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Scientific paper
Jan 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002esasp.485..357v&link_type=abstract
In: Proceedings of the First Eddington Workshop on Stellar Structure and Habitable Planet Finding, 11 - 15 June 2001, Córdoba, S
Other
Hot B Subdwarfs, Planetary Nebulae: Nuclei, White Dwarfs
Scientific paper
About 97% of the stars in the galaxy will end their evolution as white dwarfs. Two channels are presently known to lead to this ultimate stage: one goes through the planetary nebula phase (PN) and the other through the subdwarf phase. Fortunately, along these two evolutionary paths, the stars cross a number of instability strips which allow us to scrutinize their internal constitution and even follow their evolution from asteroseismology. Among the white dwarf progenitors, the variable planetary nebulae nuclei (PNNV) and pre-white dwarfs of PG1159 type (GW Vir stars) and the variable hot B subdwarfs (sdBV or EC14026 stars), offer the opportunity to understand the difference in their respective structure which make stars evolve from the horizontal branch to the white dwarf either through the asymptotic giant branch and PN phase, or directly through the subdwarf stage. Then, once on the white dwarf cooling sequence the variable DB (DBV) and DA (DAV) white dwarfs offer two additional windows to study their structure and evolution. The white dwarf cooling sequence is an important tool to determine the age of stellar populations in various environments: solar neighbourhood, open and globular clusters, galactic halo. The knowledge on the structure of these stars (the total star mass, the mass of the H and He outer layers, the rotation period, the strength of the magnetic field) and how it changes as the stars evolve along the cooling sequence are the major outputs of white dwarf asteroseismology. Great progresses in our understanding of white dwarf and subdwarf structure have been accomplished owing to ground-based asteroseismology during the last decade, mainly through the use of large telescopes and multi-site campaigns. However, these variable stars are multiperiodic pulsators of low amplitude. The power of asteroseismology in deciphering stellar structure strongly depends on the number of pulsation modes observed and identified. Those stars, whose pulsation periods are between ≍80 s for the "fastest" pulsating sdB and 1500 s for the "slowest" pulsating white dwarf, have amplitudes between a few millimagnitudes and a few tens of millimagnitudes. Many modes remain probably undetected being below the noise level. In addition, most of the observed pulsation modes are amplitude variable on time scales which have not yet been investigated from ground. Going to space to observe these pulsators during long periods should immensely enlarge the number of detected low amplitude modes, owing to the reduced noise and give clues on the amplitude variation time scales.
Charpinet Stephane
Vauclair Gerard
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