Precise Atmospheric Parameters for the Shortest Period Binary White Dwarfs: Gravitational Waves, Metals, and Pulsations

Abstract

We present a detailed spectroscopic analysis of 61 low mass white dwarfs and provide precise atmospheric parameters, masses, and updated binary system parameters based on our new model atmosphere grids and the most recent evolutionary model calculations. For the first time, we measure systematic abundances of He, Ca and Mg for metal-rich extremely low mass white dwarfs and examine the distribution of these abundances as a function of effective temperature and mass. Based on our preliminary results, we discuss the possibility that shell flashes may be responsible for the presence of the observed He and metals. We compare stellar radii derived from our spectroscopic analysis to model-independent measurements and find good agreement except for those white dwarfs with Teff < 10,000 K. We also calculate the expected gravitational wave strain for each system and discuss their significance to the eLISA space-borne gravitational wave observatory. Finally, we provide an update on the instability strip of extremely low mass white dwarf pulsators.

18 pages, 13 figures, 3 tables, accepted for publication in ApJ

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• PSR J1012+5307: a millisecond pulsar with an extremely low-mass white dwarf companion
• The ELM Survey. IX. A Complete Sample of Low Mass White Dwarf Binaries in the SDSS Footprint
• Mixing and Overshooting in Surface Convection Zones of DA White Dwarfs: First Results from ANTARES
• White dwarf and subdwarf stars in the Sloan Digital Sky Survey Data Release 16
• The ELM Survey South. I. An Effective Search for Extremely Low Mass White Dwarfs
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• A New Merging Double Degenerate Binary in the Solar Neighborhood
• Pulsating low-mass white dwarfs in the frame of new evolutionary sequences: V. Asteroseismology of ELMV white dwarf stars
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• Four new deeply-eclipsing white dwarfs in ZTF
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