A. S. Hales

National Radio Astronomy Observatory, Charlottesville, Virginia, United States

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Publications (9)17.78 Total impact

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    ABSTRACT: We have identified 17 A-type stars in the Galactic Plane that have mid-IR excesses at 8 micron. From the observed colors in the (r'-H_alpha)-(r'-i') plane, we first identified 23050 early A-type main sequence (MS) star candidates in the Isaac Newton Photometric H-Alpha Survey (IPHAS) point source database that are located in Spitzer GLIMPSE Galactic Plane fields. Imposing the requirement that they be detected in all seven 2MASS and IRAC bands led to a sample of 2692 candidate A-type stars with fully sampled 0.6 to 8 micron SEDs. Optical classification spectra of 18 of the IPHAS candidate A-type MS stars showed that all but one could be well fitted using main sequence A-type templates, with the other being an A-type supergiant. Out of the 2692 A-type candidates 17 (0.6%) were found to have 8-micron excesses above the expected photospheric values. Taking into account non-A-Type contamination estimates, the 8-micron excess fraction is adjusted to ~0.7%. The distances to these sources range from 0.7-2.5 kpc. Only 10 out of the 17 excess stars had been covered by Spitzer MIPSGAL survey fields, of which 5 had detectable excesses at 24 micron. For sources with excesses detected in at least two mid-IR wavelength bands, blackbody fits to the excess SEDs yielded temperatures ranging from 270 to 650 K, and bolometric luminosity ratios L_IR/L* from 2.2x10^{-3}-1.9x10^{-2}, with a mean value of 7.9x10^{-3} (these bolometric luminosities are lower limits as cold dust is not detectable by this survey). Both the presence of mid-IR excesses and the derived bolometric luminosity ratios are consistent with many of these systems being in the planet-building transition phase between the early protoplanetary disk phase and the later debris disk phase.
    The Astrophysical Journal 01/2009; 695(1). · 6.73 Impact Factor
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    ABSTRACT: The INT/WFC Photometric H-alpha Survey of the Northern Galactic Plane (IPHAS) is an imaging survey being carried out in H-alpha, r' and i' filters, with the Wide Field Camera (WFC) on the 2.5-metre Isaac Newton Telescope (INT) to a depth of r'=20 (10 sigma). The survey is aimed at revealing large scale structure in our local galaxy, and also the properties of key early and late populations making up the Milky Way. Mapping emission line objects enables a particular focus on objects in the young and old stages of stellar evolution ranging from early T-Tauri stars to late planetary nebulae. In this paper we present the IPHAS Initial Data Release, primarily a photometric catalogue of about 200 million unique objects, coupled with associated image data covering about 1600 square degrees in three passbands. We note how access to the primary data products has been implemented through use of standard virtual observatory publishing interfaces. Simple traditional web access is provided to the main IPHAS photometric catalogue, in addition to a number of common catalogues (such as 2MASS) which are of immediate relevance. Access through the AstroGrid VO Desktop opens up the full range of analysis options, and allows full integration with the wider range of data and services available through the Virtual Observatory. The IDR represents the largest dataset published primarily through VO interfaces to date, and so stands as an examplar of the future of survey data mining. Examples of data access are given, including a cross-matching of IPHAS photometry with sources in the UKIDSS Galactic Plane Survey that validates the existing calibration of the best data. Comment: 17 pages, 23 figures, MNRAS in press. Version with full-resolution figures can be found at http://casu.ast.cam.ac.uk/surveys-projects/iphas
    12/2007;
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    ABSTRACT: We have acquired high spectral resolution observations (R=150,000) of the planetary nebulae NGC 7009 and NGC 6153, using bHROS on Gemini South. Observations of this type may provide a key to understanding why optical recombination lines (ORLs) yield systematically higher heavy element abundances for photoionized nebulae than do the classical forbidden collisionally excited lines (CELs) emitted by the same ions; NGC 7009 and NGC 6153 have notably high ORL/CEL abundance discrepancy factors (ADFs) of 5 and 10, respectively. Due to the opposite temperature dependences of ORLs and CELs, ORLs should be preferentially emitted by colder plasma. Our bHROS observations of NGC 7009 reveal that the [O III] 4363A CEL has a FWHM linewidth that is 1.5 times larger than that shown by O II ORLs in the same spectrum, despite the fact that all of these lines are emitted by the O2+ ion. The bHROS spectra of NGC 6153 also show that its O II ORLs have significantly narrower linewidths than do the [O III] 4363A and 5007A lines but, in addition, the [O III] 4363A and 5007A lines show very different velocity profiles, implying the presence of large temperature variations in the nebula. Comment: 2 pages, 1 figure, for Proceedings of IAU Symposium No. 234, `Planetary Nebulae in our Galaxy and Beyond'
    Proceedings of the International Astronomical Union 05/2006;
