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S. M. Dougherty

Publications and source records attributed to S. M. Dougherty.

At least 19 recordsLinked 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

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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.

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arXiv2011

The complex circumstellar environment of HD142527

The recent findings of gas giant planets around young A-type stars suggest that disks surrounding Herbig Ae/Be stars will develop planetary systems. An interesting case is HD142527, for which previous observations revealed a complex circumstellar environment and an unusually high ratio of infrared to stellar luminosity. Its properties differ considerably from other Herbig Ae/Be stars. This suggests that the disk surrounding HD142527 is in an uncommon evolutionary stage. We aim for a better understanding of the geometry and evolutionary status of the circumstellar material around the Herbig Ae/Be star HD142527. We map the composition and spatial distribution of the dust around HD142527. We analyze SEST and ATCA millimeter data, VISIR N and Q-band imaging and spectroscopy. We gather additional relevant data from the literature. We use the radiative transfer code MCMax to construct a model of the geometry and density structure of the circumstellar matter, which fits all of the observables satisfactorily. We find that the disk of HD142527 has three geometrically distinct components separated by a disk gap running from 30 to 130 AU. There is a geometrically flat inner disk running from 0.3 AU up to 30 AU; an optically thin halo-like component of dust in the inner disk regions; and a massive self-shadowed outer disk running from 130 AU up to 200 AU. We derived a total dust mass in small grains of 10^-3 M0 and a vertical height of the inner wall of the outer disk of h = 60 AU. ....

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arXiv2011

The WR 140 periastron passage 2009: first results from MONS and other optical sources

We present the results from the spectroscopic follow-up of WR140 (WC7 + O4-5) during its last periastron passage in January 2009. This object is known as the archetype of colliding wind binaries and has a relatively large period (~ 8 years) and eccentricity (~ 0.89). We provide updated values for the orbital parameters, new estimates for the WR and O star masses and new constraints on the mass-loss rates.

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arXiv2011

The Orbit and Distance of WR140 - Proceedings of "Stellar Winds in Interaction" 2010

A campaign of 35 epochs of milli-arcsec resolution VLBA observations of the archetype collidingwind WR+O star binary system WR140 show the wind-collision region (WCR) as a bow-shaped arc of emission that rotates as the highly eccentric orbit progresses. The observations comprise 21 epochs from the 1993- 2001 orbit, discussed by Dougherty et al. (2005), and 14 epochs from the 2001-2009 orbit, and span orbital phase 0.43 to 0.95. Assuming the WCR is symmetric about the line-of-centres of the two stars and "points" at the WR star, this rotation shows the O star moving from SE to E of the WR star between these orbital phases. Using IR interferometry observations from IOTA that resolve both stellar components at phase 0.297 in conjunction with orbital parameters derived from radial velocity variations, the VLBA observations constrain the inclination of the orbit plane as 120°\pm4°, the longitude of the ascending node as 352°\pm2°, and the orbit semimajor axis as 9.0\pm0.1 mas. This leads to a distance estimate to WR140 of 1.81\pm0.08 kpc. Further refinements of the orbit and distance await more IR interferometric observations of the stellar components directly.

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arXiv2010

Cyg OB2 #5: When three stars are just not enough

Archival observations from the Very Large Array (VLA) at frequencies between 1.4 GHz and 43 GHz of the 6.6-day O6.5-7+O5.5-6 binary Cyg OB2 #5 over 20 years are re-examined. The aim is to determine the location and character of its known variable radio emission. The radio emission consists of a primary component associated with the binary, and a non-thermal source (NE), $0.8" to the NE. This work reveals that NE shows no evidence of variation demonstrating that the variable emission arises in the primary component. With NE constant, the radio flux from the primary can now be well determined for the first time, especially in observations that do not resolve both the primary and NE components. The variable radio emission from the primary has a period of 6.7+/-0.3 years which is described by a simple model of a non-thermal source orbiting within the stellar wind envelope of the binary. Such a model implies the presence of a third, unresolved stellar companion (Star C) orbiting the 6.6-day binary with a period of 6.7 years. The non-thermal emission arises from either a WCR between Star C and the binary system, or possibly from Star C directly. Examination of radial velocity observations suggests reflex motion of the binary due to Star C, for which a mass of 23^+22_-14 solar mass is deduced. Together with the star associated with NE, this implies that Cyg OB2 #5 is a quadruple system.

