Portal de Periódicos da CAPES

Self-Excited Radial Oscillations in Thin Stellar Envelopes. I

1966; IOP Publishing; Volume: 144; Linguagem: Inglês

10.1086/148702

1538-4357

J. P. Cox, A. N. Cox, K. H. Olsen, D. S. King, D. D. Eilers,

Astro and Planetary Science

view Abstract Citations (54) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Self-Excited Radial Oscillations in Thin Stellar Envelopes. I Cox, J. P. ; Cox, A. N. ; Olsen, K. H. ; King, D. S. ; Eilers, D. D. Abstract The computation techniques described in the preceding paper (Cox, Brownlee, and Eilers 1966) have been used to study self-excited radial pulsations in stellar-envelope models and the basic physics of the destabilizing mechanism arising from He+ ionization in the envelopes. The mass of the envelope is about 10- of the total stellar mass (=5 395 Mo for all models) and the envelopes consist of either twenty or fifty mass zones. The temperature at the bottom of the envelopes is about 10 K, high enough so that the envelopes contain the entire He+ ionization region plus enough material below to provide considerable radiative damping. The lower boundaries of the envelopes are held fixed in time (at a radius typically 0.8-09 of the total stellar radius), and a constant luminosity is fed into the envelopes from below. The envelopes are followed in time from an initial state which is in very close hydrostatic and thermal equilibrium. Unstable envelopes develop radial pulsations whose amplitude grows exponentially to some limiting amplitude in several hundred periods, whereas stable envelopes remain in equilibrium. The pulsations are practically periodic at limiting amplitude, and their properties are independent of the method of reaching the limiting amplitude. Most of the unstable envelopes studied (about 11) had R/R0 (radius semi-amplitude) 0.04A) 05, total surface velocity amplitude 60-70 km/sec, and total bolometric light range 0 3-0.4 mag. at limiting amplitude. The relative smallness of the radial variations at limiting amplitude suggests that the limiting amplitude is determined primarily by a "saturation effect" of the driving mechanism. The results show that the opacity variations (the "ic-mechanism" of Baker and Kippenhahn) and the small values of P3 - 1 in the He+ ionization zone both produce negative dissipation in and above this region. Evidence is also found for the existence of negative dissipation in the H ionization region, but this negative dissipation is "swamped" by other effects. The negative dissipation in and above the He+ ionization region and the positive dissipation below are approximately equal at limiting amplitude. Two "envelope instability strips," corresponding to two chemical compositions (a pure He/H mix and an Aller mix) and two mass-luminosity laws, are found on the Hertzsprung-Russell diagram close to the region of the classical Cepheids (see Fig. 18). If our periods were corrected for the shallowness of our envelopes, our computed instability strips for the fundamental mode of the whole star would be shifted toward lower effective temperatures (by about A log Te 0 036) and would then lie slightly to the right of the center of the Cepheid strip. Some suggestions are offered concerning some of the factors that may determine the upper and lower boundaries of the Cepheid instability strip. Publication: The Astrophysical Journal Pub Date: June 1966 DOI: 10.1086/148702 Bibcode: 1966ApJ...144.1038C full text sources ADS |