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Title: Near-UV absorption in very cool DA white dwarfs

The atmospheres of very cool, hydrogen-rich white dwarfs (WDs) (T{sub eff} < 6000 K) are challenging to model because of the increased complexity of the equation of state, chemical equilibrium, and opacity sources in a low-temperature, weakly ionized dense gas. In particular, many models that assume relatively simple models for the broadening of atomic levels and mostly ideal gas physics overestimate the flux in the blue part of their spectra. A solution to this problem that has met with some success is that additional opacity at short wavelengths comes for the extreme broadening of the Lyman α line of atomic H by collisions primarily with H{sub 2}. For the purpose of validating this model more rigorously, we acquired Hubble Space Telescope STIS spectra of eight very cool WDs (five DA and three DC stars). Combined with their known parallaxes, BVRIJHK, and Spitzer IRAC photometry, we analyze their entire spectral energy distribution (from 0.24 to 9.3 μm) with a large grid of model atmospheres and synthetic spectra. We find that the red wing of the Lyman α line reproduces the rapidly decreasing near-UV flux of these very cool stars very well. We determine better constrained values of T{sub eff} and gravitymore » as well as upper limits to the helium abundance in their atmospheres.« less
Authors:
 [1] ;  [2] ;  [3]
  1. Los Alamos National Laboratory, P.O. Box 1663, Mail Stop F663, Los Alamos, NM 87545 (United States)
  2. Lunar and Planetary Laboratory, University of Arizona, Sonett Space Sciences Building, 1541 East University Boulevard, Tucson, AZ 85721-0063 (United States)
  3. Institute of Energy and Climate Research (IEK-6), Forschungszentrum Jülich, Wilhelm-Johnen-Strasse, D-52425 Jülich (Germany)
Publication Date:
OSTI Identifier:
22365586
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 790; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ABSORPTION; ATMOSPHERES; COLLISIONS; ELEMENT ABUNDANCE; ENERGY SPECTRA; EQUATIONS OF STATE; EQUILIBRIUM; GRAVITATION; HELIUM; OPACITY; PHOTOMETRY; TELESCOPES; WAVELENGTHS; WHITE DWARF STARS