Pierre Bergeron - Universit‚e de Montr‚eal
Chr‚etien Grant Winning Proposal
In collaboration with M.-T. Ruiz (U.of Chile) and S.K. Leggett (JACH), we are obtaining
optical
and infrared photometry, as well as optical spectroscopy, of cool (Teff < 7000K) white dwarf
stars.
Effective temperatures, surface gravities, and chemical abundances of helium and hydrogen are
determined for individual objects by fitting the optical/infrared energy distributions with the
predictions
of new model atmospheres appropriate for these cool degenerate stars. Our results indicate that
the
atmospheric composition of the coolest stars can be inferred from the observed energy
distributions,
and that objects with mixed compositions (N(He)/N(H)) are easily recognizable from their
predicted
strong infrared flux deficiency. Our study reveals that the dichotomy observed in hotter white
dwarf
atmospheres, namely that the photosphere is composed either of nearly pure hydrogen or of
nearly pure
helium, persists at low effective temperatures. In particular, no white dwarf star with a
helium-to-hydrogen ratio near unity has been found, with the exception of the peculiar white
dwarf LHS
1126. High signal-to-noise spectroscopy has revealed the presence of Hà in many previously
classified
featureless stars, and it has been detected in objects as cool as Teff ÷ 4600K. When completed,
our
analysis should shed some light on the physical mechanisms at play, such as convective mixing
and
accretion from the interstellar medium, which can alter the photospheric chemical composition.
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Please e-mail any suggestions/comments to Jack Penfold
(jpenfold@mtroyal.ab.ca)