arXiv subjects

M. G. Hoare

Publications and source records attributed to M. G. Hoare.

At least 55 records · Page 3Linked to original sources

arXiv2012

The Co-ordinated Radio and Infrared Survey for High-Mass Star Formation - II. Source Catalogue

The CORNISH project is the highest resolution radio continuum survey of the Galactic plane to date. It is the 5 GHz radio continuum part of a series of multi-wavelength surveys that focus on the northern GLIMPSE region (10 deg < l < 65 deg), observed by the Spitzer satellite in the mid-infrared. Observations with the Very Large Array in B and BnA configurations have yielded a 1.5" resolution Stokes I map with a root-mean-squared noise level better than 0.4 mJy/beam. Here we describe the data-processing methods and data characteristics, and present a new, uniform catalogue of compact radio-emission. This includes an implementation of automatic deconvolution that provides much more reliable imaging than standard CLEANing. A rigorous investigation of the noise characteristics and reliability of source detection has been carried out. We show that the survey is optimised to detect emission on size scales up to 14" and for unresolved sources the catalogue is more than 90 percent complete at a flux density of 3.9 mJy. We have detected 3,062 sources above a 7-sigma detection limit and present their ensemble properties. The catalogue is highly reliable away from regions containing poorly-sampled extended emission, which comprise less than two percent of the survey area. Imaging problems have been mitigated by down-weighting the shortest spacings and potential artefacts flagged via a rigorous manual inspection with reference to the Spitzer infrared data. We present images of the most common source types found: regions, planetary nebulae and radio-galaxies. The CORNISH data and catalogue are available online at http://cornish.leeds.ac.uk

astro-ph.GA↗

arXiv2012

CO bandhead emission of massive young stellar objects: determining disc properties

Massive stars play an important role in many areas of astrophysics, but numerous details regarding their formation remain unclear. In this paper we present and analyse high resolution (R ~ 30,000) near-infrared 2.3 micron spectra of 20 massive young stellar objects from the RMS database, in the largest such study of CO first overtone bandhead emission to date. We fit the emission under the assumption it originates from a circumstellar disc in Keplerian rotation. We explore three approaches to modelling the physical conditions within the disc - a disc heated mainly via irradiation from the central star, a disc heated mainly via viscosity, and a disc in which the temperature and density are described analytically. We find that the models described by heating mechanisms are inappropriate because they do not provide good fits to the CO emission spectra. We therefore restrict our analysis to the analytic model, and obtain good fits to all objects that possess sufficiently strong CO emission, suggesting circumstellar discs are the source of this emission. On average, the temperature and density structure of the discs correspond to geometrically thin discs, spread across a wide range of inclinations. Essentially all the discs are located within the dust sublimation radius, providing strong evidence that the CO emission originates close to the central protostar, on astronomical unit scales. In addition, we show that the objects in our sample appear no different to the general population of MYSOs in the RMS database, based on their near- and mid-infrared colours. The combination of observations of a large sample of MYSOs with CO bandhead emission and our detailed modelling provide compelling evidence of the presence of small scale gaseous discs around such objects, supporting the scenario in which massive stars form via disc accretion.

astro-ph.SR↗

arXiv2012

The Co-ordinated Radio and Infrared Survey for High Mass Star Formation (The CORNISH Survey) - I. Survey Design

We describe the motivation, design and implementation of the CORNISH survey, an arcsecond resolution radio continuum survey of the inner Galactic plane at 5 GHz using the Karl G. Jansky Very Large Array (VLA). It is a blind survey co-ordinated with the northern Spitzer GLIMPSE I region covering 10 deg < l < 65 deg and |b| < 1 deg at similar resolution. We discuss in detail the strategy that we employed to control the shape of the synthesised beam across this survey that covers a wide range of fairly low declinations. Two snapshots separated by 4 hours in hour angle kept the beam elongation to less that 1.5 over 75% of the survey area and less than 2 over 98% of the survey. The prime scientific motivation is to provide an unbiased survey for ultra-compact HII regions to study this key phase in massive star formation. A sensitivity around 2 mJy will allow the automatic distinction between radio loud and quiet mid-IR sources found in the Spitzer surveys. This survey has many legacy applications beyond star formation including evolved stars, active stars and binaries, and extragalactic sources. The CORNISH survey for compact ionized sources complements other Galactic plane surveys that target diffuse and non-thermal sources as well as atomic and molecular phases to build up a complete picture of the ISM in the Galaxy.