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    ABSTRACT: We have carried out JHK polarimetric observations of 11 dusty young stars, by using the polarimeter module IRPOL2 with the near-infrared camera UIST on the 3.8-m United Kingdom Infrared Telescope (UKIRT). Our sample targeted systems for which UKIRT-resolvable discs had been predicted by model fits to their spectral energy distributions. Our observations have confirmed the presence of extended polarized emission around TW Hya and around HD 169142. HD 150193 and HD 142666 show the largest polarization values among our sample, but no extended structure was resolved. By combining our observations with Hubble Space Telescope (HST) coronographic data from the literature, we derive the J- and H-band intrinsic polarization radial dependences of the disc of TW Hya. We find the polarizing efficiency of the disc is higher at H than at J, and we confirm that the J- and H-band percentage polarizations are reasonably constant with radius in the region between 0.9 and 1.3 arcsec from the star. We find that the objects for which we have detected extended polarizations are those for which previous modelling has suggested the presence of flared discs, which are predicted to be brighter than flat discs and thus would be easier to detect polarimetrically.
    Monthly Notices of the Royal Astronomical Society 01/2006; 365(4):1348 - 1356. · 5.52 Impact Factor
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    ABSTRACT: We have carried out JHK polarimetric observations of ten stars around which dust-debris disks may be present, by using the polarimeter module IRPOL2 with the near-IR camera UIST on UKIRT. Our sample targeted systems for which UKIRT-resolvable disks were predicted by model fits to their spectral energy distribution. Since only the light from the disks should be polarized, the bright central stars are suppressed in polarized light images. Our observations have detected the presence of extended polarized emission around at least four of our ten targets, confirming the potential of imaging polarimetry for substantially raising the overall number of systems with directly imaged debris disks.
    11/2005; 343:275.
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    ABSTRACT: The Isaac Newton Telescope (INT) Photometric Hα Survey of the Northern Galactic Plane (IPHAS) is a 1800-deg2 CCD survey of the northern Milky Way spanning the latitude range −5° < b < + 5° and reaching down to r′≃ 20 (10σ). Representative observations and an assessment of point-source data from IPHAS, now underway, are presented. The data obtained are Wide Field Camera images in the Hα narrow-band, and Sloan r′ and i′ broad-band filters. We simulate IPHAS (r′−Hα, r′−i′) point-source colours using a spectrophotometric library of stellar spectra and available filter transmission profiles: this defines the expected colour properties of (i) solar metallicity stars, without Hα emission, and (ii) emission-line stars. Comparisons with observations of fields in Aquila show that the simulations of normal star colours reproduce the observations well for all spectral types earlier than M. A further comparison between colours synthesized from long-slit flux-calibrated spectra and IPHAS photometry for six objects in a Taurus field confirms the reliability of the pipeline calibration. Spectroscopic follow-up of a field in Cepheus shows that sources lying above the main stellar locus in the (r′− Hα, r′−i′) plane are confirmed to be emission-line objects with very few failures. In this same field, examples of Hα deficit objects (a white dwarf and a carbon star) are shown to be readily distinguished by their IPHAS colours. The role IPHAS can play in studies of spatially resolved northern Galactic nebulae is discussed briefly and illustrated by a continuum-subtracted mosaic image of Shajn 147 (a supernova remnant, 3° in diameter). The final catalogue of IPHAS point sources will contain photometry on about 80 million objects. Used on its own, or in combination with near-infrared photometric catalogues, IPHAS is a major resource for the study of stellar populations making up the disc of the Milky Way. The eventual yield of new northern emission-line objects from IPHAS is likely to be an order of magnitude increase on the number already known.
    Monthly Notices of the Royal Astronomical Society 08/2005; 362(3):753 - 776. · 5.52 Impact Factor
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    ABSTRACT: H-alpha emission is ubiquitous in our Galaxy. It traces ionised gas of assorted nebulae such as HII regions, planetary nebulae, Wolf-Rayet nebulae, and supernova remnants. It is a strong signature of active stars, interacting binaries, very massive stars (especially supergiants, Luminous Blue Variables and Wolf-Rayet stars), Be stars, post-AGB stars, pre-main-sequence stars and so on. These objects represent important evolutionary phases which are generally short lived, and are hence few in number and difficult to find. Their discovery is therefore well worth the effort of a concerted programme and in August 2003 a major new survey project was started using the Wide Field Camera (WFC) on the Isaac Newton Telescope (INT) to do just that. It is called the INT Photometric H-alpha Survey of the Northern Galactic Plane, or IPHAS for short.
    The Newsletter of the Isaac Newton Group of Telescopes. 02/2005; 9:3-7.
  • Minor Planet Circulars. 02/2004;
  • Minor Planet Circulars. 11/2003;

Publication Stats

100 Citations
17.78 Total Impact Points

Institutions

  • 2009
    • National Radio Astronomy Observatory
      Charlottesville, Virginia, United States
  • 2005–2006
    • University College London
      • Department of Physics and Astronomy
      London, ENG, United Kingdom