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GHRS Observations of Cool, Low-Gravity StarsUV spectra of lambda Velorum taken with the Goddard High Resolution Spectrograph (GHRS) on the Hubble Space Telescope are used to probe the structure of the outer atmospheric layers and wind and to estimate the mass-loss rate from this K5 lb-II supergiant. VLA radio observations at lambda = 3.6 cm are used to obtain an independent check on the wind velocity and mass-loss rate inferred from the UV observations, Parameters of the chromospheric structure are estimated from measurements of UV line widths, positions, and fluxes and from the UV continuum flux distribution. The ratios of optically thin C II] emission lines indicate a mean chromospheric electron density of log N(sub e) approximately equal 8.9 +/- 0.2 /cc. The profiles of these lines indicate a chromospheric turbulence (v(sub 0) approximately equal 25-36 km/s), which greatly exceeds that seen in either the photosphere or wind. The centroids of optically thin emission lines of Fe II and of the emission wings of self-reversed Fe II lines indicate that they are formed in plasma approximately at rest with respect to the photosphere of the star. This suggests that the acceleration of the wind occurs above the chromospheric regions in which these emission line photons are created. The UV continuum detected by the GHRS clearly traces the mean flux-formation temperature as it increases with height in the chromosphere from a well-defined temperature minimum of 3200 K up to about 4600 K. Emission seen in lines of C III] and Si III] provides evidence of material at higher than chromospheric temperatures in the outer atmosphere of this noncoronal star. The photon-scattering wind produces self-reversals in the strong chromospheric emission lines, which allow us to probe the velocity field of the wind. The velocities to which these self-absorptions extend increase with intrinsic line strength, and thus height in the wind, and therefore directly map the wind acceleration. The width and shape of these self-absorptions reflect a wind turbulence of approximately equal 9-21 km/s. We further characterize the wind by comparing the observations with synthetic profiles generated with the Lamers et al. Sobolev with Exact Integration (SEI) radiative transfer code, assuming simple models of the outer atmospheric structure. These comparisons indicate that the wind in 1994 can be described by a model with a wind acceleration parameter beta approximately 0.9, a terminal velocity of 29-33 km/s, and a mass-loss rate approximately 3 x 10(exp -9) solar M/yr. Modeling of the 3.6 cm radio flux observed in 1997 suggests a more slowly accelerating wind (higher beta) and/or a higher mass-loss rate than inferred from the UV line profiles. These differences may be due to temporal variations in the wind or from limitations in one or both of the models. The discrepancy is currently under investigation.
Document ID
20000033138
Acquisition Source
Goddard Space Flight Center
Document Type
Reprint (Version printed in journal)
Authors
Carpenter, Kenneth G.
(NASA Goddard Space Flight Center Greenbelt, MD United States)
Robinson, Richard D.
(NASA Goddard Space Flight Center Greenbelt, MD United States)
Harper, Graham M.
(Colorado Univ. Boulder, CO United States)
Bennett, Philip D.
(Colorado Univ. Boulder, CO United States)
Brown, Alexander
(Colorado Univ. Boulder, CO United States)
Mullan, Dermott J.
(Delaware Univ. Newark, DE United States)
Date Acquired
August 19, 2013
Publication Date
August 10, 1999
Publication Information
Publication: Astrophysical Journal
Publisher: American Astronomical Society
Volume: 521
Subject Category
Astronomy
Funding Number(s)
CONTRACT_GRANT: NAG5-4804
CONTRACT_GRANT: NAS5-26555
CONTRACT_GRANT: NAG5-3033
CONTRACT_GRANT: NAG5-6905
CONTRACT_GRANT: NAG5-3226
Distribution Limits
Public
Copyright
Other

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