arXiv subjects

M. G. Hoare

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

At least 91 records · Page 5Linked to original sources

arXiv2007

The RMS Survey: A Galaxy-wide Sample of Massive Young Stellar Objects

Here we describe the Red MSX Source (RMS) survey which is the largest, systematic, galaxy-wide search for massive young stellar objects (MYSOs) yet undertaken. Mid-IR bright point sources from the MSX satellite survey have been followed-up with ground-based radio, millimetre, and infrared observations to identify the contaminating sources and characterise the MYSOs and UCHII regions. With the initial classification now complete the distribution of sources in the galaxy will be discussed, as well as some programmes being developed to exploit our sample.

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arXiv2007

VLTI/MIDI 10 micron interferometry of the forming massive star W33A

We report on resolved interferometric observations with VLTI/MIDI of the massive young stellar object (MYSO) W33A. The MIDI observations deliver spectrally dispersed visibilities with values between 0.03 and 0.06, for a baseline of 45m over the wavelength range 8-13 micron. The visibilities indicate that W33A has a FWHM size of approximately 120AU (0.030'') at 8 micron which increases to 240AU at 13 micron, scales previously unexplored among MYSOs. This observed trend is consistent with the temperature falling off with distance. 1D dust radiative transfer models are simultaneously fit to the visibility spectrum, the strong silicate feature and the shape of the mid infrared spectral energy distribution (SED). For any powerlaw density distribution, we find that the sizes (as implied by the visibilities) and the stellar luminosity are incompatible. A reduction to a third of W33A's previously adopted luminosity is required to match the visibilities; such a reduction is consistent with new high resolution 70 micron data from Spitzer's MIPSGAL survey. We obtain best fits for models with shallow dust density distributions of r^(-0.5) and r^(-1.0) and for increased optical depth in the silicate feature produced by decreasing the ISM ratio of graphite to silicates and using optical grain properties by Ossenkopf et al. (1992).

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arXiv2007

The RMS Survey: Mid-Infrared Observations of Candidate Massive YSOs in the Southern Hemisphere

Abridged abstract: The Red MSX Source (RMS) survey is an ongoing effort to return a large, well-selected sample of massive young stellar objects (MYSOs) within our Galaxy. A series of ground-based follow-up observations are being undertaken in order to remove contaminant objects from our list of 2000 candidates, and to begin characterising these MYSOs. As a part of these follow-up observations, high resolution (~1") mid-IR imaging aids the identification of contaminant objects which are resolved (UCHII regions, PN) as opposed to those which are unresolved (YSOs, evolved stars) as well as identifying YSOs near UCHII regions and other multiple sources. We present 10.4 micron imaging observations for 346 candidate MYSOs in the RMS survey in the Southern Hemisphere, primarily outside the region covered by the GLIMPSE Spitzer Legacy Survey. These were obtained using TIMMI2 on the ESO 3.6m telescope in La Silla, Chile. Our photometric accuracy is of order 0.05Jy, and our astrometric accuracy is 0.8", which is an improvement over the nominal 2" accuracy of the MSX PSC.

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arXiv2007

The RMS Survey: 13CO observations of candidate massive YSOs in the southern hemisphere

Abridged: The Red MSX Source (RMS) survey is an ongoing multi-wavelength observational programme designed to return a large, well-selected sample of massive young stellar objects (MYSOs). Here we present 13CO observations made towards 854 MYSOs candidates located in the 3rd and 4th quadrants. We detected 13CO emission towards a total of 751 of the 854 RMS sources observed (~88%). In total 2185 emission components are detected above 3$σ$ level. Multiple emission profiles are observed towards the majority of these sources - 455 sources (~60%) - with an average of ~4 molecular clouds along the line of sight. These multiple emission features make it difficult to assign a kinematic velocity to many of our sample. We have used archival CS (J=2-1) and maser velocities to resolved the component multiplicity towards 82 sources and have derived a criterion which is used to identify the most likely component for a further 202 multiple component sources. Combined with the single component detections we have obtained unambiguous kinematic velocities towards 580 sources (~80% of the detections). The 171 sources for which we have not been able to determine the kinematic velocity will require additional line data. Using the rotation curve of Brand and Blitz (1993) and their radial velocities we calculate kinematic distances for all components detected.