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arXiv2010

The orbit and distance of WR140

A campaign of 35 epochs of milli-arcsecond resolution VLBA observations of the archetype colliding wind WR+O star binary system WR140 show the wind-collision region (WCR) as a bow-shaped arc of emission that rotates as the highly eccentric orbit progresses. The observations comprise 21 epochs from the 1993-2001 orbit, discussed by Dougherty et al. (2005), and 14 epochs from the 2001-2009 orbit, and span orbital phase 0.43 to 0.95. Assuming the WCR is symmetric about the line-of-centres of the two stars and "points" at the WR star, this rotation shows the O star moving from SE to E of the WR star between these orbital phases. Using IR interferometry observations from IOTA that resolve both stellar components at phase 0.297, in conjuction with orbital parameters derived from radial velocity variations, the VLBA observations constrain the inclination of the orbit plane as 120\degree \pm 4 \degree, the longitude of the ascending node as 352\degree \pm 2 \degree, and the orbit semimajor axis as 9.0 \pm 0.1 mas. This leads to a distance estimate to WR140 of 1.81 \pm 0.08 kpc. Further refinements of the orbit and distance await more IR interferometric observations of the stellar components directly.

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arXiv2010

Radio emission from the massive stars in Westerlund 1

The diverse massive stellar population in the young massive cluster Westerlund 1 (Wd~1) provides an ideal laboratory to observe and constrain mass-loss processes throughout the transitional phase of massive star evolution. A set of high sensitivity radio observations of Wd~1 leads to the detection of 18 cluster members, a sample dominated by cool hypergiants, but with detections among hotter OB supergiants and WR stars. Here the diverse radio properties of the detected sample are briefly described. The mass-loss rates of the detected objects are surprisingly similar across the whole transitional phase of massive star evolution, at ~10^-5 solar masses per year. Such as rate is insufficient to strip away the H-rich mantle in a massive star lifetime, unless the stars go through a period of enhanced mass-loss. The radio luminous star W9 provides an example of such an object, with evidence for two eras of mass-loss with rates of ~10^-4 solar masses per year.

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arXiv2009

Radio emission from the massive stars in the Galactic Super Star Cluster Westerlund 1

Current mass-loss rate estimates imply that main sequence winds are not sufficient to strip away the H-rich envelope to yield Wolf-Rayet (WR) stars. The rich transitional population of Westerlund 1 (Wd 1) provides an ideal laboratory to observe mass-loss processes throughout the transitional phase of stellar evolution. An analysis of deep radio continuum observations of Wd 1 is presented. We detect 18 cluster members. The radio properties of the sample are diverse, with thermal, non-thermal and composite thermal/non-thermal sources present. Mass-loss rates are ~10^{-5} solar mass/year across all spectral types, insufficient to form WRs during a massive star lifetime, and the stars must undergo a period of enhanced mass loss. The sgB[e] star W9 may provide an example, with a mass-loss rate an order of magnitude higher than the other cluster members, and an extended nebula of density ~3 times the current wind. This structure is reminiscent of luminous blue variables, and one with evidence of two eras of high, possibly eruptive, mass loss. Three OB supergiants are detected, implying unusually dense winds. They also may have composite spectra, suggesting binarity. Spatially resolved nebulae are associated with three of the four RSGs and three of the six YHGs in the cluster, which are due to quiescent mass loss rather than outbursts. For some of the cool star winds, the ionizing source may be a companion star though the cluster radiation density is sufficiently high to provide the necessary ionizing radiation. Five WR stars are detected with composite spectra, interpreted as arising in colliding-wind binaries.

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arXiv2009

Modelling the radio emission from Cyg OB2 #5: a quadruple system?

