B. L. Rachford

Publications (9)18.07 Total impact

• Source

  Available from: derckmassa.net

  Article: Far Ultraviolet Spectroscopic Explorer Observations of the Stellar Winds of Two O7 Supergiants in the Magellanic Clouds

  A. W. Fullerton, P. A. Crowther, O. De Marco, J. B. Hutchings, L. Bianchi, K. R. Brownsberger, D. L. Massa, D. C. Morton, B. L. Rachford, T. P. Snow, G. Sonneborn, J. Tumlinson, and A. J. Willis
  [show abstract] [hide abstract]
  ABSTRACT: We compare the stellar wind features in far-UV spectra of Sk -67°111, an O7 Ib(f) star in the LMC, with Sk 80, an O7 Iaf+ star in the SMC. The most striking differences are that Sk 80 has a substantially lower terminal velocity, much weaker O VI absorption, and stronger S IV emission. We have used line-blanketed, hydrodynamic, non-LTE atmospheric models to explore the origin of these differences. The far-UV spectra require systematically lower stellar temperatures than previous determinations for O7 supergiants derived from plane-parallel, hydrostatic models of photospheric line profiles. At these temperatures, the O VI in Sk -67°111 must be due primarily to shocks in the wind.
  The Astrophysical Journal 12/2008; 538(1):L43. · 6.02 Impact Factor
• Article: Far Ultraviolet Spectroscopic Explorer Observations of the HD Molecule toward HD 73882

  R. Ferlet, M. André, G. Hébrard, A. Lecavelier des Etangs, M. Lemoine, G. Pineau des Forêts, E. Roueff, B. L. Rachford, J. M. Shull, T. P. Snow, J. Tumlinson, A. Vidal-Madjar, D. G. York, and H. W. Moos
  [show abstract] [hide abstract]
  ABSTRACT: The Lyman and Werner band systems of deuterated molecular hydrogen (HD) occur in the far-UV range below 1120 Å. The high sensitivity of the FUSE mission can give access, at moderate resolution, to hot stars shining through translucent clouds in the hope of observing molecular cores in which deuterium is essentially in the form of HD. Thus, the measurement of the HD/H2 ratio may become a powerful new tool to evaluate the deuterium abundance, D/H, in the interstellar medium. We report on the detection of HD toward the high-extinction star HD 73882 (EB-V = 0.72). A preliminary analysis is presented.
  The Astrophysical Journal 12/2008; 538(1):L69. · 6.02 Impact Factor
• Source

  Available from: Roger Ferlet

  Article: Far Ultraviolet Spectroscopic Explorer Observations of Molecular Hydrogen in Translucent Interstellar Clouds: The Line of Sight toward HD 73882

  T. P. Snow, B. L. Rachford, J. Tumlinson, J. M. Shull, D. E. Welty, W. P. Blair, R. Ferlet, S. D. Friedman, C. Gry, E. B. Jenkins, A. Lecavelier, M. Lemoine, D. C. Morton, B. D. Savage, K. R. Sembach, A. Vidal-Madjar, D. G. York, B-G Andersson, P. D. Feldman, and H. W. Moos
  [show abstract] [hide abstract]
  ABSTRACT: We report the results of the initial Far Ultraviolet Spectroscopic Explorer observations of molecular hydrogen (H2) in translucent clouds. These clouds have greater optical depth than any of the diffuse clouds previously observed for far-UV H2 absorption and provide new insights into the physics and chemistry of such regions. Our initial results involve observations of HD 73882, a well-studied southern hemisphere star lying behind substantial interstellar material (EB-V = 0.72; AV = 2.44). We find a total H2 column density N(H2) = 1.2 × 1021 cm-2 about 3 times larger than the values for diffuse clouds previously measured in the far-UV. The gas kinetic temperature indicated by the ratio N(J = 1)/N(J = 0) is 58 ± 10 K. With the aid of ground-based data to calculate an appropriate multicomponent curve of growth, we have determined column densities for all rotational levels up to J = 7. The J ≥ 2 states can be reasonably fitted with a rotational excitation temperature of 307 ± 23 K. Both the kinetic and rotational temperatures are similar to those found in previous investigations of diffuse clouds. The ratios of carbonaceous molecules to hydrogen molecules are also similar to ratios in diffuse clouds, suggesting a similar chemistry for this line of sight.
  The Astrophysical Journal 12/2008; 538(1):L65. · 6.02 Impact Factor
• Source