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arXiv2007

A high-frequency radio continuum study of massive young stellar objects

We present high-resolution observations made with the Very Large Array (VLA) in its A configuration at frequencies between 5 and 43 GHz of a sample of five massive young stellar objects (YSOs): LkHa101, NGC2024-IRS2, S106-IR, W75N and S140-IRS1. The resolution varied from 0.04 arcsec (at 43 GHz) to 0.5 arcsec (at 5 GHz), corresponding to a linear resolution as high as 17 AU for our nearest source. A MERLIN observation of S106-IR at 23 GHz with 0.03-arcsec resolution is also presented. S106-IR and S140-IRS1 are elongated at 43 GHz perpendicular to their large scale bipolar outflows. This confirms the equatorial wind picture for these sources seen previously in MERLIN 5 GHz observations. The other sources are marginally resolved at 43 GHz. The spectral indices we derive for the sources in our sample range from +0.2 to +0.8, generally consistent with ionized stellar winds. We have modelled our sources as uniform, isothermal spherical winds, with LkHa101 and NGC2024-IRS2 yielding the best fits. However, in all cases our fits give wind temperatures of only 2000 to 5000 K, much less than the effective temperatures of main-sequence stars of the same luminosity, a result which is likely due to the clumpy nature of the winds.

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arXiv2006

The RMS Survey: Radio and mid-IR observations of candidate massive YSOs

The Red MSX Source (RMS) survey is a multi-wavelength programme of follow-up observations designed to distinguish between genuine massive young stellar objects (MYSOs) and other embedded or dusty objects, such as ultra compact (UC) HII regions, evolved stars and planetary nebulae (PNe), from a sample of ~2000 MYSOs candidates. These were identified by comparing the colours of objects from the MSX and 2MASS point source catalogues to those of known MYSOs, in order to develop colour selection criteria which have been used to produced the RMS sample of MYSOs candidates. Our ultimate aim is to produce a large unbiased sample of MYSOs (~500) with complementary multi-wavelength data with which to study their properties. Here we report the results of 826 radio continuum and 346 mid-IR imaging observations carried out using the Australia Telescope Compact Array and TIMMI2 on the ESO 3.6 m telescope respectively. These observations are aimed at identifying and removing contaminating sources.

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arXiv2006

The RMS Survey: Radio observations of candidate massive YSOs in the southern hemisphere

The Red MSX Source (RMS) survey is a multi-wavelength program of follow-up observations designed to distinguish between genuine massive young stellar objects (MYSOs) and other embedded or dusty objects, such as ultra compact (UC) HII regions, evolved stars and planetary nebulae (PNe). We have identified nearly 2000 massive YSOs candidates by comparing the colours of MSX and 2MASS point sources to those of known MYSOs. Unfortunately, there are several other types of embedded or dust enshrouded objects that have similar colours as MYSOs and contaminate our sample. Two sources of contamination are from UCHII regions and PNe, both of which can be identified from the radio emission emitted by their ionised nebulae. In order to identify UCHII regions and PNe that contaminate our sample we have conducted high resolution radio continuum observations at 3.6 and 6 cm of all southern MYSOs candidates ($235< l < 350$) using the Australia Telescope Compact Array (ATCA). These observations have a spatial resolution of $\sim$1--2\arcsec and typical image rms noise values of $\sim$0.3 mJy -- sensitive enough to detect a B0.5 star at the far side of the Galaxy. Of the 826 RMS sources observed we found 199 to be associated with radio emission, $\sim$25% of the sample. More significant than those sources associated with radio emission are the 627 RMS sources for which no radio emission was detected as these sources are still potential MYSOs. The Galactic distribution, morphologies and spectral indices of the radio sources associated with the RMS sources are consistent with these sources being UCHII regions. In addition to the 826 RMS sources observed we present observations of a further 190 fields. These observations were made towards MSX sources that passed cuts in earlier versions of the survey, but were later excluded.