astro-ph.GA↗

arXiv2012

The H2O southern Galactic Plane Survey(HOPS): NH3 (1,1) and (2,2) catalogues

The H2O Southern Galactic Plane Survey (HOPS) has mapped a 100 degree strip of the Galactic plane (-70deg > l > 30deg, |b| < 0.5deg) using the 22-m Mopra antenna at 12-mm wavelengths. Observations were conducted in on-the-fly mode using the Mopra spectrometer (MOPS), targeting water masers, thermal molecular emission and radio-recombination lines. Foremost among the thermal lines are the 23 GHz transitions of NH3 J,K = (1,1) and (2,2), which trace the densest parts of molecular clouds (n > 10^4 cm^{-3}). In this paper we present the NH3 (1,1) and (2,2) data, which have a resolution of 2 arcmin and cover a velocity range of +/-200 km/s. The median sensitivity of the NH3 data-cubes is sigma_Tmb = 0.20 +/1 0.06 K. For the (1,1) transition this sensitivity equates to a 3.2 kpc distance limit for detecting a 20 K, 400 Msun cloud at the 5-sigma level. Similar clouds of mass 5,000 Msun would be detected as far as the Galactic centre, while 30,000 Msun clouds would be seen across the Galaxy. We have developed an automatic emission finding procedure based on the ATNF DUCHAMP software and have used it to create a new catalogue of 669 dense molecular clouds. The catalogue is 100 percent complete at the 5-sigma detection limit (Tmb = 1.0 K). A preliminary analysis of the ensemble cloud properties suggest that the near kinematic distances are favoured. The cloud positions are consistent with current models of the Galaxy containing a long bar. Combined with other Galactic plane surveys this new molecular-line dataset constitutes a key tool for examining Galactic structure and evolution. Data-cubes, spectra and catalogues are available to the community via the HOPS website.

astro-ph.GA↗

arXiv2012

Tracers of Discs and Winds around Intermediate and High Mass Young Stellar Objects

We present a study of the kinematical properties of a small sample of nearby near-infrared bright massive and intermediate mass young stellar objects using emission lines sensitive to discs and winds. We show for the first time that the broad ($\sim500$kms$^{-1}$) symmetric line wings on the HI Brackett series lines are due to Stark broadening or electron scattering, rather than pure Doppler broadening due to high speed motion. The results are consistent with the presence of a very dense circumstellar environment. In addition, many of these lines show evidence for weak line self-absorption, suggestive of a wind or disc-wind origin for that part of the absorbing material. The weakness of the self-absorption suggests a large opening angle for such an outflow. We also study the fluorescent 1.688$μ$m FeII line, which is sensitive to dense material. We fitted a Keplerian disc model to this line, and find reasonable fits in all bar one case, in agreement with previous finding for classical Be stars that fluorescent iron transitions are reasonable disc tracers. Overall the picture is one in which these stars still have accretion discs, with a very dense inner circumstellar environment which may be tracing either the inner regions of a disc, or of a stellar wind, and in which ionised outflow is also present. The similarity with lower mass stars is striking, suggesting that at least in this mass range they form in a similar fashion.

astro-ph.SR↗

arXiv2012

Probing the envelopes of massive young stellar objects with diffraction limited mid-infrared imaging

