Tirupati K. Sridharan

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.

CH3OH and H2O masers in high-mass star-forming regions

We present a comparison of Class ii CH3OH (6.7 GHz) and H2O (22.2 GHz) masers at high spatial resolution in a sample of 29 massive star-forming regions. Absolute positions of both maser types are compared with mm dust continuum, cm continuum and mid-infrared sources. All maser features - regardless of the species - are associated with massive mm cores, but only 3 out of 18 CH3OH masers and 6 out of 22 H2O masers are associated with cm emission likely indicating the presence of a recently ignited massive star. These observations of a homogenous sample of massive, young star-forming regions confirm earlier results, obtained for each maser species separately, that both maser types are signposts of high-mass star formation in very early evolutionary stages. The data are consistent with models that explain CH3OH maser emission by radiative pumping in moderately hot cores, requiring the absence, or only weak, free-free cm continuum radiation due to recently ignited stars. Mid-infrared sources are associated with both maser types in approximately 60% of the observed fields. Thus, mid-infrared objects may power maser sites, but the detection of strong mid-infrared emission is not strictly necessary because it might be heavily extincted. A comparison of the spatial separations between the dierent observed quantities and other properties of the star-forming regions does not reveal any correlation. Our data suggest that CH3OH and H2O masers need a similar environment (dense and warm molecular gas), but that, due to the dierent excitation processes (radiative pumping for CH3OH and collisional pumping for H2O), no spatial correlations exist. Spatial associations are probably coincidences due to insucient angular resolution and projection eects. The kinematic structures we find in the dierent maser species show no recognizable pattern, and we cannot draw firm conclusions as to whether the features are produced in disks, outflows or expanding shock waves.

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.

Self-similar fragmentation regulated by magnetic fields in a region forming massive stars

Most molecular clouds are filamentary or elongated. For those forming low-mass stars (<8 solar masses), the competition between self-gravity and turbulent pressure along the dynamically dominant intercloud magnetic field (10 to 100 parsecs) shapes the clouds to be elongated either perpendicularly or parallel to the fields. A recent study also suggested that on the scales of 0.1 to 0.01 parsecs, such fields are dynamically important within cloud cores forming massive stars (>8 solar masses). But whether the core field morphologies are inherited from the intercloud medium or governed by cloud turbulence is unknown, as is the effect of magnetic fields on cloud fragmentation at scales of 10 to 0.1 parsecs. Here we report magnetic-field maps inferred from polarimetric observations of NGC 6334, a region forming massive stars, on the 100 to 0.01 parsec scale. NGC 6334 hosts young star-forming sites where fields are not severely affected by stellar feedback, and their directions do not change much over the entire scale range. This means that the fields are dynamically important. The ordered fields lead to a self-similar gas fragmentation: at all scales, there exist elongated gas structures nearly perpendicular to the fields. Many gas elongations have density peaks near the ends, which symmetrically pinch the fields. The field strength is proportional to the 0.4th power of the density, which is an indication of anisotropic gas contractions along the field. We conclude that magnetic fields have a crucial role in the fragmentation of NGC 6334.

APEX: the Atacama Pathfinder EXperiment

APEX, the Atacama Pathfinder Experiment, has been successfully commissioned and is in operation now. This novel submillimeter telescope is located at 5107 m altitude on Llano de Chajnantor in the Chilean High Andes, on what is considered one of the worlds outstanding sites for submillimeter astronomy. The primary reflector with 12 m diameter has been carefully adjusted by means of holography. Its surface smoothness of 17-18 μm makes APEX suitable for observations up to 200 μm, through all atmospheric submm windows accessible from the ground.

Mapping the outflow from G5.89-0.39 in SiO J = 5 → 4

We have mapped the ultracompact H II region, G5.89-0.39, and its molecular surroundings with the Submillimeter Array at 28 × 18 angular resolution in 1.3 mm continuum, SiO J = 5 → 4, and eight other molecular lines. We have resolved for the first time the highly energetic molecular outflow in this region. At this resolution, the outflow is definitely bipolar and appears to originate in a 1.3 mm continuum source. The continuum source peaks in the center of the H II region. The axis of the outflow lines up with a recently discovered O5 V star.