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arXiv2006

Evidence for variable outflows in the Young Stellar Object V645 Cygni

As part of the Red MSX Source Survey of Massive Young Stellar Objects (MYSOs) we have conducted multi-wavelength follow up observations of the well-known object V645 Cygni. We present our data on this object, whose near-infrared spectrum is exceptional and place these in context with previous observations. Our observations of V645 Cyg included near/mid infrared imaging observations, 13CO 2-1 line observations and high signal-to-noise velocity resolved near-infrared spectroscopy. The spectrum shows P-Cygni hydrogen Brackett emission, consistent with a high velocity stellar wind. A red-shifted emission component to a number of near-IR emission lines was also uncovered. This is associated with a similar component in the H alpha line. V645 Cyg is also found to have variable CO first overtone bandhead emission. The data clearly indicate that the outflow of V645 Cyg is variable. The unidentified feature in a previously published optical spectrum is identified with a receding outflow at 2000 km per second. The nature of this feature, which is found in hydrogen and helium atomic lines and CO molecular lines remains a puzzle.

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arXiv2006

Hydrodynamics of Cometary Compact HII Regions

We present numerical radiation-hydrodynamic simulations of cometary HII regions for a number of champagne flow and bowshock models. For the champagne flow models we study smooth density distributions with both steep and shallow gradients. We also consider cases where the ionizing star has a strong stellar wind, and cases in which the star additionally has a proper motion within the ambient density gradient. We present simulated emission-measure maps and long-slit spectra of our results. Our numerical models are not tailored to any particular object but comparison with observations from the literature shows that, in particular, the models combining density gradients and stellar winds are able to account for both the morphology and radial velocity behavior of several observed cometary HII regions, such as the well-studied object G29.96-0.02.

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arXiv2006

Ultra-Compact H II Regions and the Early Lives of Massive Stars

We review the phenomenon of ultra-compact H II regions (UCHIIs) as a key phase in the early lives of massive stars. This most visible manifestation of massive star formation begins when the Lyman continuum output from the massive young stellar object becomes sufficient to ionize the surroundings from which it was born. Knowledge of this environment is gained through an understanding of the morphologies of UCHII regions and we examine the latest developments in deep radio and mid-IR imaging. SPITZER data from the GLIMPSE survey are an important new resource in which PAH emission and the ionizing stars can be seen. We review the role played by strong stellar winds from the central stars in sweeping out central cavities and causing the limb-brightened appearance. A range of evidence from velocity structure, proper motions, the molecular environment and recent hydrodynamical modeling indicates that cometary UCHII regions require a combination of champagne flow and bow shock motion. Finally, we discuss the class of hyper-compact H II regions or broad recombination line objects. They are likely to mark the transition soon after the breakout of the Lyman continuum radiation from the young star. Models for these objects are presented, including photo-evaporating disks and ionized accretion flows that are gravitationally trapped. Evolutionary scenarios tracing young massive stars passage through these ionized phases are discussed.

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arXiv2005

A CH3CN and HCO+ survey towards southern methanol masers associated with star formation

We present the initial results of a 3-mm spectral line survey towards 83 methanol maser selected massive star-forming regions. Here we report observations of the J=5-4 and 6-5 rotational transitions of methyl cyanide (CH3CN) and the J=1-0 transition of HCO+and H13CO+. CH3CN emission is detected in 58 sources (70 %) of our sample). We estimate the temperature and column density for 37 of these using the rotational diagram method. The temperatures we derive range from 28-166 K, and are lower than previously reported temperatures, derived from higher J transitions. We find that CH3CN is brighter and more commonly detected towards ultra-compact HII (UCHII) regions than towards isolated maser sources. Detection of CH3CN towards isolated maser sources strongly suggests that these objects are internally heated and that CH3CN is excited prior to the UCHII phase of massive star-formation. HCO+ is detected towards 82 sources (99 % of our sample), many of which exhibit asymmetric line profiles compared to H13CO+. Skewed profiles are indicative of inward or outward motions, however, we find approximately equal numbers of red and blue-skewed profiles among all classes. Column densities are derived from an analysis of the HCO+ and H13CO+ line profiles. 80 sources have mid-infrared counterparts: 68 seen in emission and 12 seen in absorption as `dark clouds'. Seven of the twelve dark clouds exhibit asymmetric HCO+ profiles, six of which are skewed to the blue, indicating infalling motions. CH3CN is also common in dark clouds, where it has a 90 % detection rate.