Massive stars form whilst they are still embedded in dense envelopes. As a result, the roles of rotation, mass loss and accretion in massive star formation are not well understood. This study evaluates the source of the Q-band, lambda=19.5 microns, emission of massive young stellar objects (MYSOs). This allows us to determine the relative importance of rotation and outflow activity in shaping the circumstellar environments of MYSOs on 1000 AU scales. We obtained diffraction limited mid-infrared images of a sample of 20 MYSOs using the VLT/VISIR and Subaru/COMICS instruments. For these 8 m class telescopes and the sample selected, the diffraction limit, ~0.6", corresponds to approximately 1000 AU. We compare the images and the spectral energy distributions (SEDs) observed to a 2D, axis-symmetric dust radiative transfer model that reproduces VLTI/MIDI observations of the MYSO W33A. We vary the inclination, mass infall rate, and outflow opening angle to simultaneously recreate the behaviour of the sample of MYSOs in the spatial and spectral domains. The mid-IR emission of 70 percent of the MYSOs is spatially resolved. In the majority of cases, the spatial extent of their emission and their SEDs can be reproduced by the W33A model featuring an in-falling, rotating dusty envelope with outflow cavities. There is independent evidence that most of the sources which are not fit by the model are associated with ultracompact HII regions and are thus more evolved. We find that, in general, the diverse 20 micron morphology of MYSOs can be attributed to warm dust in the walls of outflow cavities seen at different inclinations. This implies that the warm dust in the outflow cavity walls dominates the Q-band emission of MYSOs. In turn, this emphasises that outflows are an ubiquitous feature of massive star formation.

astro-ph.SR↗

arXiv2011

VVV DR1: The First Data Release of the Milky Way Bulge and Southern Plane from the Near-Infrared ESO Public Survey VISTA Variables in the Via Lactea

The ESO Public Survey VISTA Variables in the Via Lactea (VVV) started in 2010. VVV targets 562 sq. deg in the Galactic bulge and an adjacent plane region and is expected to run for ~5 years. In this paper we describe the progress of the survey observations in the first observing season, the observing strategy and quality of the data obtained. The observations are carried out on the 4-m VISTA telescope in the ZYJHKs filters. In addition to the multi-band imaging the variability monitoring campaign in the Ks filter has started. Data reduction is carried out using the pipeline at the Cambridge Astronomical Survey Unit. The photometric and astrometric calibration is performed via the numerous 2MASS sources observed in each pointing. The first data release contains the aperture photometry and astrometric catalogues for 348 individual pointings in the ZYJHKs filters taken in the 2010 observing season. The typical image quality is ~0.9-1.0". The stringent photometric and image quality requirements of the survey are satisfied in 100% of the JHKs images in the disk area and 90% of the JHKs images in the bulge area. The completeness in the Z and Y images is 84% in the disk, and 40% in the bulge. The first season catalogues contain 1.28x10^8 stellar sources in the bulge and 1.68x10^8 in the disk area detected in at least one of the photometric bands. The combined, multi-band catalogues contain more than 1.63x10^8 stellar sources. About 10% of these are double detections due to overlapping adjacent pointings. These overlapping multiple detections are used to characterise the quality of the data. The images in the JHKs bands extend typically ~4 mag deeper than 2MASS. The magnitude limit and photometric quality depend strongly on crowding in the inner Galactic regions. The astrometry for Ks=15-18 mag has rms ~35-175 mas.

astro-ph.GA↗

arXiv2011

The RMS Survey: Resolving kinematic distance ambiguities towards a sample of compact HII regions using HI absorption

We present high-resolution HI data obtained using the Australia Telescope Compact Array to resolve the near/far distance ambiguities towards a sample of compact HII regions from the Red MSX Source (RMS) survey. The high resolution data are complemented with lower resolution archival HI data extracted from the Southern and VLA Galactic Plane surveys. We resolve the distance ambiguity for nearly all of the 105 sources where the continuum was strong enough to allow analysis of the HI absorption line structure. This represents another step in the determination of distances to the total RMS sample, which with over 1,000 massive young stellar objects and compact HII regions, is the largest and most complete sample of its kind. The full sample will allow the distribution of massive star formation in the Galaxy to be examined.