Submillimeter Array Outflow/Disk Studies in the Massive Star-forming Region IRAS 18089–1732

Submillimeter Array observations of the massive star-forming region IRAS 18089-1732 in the 1 mm and 850 ?m band reveal outflow and disk signatures in different molecular lines. The SiO (5-4) data show a collimated outflow in the northern direction. In contrast, the HCOOCH3 (20-19) line, which traces high-density gas, is confined to the very center of the region and shows a velocity gradient across the core. The HCOOCH3 velocity gradient is not exactly perpendicular to the outflow axis but between an assumed disk plane and the outflow axis. We interpret these HCOOCH3 features as originating from a rotating disk that is influenced by the outflow and infall. On the basis of the (sub)millimeter continuum emission, the mass of the central core is estimated to be around 38 M?. The dynamical mass derived from the HCOOCH3 data is 22 M?, of about the same order as the core mass. Thus, the mass of the protostar/disk/envelope system is dominated by its disk and envelope. The two frequency continuum data of the core indicate a low dust opacity index ? ~ 1.2 in the outer part, decreasing to ? ~ 0.5 on shorter spatial scales.

Interferometric mapping of magnetic fields: The ALMA view of the massive star-forming clump W43-MM1

Here we present the first results from ALMA observations of 1 mm polarized dust emission towards the W43-MM1 high mass star forming clump. We have detected a highly fragmented filament with source masses ranging from 14Msun to 312Msun, where the largest fragment, source A, is believed to be one of the most massive in our Galaxy. We found a smooth, ordered, and detailed polarization pattern throughout the filament which we used to derived magnetic field morphologies and strengths for 12 out of the 15 fragments detected ranging from 0.2 to 9 mG. The dynamical equilibrium of each fragment was evaluated finding that all the fragments are in a super-critical state which is consistent with previously detected infalling motions towards W43-MM1. Moreover, there are indications suggesting that the field is being dragged by gravity as the whole filament is collapsing.

Submillimeter Array Multiline observations of the Massive Star-forming region IRAS 18089–1732

Submillimeter Array (SMA) observations of the high-mass star-forming region IRAS 18089-1732 in the 1 mm and 850 μm band with 1 GHz bandwidth reveal a wealth of information. We present the observations of 34 lines from 16 different molecular species. Most molecular line maps show significant contributions from the outflow, and only a few molecules are confined to the inner core. We present and discuss the molecular line observations and outline the unique capabilities of the SMA for future imaging line surveys at high spatial resolution.

The Greenland Telescope (GLT): antenna status and future plans

The ALMA North America Prototype Antenna was awarded to the Smithsonian Astrophysical Observatory (SAO) in 2011. SAO and the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), SAO’s main partner for this project, are working jointly to relocate the antenna to Greenland to carry out millimeter and submillimeter VLBI observations. This paper presents the work carried out on upgrading the antenna to enable operation in the Arctic climate by the GLT Team to make this challenging project possible, with an emphasis on the unexpected telescope components that had to be either redesigned or changed. Five-years of inactivity, with the antenna laying idle in the desert of New Mexico, coupled with the extreme weather conditions of the selected site in Greenland have it necessary to significantly refurbish the antenna. We found that many components did need to be replaced, such as the antenna support cone, the azimuth bearing, the carbon fiber quadrupod, the hexapod, the HVAC, the tiltmeters, the antenna electronic enclosures housing servo and other drive components, and the cables. We selected Vertex, the original antenna manufacturer, for the main design work, which is in progress. The next coming months will see the major antenna components and subsystems shipped to a site of the US East Coast for test-fitting the major antenna components, which have been retrofitted. The following step will be to ship the components to Greenland to carry out VLBI