  Available from: articles.adsabs.harvard.edu

  Article: Early Results from the APO Diffuse Interstellar Band Survey

  B. J. McCall, B. L. Rachford, T. P. Snow, P. Sonnentrucker, S. Friedman, J. Thorburn, T. Oka, D. Welty, L. M. Hobbs, D. G. York
  [show abstract] [hide abstract]
  ABSTRACT: The diffuse interstellar bands (DIBs) are absorption features observed in the visible spectra of nearly all red- dened stars, and were first observed in the early decades of the 20th century when many lines in astronomical spectra were unassigned. As laboratory spectroscopy progressed, most of the stronger lines were identified with atomic or diatomic species the DIBs are those lines (more numerous and generally broader) that remain unidentified. Since the DIBs have remained unassigned for over 75 years despite extensive laboratory efforts, we are trying a new approach. Our goal is to obtain moderate resolution (λδλ ± 37,500), high signal-to-noise spectra of a large sample of reddened stars. We are using the ARCES echelle spectrograph at the Apache Point Observatory, which offers complete spectral coverage from 3700-1O,000Å. So far, we have taken data on 75 nights and have obtained S/N > 1000 on 63 stars, and S/N > 500 on 60 additional stars. Our hope is that this extensive DIB dataset, coupled with complementary measurements of known species at ultraviolet, visible, and infrared wavelengths, will yield new insights into the origin of the DIBs this paper presents some of onr early results.
  10/2002; -1:138.
• Article: The FUSE Survey of Molecular Hydrogen in Translucent Interstellar Clouds

  B. L. Rachford, T. P. Snow, J. Tumlinson, J. M. Shull, W. P. Blair, S. D. Friedman, R. Ferlet, E. B. Jenkins, D. C. Morton, B. D. Savage, A. Vidal-Madjar, D. E. Welty, D. G. York
  [show abstract] [hide abstract]
  ABSTRACT: We present preliminary results from the first survey of molecular hydrogen in translucent clouds undertaken with FUSE. Sixteen lines of sight toward early-type stars have been observed, covering a wide range of extinction properties (AV ~ 1--5; RV ~ 2--4). Through profile fitting of the heavily damped low-J lines, and curve-of-growth analysis of the high-J lines, we have determined rotational populations for J = 0 through J = 5, and sometimes as high as J = 7. The total H2 column densities, dominated by J = 0 and J = 1, lie within the range ~3--15 x 1020 cm-2. The amount of high-J excitation varies considerably from one line of sight to another, reflecting the wide range of UV radiation fields and cloud densities. The ratio N(J=1)/N(J=0) gives kinetic temperatures generally in the range T kin = 50--70 K, similar to the smallest values found for diffuse clouds. We derive hydrogen molecular fractions of ~0.1--0.7, again similar to those found for diffuse clouds with N(H2) ≳ 1020 cm-2. This work is based on data obtained for the Guaranteed Time Team by the NASA-CNES-CSA FUSE mission operated by the Johns Hopkins University. Financial support to U.S. participants has been provided by NASA contract NAS5-32985.
  11/2000; 197:712.
• Source

  Available from: arxiv.org

  Article: FUSE Observations of Diffuse Interstellar Molecular Hydrogen

  J. M. Shull, J. Tumlinson, E. B. Jenkins, H. W. Moos, B. L. Rachford, B. D. Savage, K. R. Sembach, T. P. Snow, G. Sonneborn, D. G. York, W. P. Blair, J. C. Green, S. D. Friedman, D. J. Sahnow
  [show abstract] [hide abstract]
  ABSTRACT: We describe a moderate-resolution FUSE mini-survey of H2 in the Milky Way and Magellanic Clouds, using four hot stars and four AGN as background sources. FUSE spectra of nearly every stellar and extragalactic source exhibit numerous absorption lines from the H2 Lyman and Werner bands between 912 and 1120 A. One extragalactic sightline, PKS 2155-304, with low N(HI) shows no detectable H2 and could be the "Lockman Hole" of molecular gas, of importance for QSO absorption-line studies. We measure H2 column densities in low rotational states (J = 0 and 1) to derive rotational and/or kinetic temperatures of diffuse interstellar gas. The higher-J abundances can constrain models of the UV radiation fields and gas densities. In three optically thick clouds toward extragalactic sources, we find n(H) ~ 30-50 cm(-3) and cloud thicknesses of 2-3 pc. The rotational temperatures for H2 at high Galactic latitude, = 107 +/- 17 K (seven sightlines) and 120 +/- 13 K (three optically thick clouds), are higher than those in the Copernicus sample composed primarily of targets in the disk. We find no evidence for great differences in the abundance or state of excitation of H2 between sight lines in the Galaxy and those in the SMC and LMC. In the future, we will probe the distribution and physical parameters of diffuse molecular gas in the disk and halo and in the lower-metallicity environs of the LMC and SMC.
  05/2000;
• Source