astro-ph.GA↗

arXiv2011

The RMS Survey: Ammonia and water maser analysis of massive star forming regions

The Red MSX Source (RMS) survey has identified a sample of ~1200 massive young stellar objects (MYSOs), compact and ultra compact HII regions from a sample of ~2000 MSX and 2MASS colour selected sources. We have used the 100 m Green Bank telescope to search for 22-24 GHz water maser and ammonia (1,1), (2,2) and (3,3) emission towards ~600 RMS sources located within the northern Galactic plane. We have identified 308 H2O masers which corresponds to an overall detection rate of ~50%. Abridged: We detect ammonia emission towards 479 of these massive young stars, which corresponds to ~80%. Ammonia is an excellent probe of high density gas allowing us to measure key parameters such as gas temperatures, opacities, and column densities, as well as providing an insight into the gas kinematics. The average kinetic temperature, FWHM line width and total NH3 column density for the sample are approximately 22 K, 2 km/s and 2x10^{15} cm^{-2}, respectively. We find that the NH3 (1,1) line width and kinetic temperature are correlated with luminosity and finding no underlying dependence of these parameters on the evolutionary phase of the embedded sources, we conclude that the observed trends in the derived parameters are more likely to be due to the energy output of the central source and/or the line width-clump mass relationship. The velocities of the peak H2O masers and the NH3 emission are in excellent agreement with each other, which would strongly suggest an association between the dense gas and the maser emission. Moreover, we find the bolometric luminosity of the embedded source and the isotropic luminosity of the H2O maser are also correlated. We conclude from the correlations of the cloud and water maser velocities and the bolometric and maser luminosity that there is a strong dynamical relationship between the embedded young massive star and the H2O maser.

astro-ph.GA↗

arXiv2011

The H_2O southern Galactic Plane Survey (HOPS): I. Techniques and H_2O maser data

We present first results of the H_2O Southern Galactic Plane Survey (HOPS), using the Mopra radiotelescope with a broad band backend and a beam size of about 2'. We have observed 100 square degrees of the southern Galactic plane at 12mm (19.5 to 27.5GHz), including spectral line emission from water masers, multiple metastable transitions of ammonia, cyanoacetylene, methanol and radio recombination lines. In this paper, we report on the characteristics of the survey and water maser emission. We find 540 water masers, of which 334 are new detections. The strongest maser is 3933Jy and the weakest is 0.7Jy, with 62 masers over 100Jy. In 14 maser sites, the spread in velocity of the water maser emission exceeds 100km/s. In one region, the water maser velocities are separated by 351.3km/s. The rms noise levels are typically between 1-2Jy, with 95% of the survey under 2Jy. We estimate completeness limits of 98% at around 8.4Jy and 50% at around 5.5Jy. We estimate that there are between 800 and 1500 water masers in the Galaxy that are detectable in a survey with similar completeness limits to HOPS. We report possible masers in NH_3 (11,9) and (8,6) emission towards G19.61-0.23 and in the NH_3 (3,3) line towards G23.33-0.30.

astro-ph.GA↗

arXiv2011

Goonhilly: a new site for e-MERLIN and the EVN

The benefits for the e-MERLIN and EVN arrays of using antennae at the satellite communication station at Goonhilly in Cornwall are discussed. The location of this site - new to astronomy - will provide an almost equal distribution of long baselines in the east-west- and north-south directions, and opens up the possibility to get significantly improved observations of equatorial fields with e-MERLIN. These additional baselines will improve the sensitivity on a set of critical spatial scales and will increase the angular resolution of e-MERLIN by a factor of two. e-MERLIN observations, including many allocated under the e-MERLIN Legacy programme, will benefit from the enhanced angular resolution and imaging capability especially for sources close to or below the celestial equator (where ESO facilities such as ALMA will operate) of including the Goonhilly telescopes. Furthermore, the baselines formed between Goonhilly and the existing stations will close the gap between the baselines of e-MERLIN and those of the European VLBI Network (EVN) and therefore enhance the legacy value of e-MERLIN datasets.

astro-ph.IM↗

arXiv2011

Expanding e-MERLIN with the Goonhilly Earth Station

A consortium of universities has recently been formed with the goal of using the decommissioned telecommunications infrastructure at the Goonhilly Earth Station in Cornwall, UK, for astronomical purposes. One particular goal is the introduction of one or more of the ~30-metre parabolic antennas into the existing e-MERLIN radio interferometer. This article introduces this scheme and presents some simulations which quantify the improvements that would be brought to the e-MERLIN system. These include an approximate doubling of the spatial resolution of the array, an increase in its N-S extent with strong implications for imaging the most well-studied equatorial fields, accessible to ESO facilities including ALMA. It also increases the overlap between the e-MERLIN array and the European VLBI Network. We also discuss briefly some niche science areas in which an e-MERLIN array which included a receptor at Goonhilly would be potentially world-leading, in addition to enhancing the existing potential of e-MERLIN in its role as a Square Kilometer Array pathfinder instrument.