Fifty archival radio observations of the supergiant binary Cyg OB2 #5 using the Very Large Array over 20 years are re-examined to determine the location and character of the previously detected variable radio emission. The radio emission from the system consists of a primary component that is associated with the binary, and a non-thermal source (NE) that has been ascribed to a wind-collision region (WCR) between the stellar winds of the binary and that of a B-type star (Star D) to the NE. NE shows no evidence of variation in 23 epochs where it is resolved separately from the primary radio component, demonstrating that the variable emission arises in the primary. Since NE is non-variable, the radio flux from the primary can now be well determined for the first time, most especially in observations that do not resolve both the primary and NE components. The variable radio emission from the primary has a period of 6.7+/-0.3 years which is described by a simple model of a non-thermal source orbiting within the stellar wind envelope of the binary. Such a model implies the presence of a third, unresolved stellar companion (Star C) orbiting the 6.6-day binary with a period of 6.7 years. The variable non-thermal emission arises from either a WCR between Star C and the binary system, or possibly from Star C directly. The model gives a mass-loss rate of 3.4 x 10^{-5} solar mass/yr for Cyg OB2 #5, unusually high for an Of supergiant and comparable to that of WR stars, and consistent with an unusually strong He I 1.083-micron emission line, also redolent of WR stars. An examination of radial velocity observations suggests reflex motion of the binary due to Star C. The natures of NE and Star D are also examined. (abridged)

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arXiv2009

Orbitally modulated dust formation by the WC7+O5 colliding-wind binary WR140

We present high-resolution infrared (2--18 micron) images of the archetypal periodic dust-making Wolf-Rayet binary system WR140 (HD 193793) taken between 2001 and 2005, and multi-colour (J -- [19.5]) photometry observed between 1989 and 2001. The images resolve the dust cloud formed by WR140 in 2001, allowing us to track its expansion and cooling, while the photometry allows tracking the average temperature and total mass of the dust. The combination of the two datasets constrains the optical properties of the dust. The most persistent dust features, two concentrations at the ends of a `bar' of emission to the south of the star, were observed to move with constant proper motions of 324+/-8 and 243+/-7 mas/y. Longer wavelength (4.68-micron and 12.5-micron) images shows dust emission from the corresponding features from the previous (1993) periastron passage and dust-formation episode. A third persistent dust concentration to the east of the binary (the `arm') was found to have a proper motion ~ 320 mas/y. Extrapolation of the motions of the concentrations back to the binary suggests that the eastern `arm' began expansion 4--5 months earlier than those in the southern `bar', consistent with the projected rotation of the binary axis and wind-collision region (WCR) on the sky. Comparison of model dust images and the observations constrain the intervals when the WCR was producing sufficiently compressed wind for dust nucleation in the WCR, and suggests that the distribution of this material was not uniform about the axis of the WCR, but more abundant in the following edge in the orbital plane.

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arXiv2008

The Full Spectrum Galactic Terrarium: MHz to TeV Observations of Various Critters

Multi-wavelength studies at radio, infrared, optical, X-ray, and TeV wavelengths have discovered probable counterparts to many Galactic sources of GeV emission detected by EGRET. These include pulsar wind nebulae, high mass X-ray binaries, and mixed morphology supernova remnants. Here we provide an overview of the observational properties of Galactic sources which emit across 19 orders of magnitude in energy. We also present new observations of several sources.

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arXiv2008

Non-thermal radio emission from O-type stars III. Is Cyg OB2 No. 9 a wind-colliding binary?

The star Cyg OB2 No. 9 is a well-known non-thermal radio emitter. Recent theoretical work suggests that all such O-stars should be in a binary or a multiple system. However, there is no spectroscopic evidence of a binary component. Re-analysis of radio observations from the VLA of this system over 25 years has revealed that the non-thermal emission varies with a period of 2.35+-0.02 yr. This is interpreted as a strong suggestion of a binary system, with the non-thermal emission arising in a wind-collision region. We derived some preliminary orbital parameters for this putative binary and revised the mass-loss rate of the primary star downward from previous estimates.

