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Title: The Physical Nature of Subdwarf A Stars: White Dwarf Impostors

Abstract

We address the physical nature of subdwarf A-type (sdA) stars and their possible link to extremely low mass (ELM) white dwarfs (WDs). The two classes of objects are confused in low-resolution spectroscopy. However, colors and proper motions indicate that sdA stars are cooler and more luminous, and thus larger in radius, than published ELM WDs. We demonstrate that surface gravities derived from pure hydrogen models suffer a systematic ∼1 dex error for sdA stars, likely explained by metal line blanketing below 9000 K. A detailed study of five eclipsing binaries with radial velocity orbital solutions and infrared excess establishes that these sdA stars are metal-poor ≃1.2 M {sub ⊙} main sequence stars with ≃0.8 M {sub ⊙} companions. While WDs must exist at sdA temperatures, only ∼1% of a magnitude-limited sdA sample should be ELM WDs. We conclude that the majority of sdA stars are metal-poor A–F type stars in the halo, and that recently discovered pulsating ELM WD-like stars with no obvious radial velocity variations may be SX Phe variables, not pulsating WDs.

Authors:
 [1]; ;  [2]
  1. Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, MA 02138 (United States)
  2. Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 W. Brooks Street, Norman, OK, 73019 (United States)
Publication Date:
OSTI Identifier:
22661162
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 839; 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; ATMOSPHERES; EDGE LOCALIZED MODES; GALAXIES; GRAVITATION; HEAT EXCHANGERS; HYDROGEN; MAIN SEQUENCE STARS; MASS; METALS; PROPER MOTION; RADIAL VELOCITY; SURFACES; WHITE DWARF STARS

Citation Formats

Brown, Warren R., Kilic, Mukremin, and Gianninas, A., E-mail: wbrown@cfa.harvard.edu, E-mail: kilic@ou.edu, E-mail: alexg@nhn.ou.edu. The Physical Nature of Subdwarf A Stars: White Dwarf Impostors. United States: N. p., 2017. Web. doi:10.3847/1538-4357/AA67E4.
Brown, Warren R., Kilic, Mukremin, & Gianninas, A., E-mail: wbrown@cfa.harvard.edu, E-mail: kilic@ou.edu, E-mail: alexg@nhn.ou.edu. The Physical Nature of Subdwarf A Stars: White Dwarf Impostors. United States. doi:10.3847/1538-4357/AA67E4.
Brown, Warren R., Kilic, Mukremin, and Gianninas, A., E-mail: wbrown@cfa.harvard.edu, E-mail: kilic@ou.edu, E-mail: alexg@nhn.ou.edu. Mon . "The Physical Nature of Subdwarf A Stars: White Dwarf Impostors". United States. doi:10.3847/1538-4357/AA67E4.
@article{osti_22661162,
title = {The Physical Nature of Subdwarf A Stars: White Dwarf Impostors},
author = {Brown, Warren R. and Kilic, Mukremin and Gianninas, A., E-mail: wbrown@cfa.harvard.edu, E-mail: kilic@ou.edu, E-mail: alexg@nhn.ou.edu},
abstractNote = {We address the physical nature of subdwarf A-type (sdA) stars and their possible link to extremely low mass (ELM) white dwarfs (WDs). The two classes of objects are confused in low-resolution spectroscopy. However, colors and proper motions indicate that sdA stars are cooler and more luminous, and thus larger in radius, than published ELM WDs. We demonstrate that surface gravities derived from pure hydrogen models suffer a systematic ∼1 dex error for sdA stars, likely explained by metal line blanketing below 9000 K. A detailed study of five eclipsing binaries with radial velocity orbital solutions and infrared excess establishes that these sdA stars are metal-poor ≃1.2 M {sub ⊙} main sequence stars with ≃0.8 M {sub ⊙} companions. While WDs must exist at sdA temperatures, only ∼1% of a magnitude-limited sdA sample should be ELM WDs. We conclude that the majority of sdA stars are metal-poor A–F type stars in the halo, and that recently discovered pulsating ELM WD-like stars with no obvious radial velocity variations may be SX Phe variables, not pulsating WDs.},
doi = {10.3847/1538-4357/AA67E4},
journal = {Astrophysical Journal},
number = 1,
volume = 839,
place = {United States},
year = {Mon Apr 10 00:00:00 EDT 2017},
month = {Mon Apr 10 00:00:00 EDT 2017}
}