Gravity Mode Instabilities in DA White Dwarfs.

Details Gravity Mode Instabilities in DA White Dwarfs. Gravity Mode Instabilities in DA White Dwarfs.

Mar 1982

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1982phdt........27w&link_type=abstract

Thesis (PH.D.)--THE UNIVERSITY OF ROCHESTER, 1982.Source: Dissertation Abstracts International, Volume: 43-01, Section: B, page:

Physics

9

Scientific paper

We have investigated the stability of nonradial g-mode oscillations in DA white dwarfs through the numerical solution of the equations describing linear, nonadiabatic, nonradial oscillations for theoretical models. We report the results of calculations for modes corresponding to spherical harmonic index l = 1 through 3 for an extensive grid of evolutionary white dwarf models incorporating stratified (H/He/C) envelopes consistent with diffusion equilibrium. We have found a theoretical instability strip which is the result of the (kappa)-mechanism operating in the H partial ionization zone. The "blue edge" for this instability strip occurs at T(,e)('(VBAR))(,blue) (TURN) 11,000 K, depending on model parameters such as H and He layer masses, total stellar mass, thickness of the transition zones, and in particular convective efficiency; increases in assumed convective efficiency lead to significantly higher values of T(,e)('(VBAR))(,blue). The location of, as well as the crucial role of H-driving in, this theoretical instability strip suggests identification with the observed ZZ Ceti instability strip. Comparison with the observed value of T(,e)('(VBAR))(,blue) implies 10('-12)(, )<(, )M(,H)/M(,* )<(, )10('-8) for the ZZ Cetis. The observational result that most, and possibly all, of the DA white dwarfs in the temperature range of the ZZ Cetis are variable suggests that these constraints on M(,H) may apply to DA white dwarfs in general. For models within this instability strip, we find that the most strongly driven modes, for a given value of l, tend to have the longest periods; correlating well with the observed features of the ZZ Cetis. In addition, the periods of the most unstable modes increase with decreasing T(,e), suggesting that a period-T(,e) correlation may be found in the ZZ Cetis when more accurate observational determinations of T(,e) become available. For models with M(,H)/M(,*) < 10('-10), we have also found a second narrow instability strip resulting from He-driving near T(,e) (TURN) 19,000 K; an observational search for variable DA's near this temperature may thus place more stringent bounds on M(,H). It is demonstrated that our results for DA models can be understood on the basis of simple physical considerations. These considerations are extended to DB white dwarfs, and it is suggested that He-driving may produce an instability strip for these stars near T(,e) (TURN) 20,000 K.

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