Supergiant variability - Amplitudes and pulsation constants in relation with mass loss and convection

Details Supergiant variability - Amplitudes and pulsation constants in relation with mass loss and convection Supergiant variability - Amplitudes and pulsation constants in relation with mass loss and convection

Oct 1980

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1980a%26a....90..311m&link_type=abstract

Astronomy and Astrophysics, vol. 90, no. 3, Oct. 1980, p. 311-317.

Astronomy and Astrophysics

Astrophysics

113

Hertzsprung-Russell Diagram, Stellar Mass Ejection, Supergiant Stars, Variable Stars, A Stars, B Stars, Stellar Luminosity, Stellar Radiation, Stellar Structure

Scientific paper

The distribution in the HR diagram of the amplitudes of light variations for supergiants is established on the basis of 2420 observations of supergiants made over the last 20 yr in the Geneva photometry (Rufener, 1979). It is shown that a) for any spectral type the amplitudes increase with the luminosity, b) for Ia supergiants there is a small local maximum of amplitudes in B-type stars, c) for G-M supergiants the higher is the luminosity, the earlier is the spectral type at which important amplitudes (> 0m.1) appear. This last effect closely corresponds to the limit in the HR diagram for the existence of very deep convective zones in supergiants.
A relation is found between the amplitudes of light variations and the rate of mass loss Mṡ for B and A supergiants; it indicates an exponential increase of Mṡ with the amplitudes.
Empirical pulsation constants Q for B-G supergiants are determined and they are found to be systematically larger than the theoretical Q for the fundamental mode of radial oscillation; it is not very likely that these differences are entirely due to mass loss. It is suggested that the intermediate term variations of B-G supergiants are due to non-radial oscillations in gravity modes. Various possibilities, as emphasized by Ledoux, may be envisaged, such as motions of periodic character due to g¯ modes in presence of some extra-restoring force (e. g. rotation) or vibrational excitation of g+ modes. The instabilities may be connected with the over-adiabacity in the outer layers of supergiants. Indeed, present computations show that large external convective zones, which usually exist in stars near the Hayashi line, are present at high luminosities (L > 104.5 Lsun) in supergiants over most of the HR diagram, due to radiation pressure effects.
The external convective zones could play an important role, as source of mechanical energy for the heating of coronae in supergiants of various types.

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