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arXiv2005

Resolving the Kinematic Distance Ambiguity of Southern Massive Young Stellar Object Candidates

We investigate the use of HI data to resolve the near/far ambiguity in kinematic distances of massive young stellar object (MYSO) candidates. Kinematic distances were obtained from 13CO 1-0 spectral line observations with the Mopra Telescope towards 94 candidates selected from the Red MSX Source (RMS) survey in the fourth Galactic quadrant. HI data from the Southern Galactic Plane Survey (SPGS) was used in conjunction with the HI self-absorption technique to determine the near or far distance. We resolved the kinematic distance ambiguity to 70% of the sources. We can also simultaneously solve for any multiple line-of-sight component sources. We discuss the advantages and disadvantages of this technique in comparison with other methods, and also perform confidence checks on the reliability of using the HI self-absorption technique. We examined the projected location of these sources in both the Galactic plane and longitude-velocity diagrams to ascertain any recognisable spiral arm pattern. Although no obvious spiral pattern was found when compared to that proposed by Cordes & Lazio, far distance sources tended to lie on or near spiral arm loci. Near distance sources, however, had peculiar velocity uncertainties consistent with the separation between the spiral arms themselves. The longitude-velocity plot shows a more consistent picture, with tangent points of the spiral arm loci easily seen. We conclude that using the HI self-absorption technique to determine kinematic distance ambiguities is a quick and reliable method in most cases, with an 80% success rate in determining the correct designation of whether an object is at the near or far distance.

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arXiv2004

Star Formation at High Angular Resolution

The role of the SKA in high resolution observations of the formation of low and high mass stars in the Galaxy is examined. The large collecting area will have a large impact on dynamical studies at high resolution using spectral lines. The SKA will allow dramatic progress in the investigation of the ionised, atomic and molecular phases over a range of evolutionary stages. Attention is focused here on the potential of radio recombination lines, H I 21 cm line and molecular Zeeman measurements. This combination will unveil the physical processes that drive jets and winds from young stars. Other important areas such as pre-biotic molecules, disc ablation in OB clusters and massive star formation in nearby galaxies are also briefly discussed.

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arXiv2002

Helium and Hydrogen Line Ratios and The Stellar Content of Compact HII Regions

We present observations and models of the behaviour of the HI and HeI lines between 1.6 and 2.2um in a small sample of compact HII regions. As in our previous papers on planetary nebulae, we find that the `pure' 1.7007um 4^3D-3^3P and 2.16475um 7^(3,1)G-4^(3,1)F HeI recombination lines behave approximately as expected as the effective temperature of the central exciting star(s) increases. However, the 2.058um 2^1P-2^1S HeI line does not behave as the model predicts, or as seen in planetary nebulae. Both models and planetary nebulae showed a decrease in the HeI 2^1P-2^1S/HI Br gamma ratio above an effective temperature of 40000K. The compact HII regions do not show any such decrease. The problem with this line ratio is probably due to the fact that the photoionisation model does not account correctly for the high densities seen in these HII regions, and that we are therefore seeing more collisional excitation of the 2^1P level than the model predicts. It may also reflect some deeper problem in the assumed model stellar atmospheres. In any event, although the normal HeI recombination lines can be used to place constraints on the temperature of the hottest star present, the HeI 2^1P-2^1S/HI Br gamma ratio should not be used for this purpose in either Galactic HII regions or in starburst galaxies, and conclusions from previous work using this ratio should be regarded with extreme caution. We also show that the combination of the near infrared `pure' recombination line ratios with mid-infrared forbidden line data provides a good discriminant of the form of the far ultraviolet spectral energy distribution of the exciting star(s). From this we conclude that CoStar models are a poor match to the available data for our sources, though the more recent WM-basic models are a better fit.

