PI: Pier-Emmanuel Tremblay, University of Montreal, firstname.lastname@example.org
Address: Department of Physics, C.P. 6128, succ. centre-ville, Montreal, QC H3C 3J7, Canada
CoI: Jason Kalirai, Space Telescope Science Institute
CoI: Pierre Bergeron, University of Montreal
Title: Direct Detection of Helium in Cool DA White Dwarfs
Abstract: We propose to observe several hydrogen-line (DA) white dwarfs below T_\rm eff~ 12,000 K and search for the presence of a weak HeI (lambda)5877 line using echelle spectroscopy. Such a weak feature has recently been reported in the metal-rich DA white dwarf GD 362 at T_\rm eff~10,000 K. This was an unexpected result since helium is believed to become spectroscopically invisible below 12,000 K (white dwarf stars have \log g~8). This is also an interesting result since the presence of helium in the atmosphere of cool DA stars, brought to the surface by convective mixing, has been invoked repeatedly to account for the larger-than-average surface gravities measured in cool DA stars using the spectroscopic method. Indeed, the effects produced on the hydrogen lines at high \log g cannot be distinguished from those produced by the presence of large amounts of helium. The presence, or absence, of this absorption feature will thus help to shed some light on the physics of cool white dwarfs and on their atmospheric composition. This long-standing problem with the physics of cool white dwarfs represents a serious hurdle at obtaining reliable atmospheric parameters at the cool end of the white dwarf evolutionary sequence, which in turn affects our determination of the white dwarf luminosity function and our ability to use white dwarfs as cosmochronometers and distance indicators.
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