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تسجيل مستخدم جديدThe evolution of helium white dwarfs: II. Thermal instabilities
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T. Driebe
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We calculated a grid of evolutionary models for white dwarfs with helium cores (He-WDs) and investigated the occurrence of hydrogen-shell flashes due to unstable hydrogen burning via CNO cycling. Our calculations show that such thermal instabilities are restricted to a certain mass range (M=0.21...0.30Msun), consistent with earlier studies. Models within this mass range undergo the more hydrogen shell flashes the less massive they are. This is caused by the strong dependence of the envelope mass on the white dwarf core mass. The maximum luminosities from hydrogen burning during the flashes are of the order of 10^5 Lsun. Because of the development of a pulse-driven convection zone whose upper boundary temporarily reaches the surface layers, the envelopes hydrogen content decreases by Delta(X)=0.06 per flash. Our study further shows that an additional high mass-loss episode during a flash-driven Roche lobe overflow to the white dwarfs companion does not affect the final cooling behaviour of the models. Independent of hydrogen shell flashes the evolution along the final white dwarf cooling branch is determined by hydrogen burning via pp-reactions down to effective temperatures as low as 8000 K.
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We present a grid of evolutionary tracks for low-mass white dwarfs with helium cores in the mass range from 0.179 to 0.414 M_sun. The lower mass limit is well suited for comparison with white dwarf companions of millisecond pulsars (MSP). The derived
cooling ages are of the order of 10^9 yrs due to residual nuclear burning. The cooling ages are consistent with age estimations of MSP systems based on the pulsars spin-down. For example, for the system PSR 1012+5307 we derived a white dwarf cooling age of 6 +/-1 Gyr in good agreement with the spin-down age of 7 Gyr. For the companion mass we found M=0.19 +/- 0.02 M_sun. We studied other MSP systems as well selecting only systems with well given ages and/or masses, and determined the effective temperatures and surface gravities of the companion white dwarfs with the present evolutionary models.
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T. Rauch (Dr.-Remeis-Sternwarte
, Bamberg
, Germany; Institut fuern Astronomie und Astrophysik
2002
Previous investigations on hydrogen-rich white dwarfs generally yield only very small rotational velocities (v_rot sin i). We have analyzed line profiles in high-resolution optical spectra of eight hydrogen-deficient (pre-) white dwarfs and find devi
ations from the dominant Stark line broadening in five cases which, interpreted as an effect of stellar rotation, indicate projected rotational velocities of 40 - 70 km/sec. For the three least luminous stars upper limits of v_rot sin i = 15 - 25 km/sec could be derived only. The resulting velocities correlate with luminosity and mass. However, since the mass-loss rate is correlated to the luminosity of a star, the observed line profiles may be affected by a stellar wind as well. In the case of RX J2117.1+3412, this would solve discrepancies to results of pulsational modeling (v_rot sin i ~ 0).
We present a grid of evolutionary tracks for low-mass white dwarfs with helium cores in the mass range from 0.179 to 0.414 Msol. The lower mass limit is well-suited for comparison with white dwarf companions of millisecond pulsars. The tracks are bas
ed on a 1 Msol model sequence extending from the pre-main sequence stage up to the tip of the red-giant branch. Applying large mass loss rates at appropriate positions forced the models to move off the giant branch. The further evolution was then followed across the Hertzsprung-Russell diagram and down the cooling branch. At maximum effective temperature the envelope masses above the helium cores increase from 0.6 to 5.4 x 10^{-3} Msol for decreasing mass. We carefully checked for the occurrence of thermal instabilities of the hydrogen shell by adjusting the computational time steps accordingly. Hydrogen flashes have been found to take place only in the mass interval 0.21 < M/Msol < 0.3. The models show that hydrogen shell burning contributes significantly to the luminosity budget of white dwarfs with helium cores. For very low masses the hydrogen shell luminosity remains to be dominant even down to effective temperatures well below 10000K. Accordingly, the corresponding cooling ages are significantly larger than those gained from model calculations which neglect nuclear burning or the white dwarf progenitor evolution. Using the atmospheric parameters of the white dwarf in the PSR J1012+5307 system we determined a mass of M=0.19 +/- 0.02 Msol and a cooling age of 6 +/- 1 Gyr, in good agreement with the spin-down age, 7 Gyr, of the pulsar.
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