  Available from: Roger Ferlet

  Article: FUSE Observations of Molecular Hydrogen in Translucent Interstellar Clouds: The Line of Sight Toward HD 73882

  T. P. Snow, B. L. Rachford, J. Tumlinson, J. M. Shull, D. E. Welty, W. P. Blair, R. Ferlet, S. D. Friedman, C. Gry, E. B. Jenkins, A. Lecavelier, M. Lemoine, D. C. Morton, B. D. Savage, K. R. Sembach, A. Vidal-Madjar, D. G. York, B.-G. Andersson, P. D. Feldman, H. W. Moos
  [show abstract] [hide abstract]
  ABSTRACT: We report the results of initial FUSE observations of molecular hydrogen (H2) in translucent clouds. These clouds have greater optical depth than any of the diffuse clouds previously observed for far-UV H2 absorption, and provide new insights into the physics and chemistry of such regions. Our initial results involve observations of HD 73882, a well-studied southern hemisphere star lying behind substantial interstellar material (E(B-V) = 0.72; A_V = 2.44). We find a total H2 column density, N(H2) = 1.2 x 10^{21} cm^{-2}, about three times larger than the values for diffuse clouds previously measured in the far-UV. The gas kinetic temperature indicated by the ratio N(J=1)/N(J=0) is 58 +/- 10 K. With the aid of ground-based data to calculate an appropriate multi-component curve of growth, we have determined column densities for all rotational levels up to J = 7. The J >= 2 states can be reasonably fitted with a rotational excitation temperature of 307 +/- 23 K. Both the kinetic and rotational temperatures are similar to those found in previous investigations of diffuse clouds. The ratios of carbonaceous molecules to hydrogen molecules are also similar to ratios in diffuse clouds, suggesting a similar chemistry for this line of sight. Comment: 7 pages, 3 figures, to appear in ApJ Letters (FUSE first-results issue)
  05/2000;
• Source

  Available from: arxiv.org

  Article: FUSE Observations of the Stellar Winds of Two O7 Supergiants in the Magellanic Clouds

  A. W. Fullerton, P. A. Crowther, O. De Marco, J. B. Hutchings, L. Bianchi, K. R. Brownsberger, D. L. Massa, D. C. Morton, B. L. Rachford, T. P. Snow, G. Sonneborn, J. Tumlinson, A. J. Willis
  [show abstract] [hide abstract]
  ABSTRACT: We compare the stellar wind features in far-UV spectra of Sk -67 111, an O7 Ib(f) star in the LMC, with Sk 80, an O7 Iaf+ star in the SMC. The most striking differences are that Sk 80 has a substantially lower terminal velocity, much weaker O VI absorption, and stronger S IV emission. We have used line-blanketed, hydrodynamic, non-LTE atmospheric models to explore the origin of these differences. The far-UV spectra require systematically lower stellar temperatures than previous determinations for O7 supergiants derived from plane-parallel, hydrostatic models of photospheric line profiles. At these temperatures, the O VI in Sk -67 111 must be due primarily to shocks in the wind. Comment: 10 pages, 2 figures. AASTeX preprint format. This paper will appear in a special issue of Astrophysical Journal Letters devoted to the first scientific results from the FUSE mission
  05/2000;
• Article: FUSE observation of HD molecule toward HD 73882

  M. Andre, R. Ferlet, A. Vidal-Madjar, G. Hebrard, A. Lecavelier, C. Gry, M. Lemoine, G. Pineau des Forets, E. Roueff, B. L. Rachford, T. P. Snow, J. Tumlinson, FUSE Science Team
  [show abstract] [hide abstract]
  ABSTRACT: We present FUSE observations of HD 73882 obtained with the 30"x30" large aperture and moderate resolution. The Far UV spectrum of this high extinction O9III star (E(B-V)=0.7) between 904 and 1189 Angstrom shows strong H2 lines from a translucent cloud (J=0 to J=8). Absorption lines of HD (J=0) were also detected. We infer a very low HD / H2 ratio. Preliminary analysis is presented. This research is supported by NASA contract NAS5-32985.
  04/2000; 32:882.