astro-ph.IM↗

arXiv2010

Mid-infrared interferometry towards the massive young stellar object CRL 2136: inside the dust rim

We apply mid-infrared spectro-interferometry to the massive young stellar object CRL2136. The observations were performed with the Very Large Telescope Interferometer and the MIDI instrument at a 42m baseline probing angular scales of 50 milli-arcseconds. We model the observed visibilities in parallel with diffraction-limited images at both 24.5micron and in the N-band (with resolutions of 0.6" and 0.3", respectively), as well as the spectral energy distribution. The arcsec-scale spatial information reveals the well-resolved emission from the dusty envelope. By simultaneously modelling the spatial and spectral data, we find that the bulk of the dust emission occurs at several dust sublimation radii (approximately 170 AU). This reproduces the high mid-infrared fluxes and at the same time the low visibilities observed in the MIDI data for wavelengths longward of 8.5micron. However, shortward of this wavelength the visibility data show a sharp up-turn indicative of compact emission. We discuss various potential sources of this emission. We exclude a dust disk being responsible for the observed spectral imprint on the visibilities. A cool supergiant star and an accretion disk are considered and both shown to be viable origins of the compact mid-infrared emission. We propose that CRL2136 is embedded in a dusty envelope, which truncates at several times the dust sublimation radius. A dust torus is manifest in the equatorial region. We find that the spectro-interferometric N-band signal can be reproduced by either a gaseous disk or a bloated central star. If the disk extends to the stellar surface, it accretes at a rate of 3.0 10^(-3) Msun/yr.

astro-ph.GA↗

arXiv2010

The interactions of winds from massive young stellar objects

The supersonic stellar and disk winds possessed by massive young stellar objects will produce shocks when they collide against the interior of a pre-existing bipolar cavity (resulting from an earlier phase of jet activity). The shock heated gas emits thermal X-rays which may be observable by spaceborne observa- tories such as the Chandra X-ray Observatory. Hydrodynamical models are used to explore the wind-cavity interaction. Radiative transfer calculations are performed on the simulation output to produce synthetic X-ray observations, allowing constraints to be placed on model parameters through comparisons with observations. The model reveals an intricate interplay between the inflowing and outflowing material and is successful in reproducing the observed X-ray count rates from massive young stellar objects.

astro-ph.HE↗

arXiv2010

The RMS Survey: The Bolometric Fluxes and Luminosity Distributions of Young Massive Stars

Context: The Red MSX Source (RMS) survey is returning a large sample of massive young stellar objects (MYSOs) and ultra-compact (UC) \HII{} regions using follow-up observations of colour-selected candidates from the MSX point source catalogue. Aims: To obtain the bolometric fluxes and, using kinematic distance information, the luminosities for young RMS sources with far-infrared fluxes. Methods: We use a model spectral energy distribution (SED) fitter to obtain the bolometric flux for our sources, given flux data from our work and the literature. The inputs to the model fitter were optimised by a series of investigations designed to reveal the effect varying these inputs had on the resulting bolometric flux. Kinematic distances derived from molecular line observations were then used to calculate the luminosity of each source. Results: Bolometric fluxes are obtained for 1173 young RMS sources, of which 1069 have uniquely constrained kinematic distances and good SED fits. A comparison of the bolometric fluxes obtained using SED fitting with trapezium rule integration and two component greybody fits was also undertaken, and showed that both produce considerable scatter compared to the method used here. Conclusions: The bolometric flux results allowed us to obtain the luminosity distributions of YSOs and UC\HII{} regions in the RMS sample, which we find to be different. We also find that there are few MYSOs with L $\geq$ 10$^{5}$\lsol{}, despite finding many MYSOs with 10$^{4}$\lsol{} $\geq$ L $\geq$ 10$^{5}$\lsol{}.

