of B and C suggest that the molecular gas outside the main component A appears to be dominated by the bipolar outflow originated from the dust core with a dynamical age of >3 x 104 yr. Our analysis, based on the observations at wavelengths from millimeter, submillimeter, to mid-IR, suggest that the molecular core W3-SE hosts a group of newly formed young stars and protostars.


Zhu, Lei; Wright, M. C. H.; Zhao, Jun-Hui; Wu, Yuefang, 2010, ApJ, 712, 674

  • Nature of W51e2: Massive Cores at Different Phases of Star Formation

Abstract We present high-resolution continuum images of the W51e2 complex processed from archival data of the Submillimeter Array (SMA) at 0.85 and 1.3 mm and the Very Large Array at 7 and 13 mm. We also made line images and profiles of W51e2 for three hydrogen radio recombination lines (RRLs; H26alpha, H53alpha, and H66alpha) and absorption of two molecular lines of HCN(4-3) and CO(2-1). At least four distinct continuum components have been detected in the 3" region of W51e2 from the SMA continuum images at 0.85 and 1.3 mm with resolutions of 0.3"x0.2" and 1.4"x 0.7", respectively. The west component, W51e2-W, coincides with the ultracompact H II region reported from previous radio observations. The H26alpha line observation reveals an unresolved hyper-compact ionized core (<0.06" or < 310 AU) with a high electron temperature of 1.2 x 104 K, with the corresponding emission measure EM > 7 x 1010 pc cm-6 and the electron density Ne > 7 x 106 cm-3. The inferred Lyman continuum flux implies that the H II region W51e2-W requires a newly formed massive star, an O8 star or a cluster of B-type stars, to maintain the ionization. W51e2-E, the brightest component at 0.85 mm, is located 0.9" east from the hyper-compact ionized core. It has a total mass of ~140 M sun according to our spectral energy distribution analysis and a large infall rate of > 1.3 x 10-3 M sun yr-1 inferred from the absorption of HCN. W51e2-E appears to be the accretion center in W51e2. Given the fact that no free-free emission and no RRLs have been detected, the massive core of W51e2-E appears to host one or more growing massive proto-stars. Located 2" northwest from W51e2-E, W51e2-NW is detected in the continuum emission at 0.85 and 1.3 mm. No continuum emission has been detected at wavelengths longer than 7 mm. Along with the maser activities previously observed, our analysis suggests that W51e2-NW is at an earlier phase of star formation. W51e2-N is located 2" north of W51e2-E and has only been detected at 1.3 mm with a lower angular resolution (~1"), suggesting that it is a primordial, massive gas clump in the W51e2 complex.


Shi, Hui; Zhao, Jun-Hui; Han, J. L., 2010, ApJ, 710, 843

  • A Bipolar Outflow from the Massive Protostellar Core W51e2-E

Abstract We present high-resolution images of the bipolar outflow from W51e2, which are produced from the Submillimeter Array archival data observed for CO(3-2) and HCN(4-3) lines with angular resolutions of 0.8" x 0.6" and 0.3" x 0.2", respectively. The images show that the powerful outflow originates from the protostellar core W51e2-E rather than from the ultracompact H II region W51e2-W. The kinematic timescale of the outflow from W51e2-E is about 1000 yr, younger than the age (~5000 yr) of the ultracompact H II region W51e2-W. A large mass-loss rate of ~1 x 10-3 Msun yr-1 and a high mechanical power of 120 Lsun are inferred, suggesting that an O star or a cluster of B stars are forming in W51e2-E. The observed outflow activity along with the inferred large accretion rate indicates that at present W51e2-E is in a rapid phase of star formation.


Shi, Hui; Zhao, Jun-Hui; Han, J. L., 2010, ApJ, 718, L181