Early Post Asymptotic Giant Branch Instability: Does it Affect White Dwarf Hydrogen Envelope Mass?

Abstract

Although most white dwarf stars have hydrogen-dominated atmospheres, a significant fraction have atmospheres in which hydrogen is spectroscopically absent, with the fraction of hydrogen-free atmospheres varying with effective temperature. Estimates of the total mass of hydrogen, MH, in the stellar envelope from either asteroseismology or spectral evolution are at odds with predicted values from theoretical stellar evolution modeling. Recent work has found that models in the early post Asymptotic Giant Branch (AGB) phase of evolution can exhibit thermally and dynamical unstable behavior. Here we investigate whether this Early Post AGB Instability (EPAGBI) can help resolve the conflict in MH values determined from white dwarf spectral evolution, analysis of DAV pulsations and canonical stellar evolution modeling, by evolving models of mass 1 and 2Msun through the AGB phases and to the white dwarf cooling track. The MH values at the end of the calculations are in the range consistent with asteroseismic determinations. The major impact of EPAGBIs is that they cause loops in the HRD, which are absent when the EPAGBI is suppressed. Such loops might be detectable in a long-term monitoring program, or by their imprint on planetary nebula morphology imparted by the cyclically varying mass loss rate. Since the characteristic timescale of the looping in the HRD depends on the stellar mass, it could provide a way to determine the stellar mass just after AGB departure. Another EPAGBI signature is the production of Li by the Cameron-Fowler process. During the EPAGBI phase the photospheric temperature is always too high for the Li I resonance line to be detected. However, 7Be is convected to the photosphere in significant amounts (up to 400 times the solar photospheric mass fraction) at various times in the EPAGBI phase, which may be detectable by observing the Be II resonance doublet.