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arXiv2007

The distance and neutral environment of the massive stellar cluster Westerlund 1

The goal of this study is to determine a distance to Westerlund 1 independent of the characteristics of the stellar population and to study its neutral environment, using observations of atomic hydrogen. The HI observations are taken from the Southern Galactic Plane Survey to study HI absorption in the direction of the HII region created by the members of Westerlund 1 and to investigate its environment as observed in the HI line emission. A Galactic rotation curve was derived using the recently revised values for the Galactic centre distance of $R_\odot = 7.6$ kpc, and the velocity of the Sun around the Galactic centre of $Θ_\odot = 214$ km s$^{-1}$. The newly determined rotation model leads us to derive a distance of $3.9\pm 0.7$ kpc to Westerlund 1, consistent with a location in the Scutum-Crux Arm. Included in this estimate is a very careful investigation of possible sources of error for the Galactic rotation curve. We also report on small expanding HI features around the cluster with a maximum dynamic age of 600,000 years and a larger bubble which has a minimum dynamic age of 2.5 million years. Additionally we re-calculated the kinematic distances to nearby HII regions and supernova remnants based on our new Galaxic rotation curve. We propose that in the early stages of the development of Wd 1 a large interstellar bubble of diameter about 50 pc was created by the cluster members. This bubble has a dynamic age similar to the age of the cluster. Small expanding bubbles, with dynamical ages $\sim 0.6$ Myr are found around Wd 1, which we suggest consist of recombined material lost by cluster members through their winds.

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arXiv2007

Particle acceleration in the colliding winds binary WR140

Massive WR+O star systems produce high-temperature, shock-heated plasma where the wind of the WR star and that of its binary companion collide - the wind-collision region (WCR). The WCR is a source of thermal (e.g. hard X-rays) and non-thermal (e.g. synchrotron) emission, the latter arising from electrons and ions accelerated to relativistic energies. These colliding wind binaries provide an excellent laboratory for the study of particle acceleration at higher mass, photon and magnetic energy densities than exist in SNRs. Recent models of the non-thermal emission from WR 140 have provided insight into this process.

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arXiv2006

Radio, X-ray, and gamma-ray Emission Models of the Colliding Winds Binary WR 140

We use hydrodynamical models of the wind-collision region (WCR) in the archetype colliding-wind system WR140 to determine the spatial and spectral distribution of the radio, X-ray and gamma-ray emission from shock accelerated electrons. Our calculations are for orbital phase 0.837 when the observed radio emission is close to maximum. Using the observed thermal X-ray emission together with the radio emission to constrain the mass-loss rates, we find that the O-star mass-loss rate is consistent with recent reductions for O4-5 supergiants. We demonstrate that radio VLBI observations of the WCR fail to constrain the opening angle. The observed low frequency turnover at ~3 GHz in the radio emission is due to free-free absorption, since models based on the Razin effect have an unacceptably large fraction of energy in non-thermal particles. The index of the non-thermal electron energy distribution is flatter than the canonical value for diffusive shock acceleration, namely p<2. Several mechanisms are discussed that could lead to such an index. Tighter constraints on p and the nature of the shocks in WR140 will be obtained from future observations at MeV and GeV energies, for which we generally predict lower fluxes than previous work. Since the high stellar photon fluxes prevent the acceleration of electrons beyond gamma > 1e5-1e6, TeV emission from CWB systems will provide unambiguous evidence of pion-decay emission from accelerated ions. We finish by commenting on the emission and physics of the multiple wind collisions in dense stellar clusters, paying particular attention to the Galactic Centre (abridged).

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arXiv2006

A Neutron Star with a Massive Progenitor in Westerlund 1

We report the discovery of an X-ray pulsar in the young, massive Galactic star cluster Westerlund 1. We detected a coherent signal from the brightest X-ray source in the cluster, CXO J164710.2-455216, during two Chandra observations on 2005 May 22 and June 18. The period of the pulsar is 10.6107(1) s. We place an upper limit to the period derivative of Pdot<2e-10 s/s, which implies that the spin-down luminosity is Edot<3e33 erg/s. The X-ray luminosity of the pulsar is L_X = 3(+10,-2)e33 (D/5 kpc)^2 erg/s, and the spectrum can be described by a kT = 0.61+/-0.02 keV blackbody with a radius of R_bb = 0.27+/-0.03 (D/5 kpc}) km. Deep infrared observations reveal no counterpart with K<18.5, which rules out binary companions with M>1 Msun. Taken together, the properties of the pulsar indicate that it is a magnetar. The rarity of slow X-ray pulsars and the position of CXO J164710.2-455216 only 1.6' from the core of Westerlund 1 indicates that it is a member of the cluster with >99.97% confidence. Westerlund 1 contains 07V stars with initial masses M_i=35 Msun and >50 post-main-sequence stars that indicate the cluster is 4+/-1 Myr old. Therefore, the progenitor to this pulsar had an initial mass M_i>40 Msun. This is the most secure result among a handful of observational limits to the masses of the progenitors to neutron stars.

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