Astronomy and Astrophysics – Astrophysics
Scientific paper
Oct 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995a%26a...302..811n&link_type=abstract
Astronomy and Astrophysics, v.302, p.811
Astronomy and Astrophysics
Astrophysics
23
Hydrodynamics, Shock Waves, Stars: Mass Loss, Supergiant Stars, Turbulence
Scientific paper
The goal of this paper is to study instability regions in the HR diagram, through a calculation of the atmospheric accelerations for spherically symmetric stars, in dynamic equilibrium, without using detailed atmospheric models. The input data are five primary data, viz.: the stellar luminosity L, the effective temperature T_eff_, the mass M, the rate of mass loss ˙(M), and the microturbulent velocity component ζmu_, while we assume the temperature for a reference atmospheric layer, an assumption that appears not to be critical. An iterative solution of the momentum equation, simultaneous with some other equations, yields values for the various accelerations acting on a stellar atmosphere and their algebraic sum g_eff_', the predicted effective acceleration. In the first part of the paper we compare this latter quantity with the g_eff_-value derived observationally from spectral studies of nine program stars and we find overall fair agreement. This supports the method as well as the values of the five input data. In part 2 we determine g'_eff_ in same way for the whole upper part of the Hertzsprung-Russell diagram by using statistical primary data on the mass (based on evolutionary calculations), on mass-loss and on microturbulence (shock-strengths). We find as a fairly general rule that, as stars move along their evolutionary track, and for time scales longer than the dynamic time scale of the atmosphere, the atmosphere continuously adapts to the new (L,T_eff_)-values and essentially remains stable. Current practice of determining the stability limit of stellar atmospheres by extrapolating hydrostatic models to the Eddington limit is not justified by this study. There is one exception: we find a small area around T_eff_=8300K and log(L/Lsun_)=5.7, where no solution is possible for evolved stars on their blueward evolutionary track; the stars in this area have in any case effective accelerations <1mm/s^2^: the "Yellow Evolutionary Void". In the third part we estimate approximately the pulsational (in-)stability of stars in the upper part of the Hertzsprung-Russell diagram, by comparing the g'_eff_ values determined in part 2 with the average outward pulsational acceleration. We thus confirm the 'Yellow Evolutionary Void' for blueward evolving stars, and also find an instability region for blueward evolving stars in the area occupied by the Wolf-Rayet stars. This seems to agree with the observations that the low-temperature boundary of the Yellow Evolutionary Void appears to coincide with the region where the yellow hypergiants are clustering. The yellow hypergiants are therefore interpreted as blueward moving stars with ZAMS masses of about 25-40Msun_, and actual masses between 15 and 25Msun_. For our galaxy it is found that only a few stars are situated within the 'Yellow Evolutionary Void', in accordance with our expectation.
de Jager Cornelis
Nieuwenhuijzen Hans
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