Relaxation oscillations of a helium-burning star - Helium shell flashes not triggered by the accumulation of a critical mass

Details Relaxation oscillations of a helium-burning star - Helium shell flashes not triggered by the accumulation of a critical mass Relaxation oscillations of a helium-burning star - Helium shell flashes not triggered by the accumulation of a critical mass

May 1986

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986apj...304..217i&link_type=abstract

Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 304, May 1, 1986, p. 217-230. Research supported by the Ministry of Educati

Astronomy and Astrophysics

Astrophysics

16

Binary Stars, Relaxation Oscillators, Stellar Evolution, Stellar Oscillations, Helium, Hertzsprung-Russell Diagram, Light Curve, Thermodynamic Properties, Time Response

Scientific paper

Following the exhaustion of helium at its center, the compact remnant of an intermediate-mass star in a close binary experiences thermal pulses that are different in character from those experienced by a single asymptotic giant branch star. In this particular study, the mass of the main-sequence progenitor of the remnant is 3 Msun, and mass loss by Roche-lobe overflow begins just prior to the ignition of helium at the center and ends when ˜3.5 x to 10-3 Msun of hydrogen remains near the surface; the mass of the remnant is 0.378 Msun, and the mass of the helium zone is 0.18 Msun at the onset of the pulses. The amount of fresh helium that is produced by hydrogen burning between pulses is ˜100 times smaller than the amount of helium consumed during a pulse (1.5 × 10-3 Msun); thus it is not the accumulation of a critical mass of helium that is responsible for triggering pulses. Instead, the star behaves as a classic self-excited relaxation oscillator. It contracts and heats following each pulse, the matter near the base of the helium-containing region entering of its own accord into a domain of thermal instability in the density-temperature plane. Most of the energy released in the ensuing short burst of nuclear burning goes directly into the work necessary to expand matter in the large convective shell formed in consequence of the high fluxes produced in the burst. Matter in the degenerate core and in the radiative envelope expands adiabatically. The thermal instability is quenched because of the expansion of the core, and, in further contrast to the situation in asymptotic giant branch stars, a subsequent long phase of quiescent helium burning does not occur. The energy converted into gravitational potential energy during the pulse is released slowly during the interpulse phase in the form of escaping photons. Thus, during most of the oscillation cycle, the star contracts and shines because of the release of gravitational potential energy which has been built up at the expense of nuclear energy released over only a very brief portion of the cycle. The oscillations continue with steadily decreasing amplitude for ˜30 cycles. Eventually, the domain of instability is avoided and nonoscillatory shell helium burning continues for a time comparable to the time spent in the thermally pulsing stage (˜5 × 107 yr). After two final hydrogen shell flashes, which consume most of the hydrogen remaining near the surface, the remnant settles onto a normal cooling sequence for a low-mass DA white dwarf. The lifetimes of both the core helium-burning phase and the shell helium-burning phase are large enough that real counterparts may lose all of their surface hydrogen through stellar winds during these phases. We explore the possibility that a substantial fraction of all sdO and sdB stars are in one of the two helium-burning phases of a binary component whose companion has already become a cool white dwarf.

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