J. M. C. Rawlings

The James Clerk Maxwell telescope legacy survey of nearby star-forming regions in the gould belt

This paper describes a James Clerk Maxwell Telescope (JCMT) legacy survey that has been awarded roughly 500 hr of observing time to be carried out from 2007 to 2009. In this survey, we will map with SCUBA-2 (Submillimetre Common-User Bolometer Array 2) almost all of the well-known low-mass and intermediate-mass star-forming regions within 0.5 kpc that are accessible from the JCMT. Most of these locations are associated with the Gould Belt. From these observations, we will produce a flux-limited snapshot of star formation near the Sun, providing a legacy of images, as well as point-source and extended-source catalogs, over almost 700 deg(2) of sky. The resulting images will yield the first catalog of prestellar and protostellar sources selected by submillimeter continuum emission, and should increase the number of known sources by more than an order of magnitude. We will also obtain with the array receiver HARP (Heterodyne Array Receiver Program) CO maps, in three CO isotopologues, of a large typical sample of prestellar and protostellar sources. We will then map the brightest hundred sources with the SCUBA-2 polarimeter (POL-2), producing the first statistically significant set of polarization maps in the submillimeter. The images and source catalogs will be a powerful reference set for astronomers, providing a detailed legacy archive for future telescopes, including ALMA, Herschel, and JWST.

The JCMT Legacy Survey of the Gould Belt: a first look at Orion B with HARP

The Gould Belt Legacy Survey will survey nearby star-forming regions (within 500 pc), using HARP (Heterodyne Array Receiver Programme), SCUBA-2 (Submillimetre CommonUser Bolometer Array 2) and POL-2 (Polarimeter 2) on the James Clerk Maxwell Telescope (JCMT). This paper describes the initial data obtained using HARP to observe 12 CO, 13 CO and C 18 O J = 3! 2 towards two regions in Orion B, NGC 2024 and NGC 2071. We describe the physical characteristics of the two clouds, calculating temperatures and opacities utilizing all three isotopologues. We find good agreement between temperatures calculated from CO and from dust emission in the dense, energetic regions. We determine the mass and energetics of the clouds, and of the high-velocity material seen in 12 CO emission, and compare the relative energetics of the high- and low-velocity material in the two clouds. We present a CLUMPFIND analysis of the 13 CO condensations. The slope of the condensation mass functions, at the high-mass ends, is similar to the slope of the initial mass function.

HCO+ emission excess in bipolar outflows

A plausible model is proposed for the enhancement of the abundance of molecular species in bipolar outflow sources. In this model, levels of HCO + enhancement are considered based on previous chemical calculations, which are assumed to result from shock-induced desorption and photoprocessing of dust grain ice mantles in the boundary layer between the outflow jet and the surrounding envelope. A radiative transfer simulation that incorporates chemical variations within the flow shows that the proposed abundance enhancements in the boundary layer are capable of reproducing the observed characteristics of the outflow seen in HCO + emission in the star-forming core L1527. The radiative transfer simulation also shows that the emission lines from the enhanced molecular species, which trace the boundary layer of the outflow, exhibit complex line profiles, indicating that detailed spatial maps of the line profiles are essential in any attempt to identify the kinematics of potential infall/outflow sources. This study is one of the first applications of a full three-dimensional radiative transfer code which incorporates chemical variations within the source.

The JCMT Gould Belt Survey: first results from the SCUBA-2 observations of the Ophiuchus molecular cloud and a virial analysis of its prestellar core population

In this paper, we present the first observations of the Ophiuchus molecular cloud performed as part of the James Clerk Maxwell Telescope (JCMT) Gould Belt Survey (GBS) with the SCUBA-2 instrument. We demonstrate methods for combining these data with previous HARP CO, Herschel, and IRAM N2H+ observations in order to accurately quantify the properties of the SCUBA-2 sources in Ophiuchus. We produce a catalogue of all of the sources found by SCUBA-2. We separate these into protostars and starless cores. We list all of the starless cores and perform a full virial analysis, including external pressure. This is the first time that external pressure has been included in this level of detail. We find that the majority of our cores are either bound or virialized. Gravitational energy and external pressure are on average of a similar order of magnitude, but with some variation from region to region. We find that cores in the Oph A region are gravitationally bound prestellar cores, while cores in the Oph C and E regions are pressure-confined. We determine that N2H+ is a good tracer of the bound material of prestellar cores, although we find some evidence for N2H+ freeze-out at the very highest core densities. We find that non-thermal linewidths decrease substantially between the gas traced by C18O and that traced by N2H+, indicating the dissipation of turbulence at higher densities. We find that the critical Bonnor–Ebert stability criterion is not a good indicator of the boundedness of our cores. We detect the pre-brown dwarf candidate Oph B-11 and find a flux density and mass consistent with previous work. We discuss regional variations in the nature of the cores and find further support for our previous hypothesis of a global evolutionary gradient across the cloud from south-west to north-east, indicating sequential star formation across the region.

Infrared spectroscopy of Nova Cassiopeiae 1993 — IV. A closer look at the dust

Nova Cassiopeiae 1993 (V705 Cas) was an archetypical dust-forming nova. It displayed a deep minimum in the visual light curve, and spectroscopic evidence for carbon, hydrocarbon and silicate dust. We report the results of fitting the infrared (IR) spectral energy distribution (SED) with the DUSTY code, which we use to determine the properties and geometry of the emitting dust. The emission is well described as originating in a thin shell whose dust has a carbon:silicate ratio of 2:1 by number (∼ 1.26:1 by mass) and a relatively flat size distribution. The 9.7- and 18-μm silicate features are consistent with freshly condensed dust and, while the lower limit to the grain size distribution is not well constrained, the largest grains have dimensions ∼0.06 μm; unless the grains in V705 Cas were anomalously small, the sizes of grains produced in nova eruptions may previously have been overestimated in novae with optically thick dust shells. Laboratory work by Grishko & Duley may provide clues to the apparently unique nature of nova unidentified infrared (UIR) features.

