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

Abstract

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

Citation Formats

Saumon, D., Holberg, J. B., and Kowalski, P. M., E-mail: dsaumon@lanl.gov, E-mail: holberg@argus.lpl.arizona.edu, E-mail: p.kowalski@fz-juelich.de. Near-UV absorption in very cool DA white dwarfs. United States: N. p., 2014. Web. doi:10.1088/0004-637X/790/1/50.
Saumon, D., Holberg, J. B., & Kowalski, P. M., E-mail: dsaumon@lanl.gov, E-mail: holberg@argus.lpl.arizona.edu, E-mail: p.kowalski@fz-juelich.de. Near-UV absorption in very cool DA white dwarfs. United States. doi:10.1088/0004-637X/790/1/50.
Saumon, D., Holberg, J. B., and Kowalski, P. M., E-mail: dsaumon@lanl.gov, E-mail: holberg@argus.lpl.arizona.edu, E-mail: p.kowalski@fz-juelich.de. 2014. "Near-UV absorption in very cool DA white dwarfs". United States. doi:10.1088/0004-637X/790/1/50.
@article{osti_22365586,
title = {Near-UV absorption in very cool DA white dwarfs},
author = {Saumon, D. and Holberg, J. B. and Kowalski, P. M., E-mail: dsaumon@lanl.gov, E-mail: holberg@argus.lpl.arizona.edu, E-mail: p.kowalski@fz-juelich.de},
abstractNote = {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 gravity as well as upper limits to the helium abundance in their atmospheres.},
doi = {10.1088/0004-637X/790/1/50},
journal = {Astrophysical Journal},
number = 1,
volume = 790,
place = {United States},
year = 2014,
month = 7
}
  • We present near-infrared photometric observations of 15 and spectroscopic observations of 38 cool white dwarfs (WDs). This is the largest near-infrared spectroscopic survey of cool WDs to date. Combining the Sloan Digital Sky Survey photometry and our near-infrared data, we perform a detailed model atmosphere analysis. The spectral energy distributions of our objects are explained fairly well by model atmospheres with temperatures ranging from 6300 K down to 4200 K. Two WDs show significant absorption in the infrared, and are best explained with mixed H/He atmosphere models. Based on the up-to-date model atmosphere calculations by Kowalski and Saumon, we findmore » that the majority of the stars in our sample have hydrogen-rich atmospheres. We do not find any pure helium atmosphere WDs below 5000 K, and we find a trend of increasing hydrogen to helium ratio with decreasing temperature. These findings present an important challenge to understanding the spectral evolution of WDs.« less
  • A determination of the He abundance in 37 cool DA white dwarfs is presented. A spectroscopic analysis of the high Balmer lines reveals that the atmospheres of most objects below Te of about 11,500 K are contaminated by significant amounts of He, with abundances sometimes as high as N(He)/N(H) of about 20. This result is interpreted as the result of convective mixing between the thin superficial H layer with the more massive underlying He envelope. The lack of H-rich objects in the sample in the range Te = 7500-11,500 K indicates that most DA white-dwarfs mix near Te of aboutmore » 11,500 K, a result which is consistent with the location of the red edge of the ZZ Ceti instability strip. A definite trend for cooler objects to have lower He abundances is also observed. 37 refs.« less