In the first part of the paper, we study the evolution of hot horizontal-branch models which miss the asymptotic branch (AGB) phase to produce ''low-mass'' white dwarfs (WDs) with masses of the order of 0.5 M.. One finds that for each given metallicity there exists a maximum value for the mass of the H-rich envelope allowing the models to approach the cooling phase without the onset of CNO H-burning flashes; the larger the metallicity, the lower the mass of such a ''critical envelope.'' Moreover, we find that above log L approximately 1 the cooling rate appears drastically affected by the previous evolutionary history, allowing a meaningful discussion of the cooling laws only below such a luminosity. The dependence of the cooling rates on the mass of the H-rich envelope is investigated for suitable assumptions about the value of this parameter, discussing the contribution of both CNO and p-p H burning to the evolution. The investigation has been further extended to stars with larger He cores, showing that the final cooling structures closely approach available evaluations of post-AGB CO dwarfs, as the result of a convergence toward ''canonical'' self-consistent WD structures. On this basis, we discuss the influence of WD masses on both the cooling rate and the H-R diagram location of the cooling sequence. The application of this theoretical scenario to the WD population in stellar clusters is finally discussed.
COOLING WHITE-DWARFS IN GALACTIC GLOBULARS
DEGL'INNOCENTI, SCILLA;
1994-01-01
Abstract
In the first part of the paper, we study the evolution of hot horizontal-branch models which miss the asymptotic branch (AGB) phase to produce ''low-mass'' white dwarfs (WDs) with masses of the order of 0.5 M.. One finds that for each given metallicity there exists a maximum value for the mass of the H-rich envelope allowing the models to approach the cooling phase without the onset of CNO H-burning flashes; the larger the metallicity, the lower the mass of such a ''critical envelope.'' Moreover, we find that above log L approximately 1 the cooling rate appears drastically affected by the previous evolutionary history, allowing a meaningful discussion of the cooling laws only below such a luminosity. The dependence of the cooling rates on the mass of the H-rich envelope is investigated for suitable assumptions about the value of this parameter, discussing the contribution of both CNO and p-p H burning to the evolution. The investigation has been further extended to stars with larger He cores, showing that the final cooling structures closely approach available evaluations of post-AGB CO dwarfs, as the result of a convergence toward ''canonical'' self-consistent WD structures. On this basis, we discuss the influence of WD masses on both the cooling rate and the H-R diagram location of the cooling sequence. The application of this theoretical scenario to the WD population in stellar clusters is finally discussed.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