astro-ph.GA↗

arXiv2010

The RMS Survey: Distribution and properties of a sample of massive young stars

The Red MSX Source (RMS) survey has identified a large sample of massive young stellar objects (MYSOs) and ultra compact (UC) HII regions from a sample of ~2000 MSX and 2MASS colour selected sources. Using a recent catalogue of molecular clouds derived from the Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey (GRS), and by applying a Galactic scaleheight cut off of 120 pc, we solve the distance ambiguity for RMS sources located within 18\degr < |l| > 54\degr. These two steps yield kinematic distances to 291 sources out of a possible 326 located within the GRS longitude range. Combining distances and integrated fluxes derived from spectral energy distributions, we estimate luminosities to these sources and find that > 90% are indicative of the presence of a massive star. We find the completeness limit of our sample is ~10^4 Lsun, which corresponds to a zero age main sequence (ZAMS) star with a mass of ~12 Msun. Selecting only these sources, we construct a complete sample of 196 sources. Comparing the properties of the sample of young massive stars with the general population, we find the RMS-clouds are generally larger, more massive, and more turbulent. We examine the distribution of this sub-sample with respect to the location of the spiral arms and the Galactic bar and find them to be spatially correlated. We identify three significant peaks in the source surface density at Galactocentric radii of approximately 4, 6 and 8 kpc, which correspond to the proposed positions of the Scutum, Sagittarius and Perseus spiral arms, respectively. Fitting a scale height to the data we obtain an average value of ~29+-0.5 pc, which agrees well with other reported values in the literature, however, we note a dependence of the scale height on galactocentric radius with it increases from 30 pc to 45 pc between 2.5 and 8.5 kpc.

astro-ph.GA↗

arXiv2010

Characterisation of the Mopra Radio Telescope at 16--50 GHz

We present the results of a programme of scanning and mapping observations of astronomical masers and Jupiter designed to characterise the performance of the Mopra Radio Telescope at frequencies between 16-50 GHz using the 12-mm and 7-mm receivers. We use these observations to determine the telescope beam size, beam shape and overall telescope beam efficiency as a function of frequency. We find that the beam size is well fit by $λ$/$D$ over the frequency range with a correlation coefficient of ~90%. We determine the telescope main beam efficiencies are between ~48-64% for the 12-mm receiver and reasonably flat at ~50% for the 7-mm receiver. Beam maps of strong H$_2$O (22 GHz) and SiO masers (43 GHz) provide a means to examine the radial beam pattern of the telescope. At both frequencies the radial beam pattern reveals the presence of three components, a central `core', which is well fit by a Gaussian and constitutes the telescopes main beam, and inner and outer error beams. At both frequencies the inner and outer error beams extend out to approximately 2 and 3.4 times the full-width half maximum of the main beam respectively. Sources with angular sizes a factor of two or more larger than the telescope main beam will couple to the main and error beams, and therefore the power contributed by the error beams needs to be considered. From measurements of the radial beam power pattern we estimate the amount of power contained in the inner and outer error beams is of order one-fifth at 22 GHz rising slightly to one-third at 43 GHz.

astro-ph.GA↗

arXiv2010

Probing discs around massive young stellar objects with CO first overtone emission

We present high resolution (R~50,000) spectroastrometry over the CO 1st overtone bandhead of a sample of seven intermediate/massive young stellar objects. These are primarily drawn from the red MSX source (RMS) survey, a systematic search for young massive stars which has returned a large, well selected sample of such objects. The mean luminosity of the sample is approximately 5 times 10^4 L_\odot, indicating the objects typically have a mass of ~15 solar masses. We fit the observed bandhead profiles with a model of a circumstellar disc, and find good agreement between the models and observations for all but one object. We compare the high angular precision (0.2-0.8 mas) spectroastrometric data to the spatial distribution of the emitting material in the best-fitting models. No spatial signatures of discs are detected, which is entirely consistent with the properties of the best-fitting models. Therefore, the observations suggest that the CO bandhead emission of massive young stellar objects originates in small-scale disks, in agreement with previous work. This provides further evidence that massive stars form via disc accretion, as suggested by recent simulations.

astro-ph.SR↗