The Wind Momentum - Luminosity Relationship of Galactic A- and B-\n Supergiants

Abstract

The Balmer lines of four A Ia - supergiants (spectral type A0 to A3) and\nfourteen B Ia and Ib - supergiants (spectral type B0 to B3) in the solar\nneighbourhood are analyzed by means of NLTE unified model atmospheres to\ndetermine the properties of their stellar winds, in particular their wind\nmomenta. As in previous work for O-stars (Puls et al.) a tight relationship\nbetween stellar wind momentum and luminosity (`WLR'') is found. However, the\nWLR varies as function of spectral type. Wind momenta are strongest for\nO-supergiants, then decrease from early B (B0 and B1) to mid B (B1.5 to B3)\nspectral types and become stronger again for A-supergiants. The slope of the\nWLR appears to be steeper for A- and mid B-supergiants than for O-supergiants.\nThe spectral type dependence is interpreted as an effect of ionization changing\nthe effective number and the line strength distribution function of spectral\nlines absorbing photon momentum around the stellar flux maximum. This\ninterpretation needs to be confirmed by theoretical calculations for radiation\ndriven winds.\n A-supergiants in M31 observed with HIRES at the Keck telescope have wind\nmomenta compatible with their galactic counterparts.\n The potential of the WLR as a new, independent extragalactic distance\nindicator is discussed. It is concluded that with ten to twenty objects,\nphotometry with HST and medium resolution spectroscopy with 8m-telescopes from\nthe ground distance moduli can be obtained with an accuracy of about 0.1m out\nto the Virgo and Fornax clusters of galaxies.\n