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arXiv2002

A detailed study of G35.2-0.7N: collimated outflows in a cluster of high-mass young stellar objects

We present a series of JCMT, BIMA and VLA observations of the massive star-forming region associated with G35.2--0.7N. These new observations shed considerable light on the nature of the outflows in this region. The combination of our CO, SiO and radio data suggest that there are perhaps as many as four outflows emanating from the core containing G35.2--0.7N. The most striking result is that the radio jet is ruled out as the driving source of the large-scale CO flow. The jet drives its own highly-collimated CO outflow. Molecular line emission from H13CN and H13CO+ peaks towards the centre of the large-scale CO outflow, but no radio source is detected within. A third flow may be seen in SiO emission, which appears to be centred on another peak in H13CO+. The fourth flow is not seen in CO or SiO, but is inferred from previous L'-band data and the discovery of a radio source at the proposed centre of this flow.

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arXiv2002

The Population of the Galactic Plane as Seen by MSX

The combination of mid-infrared data from the MSX satellite mission and ground-based near-infrared photometry is used to characterise the properties of the mid-infrared population of the Galactic plane. The colours of the youngest sources still heavily embedded within their natal molecular clouds are in general different from evolved stars shrouded within their own dust shells. Our main motivation is to use MSX for an unbiased search for a large (~1000) sample of massive young stellar objects (MYSOs). A simple analysis shows that the MSX point source catalogue should contain most of the MYSOs within our Galaxy. We develop colour selection criteria using combined near- and mid-infrared data for MYSOs, which produces a list of 3071 objects, excluding the galactic centre region. The programme of follow-up observations already underway to separate the MYSOs from compact H II regions and other remaining objects is briefly described. We also show that these data can be used, just as IRAS data has been previously, to provide a separation between evolved stars with carbon rich and oxygen rich dust. These data may also be used to search for evidence of dust around normal main sequence stars, such as low mass pre-main sequence stars or the Vega-excess class of objects where debris disks are presumed to remain from the planet formation process. We discuss the accuracy and completeness of the MSX point source catalogue, and show that the errors present tend to be of a kind that is not significant for the main stellar populations we discuss in this paper.

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arXiv2001

Near Infrared Spectra of Compact Planetary Nebulae

This paper continues our study of the behaviour of near infrared helium recombination lines in planetary nebula. We find that the 1.7007um 4^3D-3^3P HeI line is a good measure of the HeI recombination rate, since it varies smoothly with the effective temperature of the central star. We were unable to reproduce the observed data using detailed photoionisation models at both low and high effective temperatures, but plausible explanations for the difference exist for both. We therefore conclude that this line could be used as an indicator of the effective temperature in obscured nebula. We also characterised the nature of the molecular hydrogen emission present in a smaller subset of our sample. The results are consistent with previous data indicating that ultraviolet excitation rather than shocks is the main cause of the molecular hydrogen emission in planetary nebulae.

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arXiv2000

Infrared Helium-Hydrogen Line Ratios as a Measure of Stellar Effective Temperature

We have observed a large sample of compact planetary nebulae in the near-infrared to determine how the 2^1P-2^1S HeI line at 2.058um varies as a function of stellar effective temperature, Teff. The ratio of this line with HI Br g at 2.166um has often been used as a measure of the highest Teff present in a stellar cluster, and hence on whether there is a cut-off in the stellar initial mass function at high masses. However, recent photoionisation modelling has revealed that the behaviour of this line is more complex than previously anticipated. Our work shows that in most aspects the photoionisation models are correct. In particular, we confirm the weakening of the 2^1P-2^1S as Teff increases beyond 40000K. However, in many cases the model underpredicts the observed ratio when we consider the detailed physical conditions in the individual planetary nebulae. Furthermore, there is evidence that there is still significant 2^1P-2^1S HeI line emission even in the planetary nebulae with very hot (Teff>100000K) central stars. It is clear from our work that this ratio cannot be considered as a reliable measure of effective temperature on its own.

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