The DB Gap and Pulsations of White Dwarfs

Details The DB Gap and Pulsations of White Dwarfs The DB Gap and Pulsations of White Dwarfs

Nov 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007aipc..948...35s&link_type=abstract

UNSOLVED PROBLEMS IN STELLAR PHYSICS: A Conference in Honor of Douglas Gough. AIP Conference Proceedings, Volume 948, pp. 35-42

Physics

4

Pulsations, Oscillations, And Stellar Seismology, Stellar Structure, Interiors, Evolution, Nucleosynthesis, Ages, Faint Blue Stars, White Dwarfs, Degenerate Stars, Nuclei Of Planetary Nebulae

Scientific paper

In terms of surface composition, the majority of white dwarfs are divided into two major subgroups: those with hydrogen atmospheres (DAs), which constitute about 80% of all white dwarfs, and those with helium atmospheres (DOs and DBs). The ratio of the memberships of these two subgroups is sensitive to the effective temperature. There are few white dwarfs showing helium lines between 45 000 K and 30 000 K, and this exclusion zone is known as the ``DB gap.'' Since the temperatures of the hotter and the cooler ends of the DB gap coincide with the effective temperatures where the He II/III and the He I/II convection zones show up respectively, convective mixing is suspected to be the cause of the appearance of DOs and DBs outside the DB gap. Conversely, chemical separation due to gravitational settling in the convectively stable atmosphere is suspected to be the cause of the presence of the DB gap. If one adopts this scenario, the potential DB white dwarfs near the cool end of the DB gap are expected to have a superadiabatic layer that is nonetheless convectively stable due to a steep chemical gradient. Such stars are pulsationally unstable due to radiative heat exchange, thus a new type of white dwarf variables is predicted near the cool end of the DB gap.

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