Molecular gas freeze‐out in the pre‐stellar core L1689B

C 1 7 O J = 2 → 1 observations have been carried out towards the pre-stellar core L1689B. By comparing the relative strengths of the hyperfine components of this line, the emission is shown to be optically thin. This allows accurate CO column densities to be determined and, for reference, this calculation is described in detail. The hydrogen column densities that these measurements imply are substantially smaller than those calculated from SCUBA dust emission data. Furthermore, the C 1 7 O J = 2 → 1 column densities are approximately constant across L1689B, whereas the SCUBA column densities are peaked towards the centre. The most likely explanation is that CO is depleted from the central regions of L1689B. Simple models of pre-stellar cores with an inner depleted region are compared with the results. This enables the magnitude of the CO depletion to be quantified and also allows the spatial extent of the freeze-out to be firmly established. We estimate that within about 5000 au of the centre of L1689B, over 90 per cent of the CO has frozen on to grains. This level of depletion can only be achieved after a duration that is at least comparable to the free-fall time-scale.

The JCMT Legacy Survey of the Gould Belt: a first look at Serpens with HARP: GBS: first look at Serpens

The Gould Belt Legacy Survey will survey nearby star-forming regions (within 500 pc), using HARP (Heterodyne Array Receiver Programme), SCUBA-2 (Submillimetre Common- User Bolometer Array 2) and POL-2 (Polarimeter 2) on the James Clerk Maxwell Telescope (JCMT). This paper describes the initial data obtained using HARP to observe 12CO, 13CO and C18O J = 3 - 2 towards two regions in Orion B, NGC 2024 and NGC 2071. We describe the physical characteristics of the two clouds, calculating temperatures and opacities utilizing all three isotopologues. We find good agreement between temperatures calculated from CO and from dust emission in the dense, energetic regions. We determine the mass and energetics of the clouds, and of the high-velocity material seen in 12CO emission, and compare the relative energetics of the high- and low-velocity material in the two clouds. We present a CLUMPFIND analysis of the 13CO condensations. The slope of the condensation mass functions, at the high-mass ends, is similar to the slope of the initial mass function.

Molecular tracers of high-mass star formation in external galaxies

Hot-core molecules should be detectable in external active galaxies out to high redshift. We present here a detailed study of the chemistry of star-forming regions under physical conditions that differ significantly from those likely to be appropriate in the Milky Way Galaxy. We examine, in particular, the trends in molecular abundances as a function of time with respect to changes in the relevant physical parameters. These parameters include metallicity, dust:gas mass ratio, the H2 formation rate, relative initial elemental abundances, the cosmic-ray ionization rate, and the temperature of hot cores. These trends indicate how different tracers provide information on the physical conditions and on evolutionary age. We identify hot-core tracers for several observed galaxies that are considered to represent spiral galaxies, active galaxies, low-metallicity galaxies, and high-redshift galaxies. Even in low-metallicity examples, many potential molecular tracers should be present at levels high enough to allow unresolved detection of active galaxies at high redshift containing large numbers of hot cores.

Rotation of the pre-stellar core L1689B

The search for the onset of star formation in pre-stellar cores has focused on the identification of an infall signature in the molecular line profiles of tracer species. The classic infall signature is a double-peaked line profile with an asymmetry in the strength of the peaks such that the blue peak is stronger. L1689B is a pre-stellar core and infall candidate, but new James Clerk Maxwell Telescope (JCMT) HCO+ line profile data, presented here, confirm that both blue and red asymmetric line profiles are present in this source. Moreover, a dividing line can be drawn between the locations where each type of profile is found. It is argued that it is unlikely that the line profiles can be interpreted with simple models of infall or outflow, and that rotation of the inner regions is the most likely explanation. A rotational model is developed in detail with a new three-dimensional molecular line transport code, and it is found that the best type of model is one in which the rotational velocity profile is in between solid-body and Keplerian. It is first shown that red and blue asymmetric line profiles can be generated with a rotation model entirely in the absence of any infall motion. The model is then quantitively compared with the JCMT data, and an iteration over a range of parameters is performed to minimize the difference between the data and the model. The results indicate that rotation can dominate the line profile shape even before the onset of infall.

MOLECULAR TRACERS OF PDR-DOMINATED GALAXIES

Photon-dominated regions (PDRs) are powerful molecular line emitters in external galaxies. They are expected in galaxies with high rates of massive star formation due to either starburst (SB) events or SB coupled with active galactic nuclei (AGNs) events. We have explored the PDR chemistry for a range of physical conditions representing a variety of galaxy types. Our main result is a demonstration of the sensitivity of the chemistry to changes in the physical conditions. We adopt crude estimates of relevant physical parameters for several galaxy types and use our models to predict suitable molecular tracers of those conditions. The set of recommended molecular tracers differs from that which we recommended for use in galaxies with embedded massive stars. Thus, molecular observations can in principle be used to distinguish between excitation by SB and by SB+AGN in distant galaxies. Our recommendations are intended to be useful in preparing Herschel and ALMA proposals to identify sources of excitation in galaxies.