The wind momentum-luminosity relationship of galactic A- and B-supergiants

The Balmer lines of four A Ia-supergiants (spectral type AO to A3) and four teen B la and Ib-supergiants (spectral type B0 to B3) in the solar neighbou rhood are analyzed by means of NLTE unified model atmospheres to determine the properties of their stellar winds, in particular their wind momenta As in previous work for O-stars (Puls et al. 1996) a tight relationship betwee n stellar wind momentum and luminosity ("WLR") is found. However, the WLR v aries as function of spectral type. Wind momenta are strongest for O-superg iants, then decrease from early B (B0 and B1) to mid B (B 1.5 to B3) spectr al types and become stronger again for A-supergiants. The slope of the WLR appears to be steeper forA- and mid B-supergiants than for O-supergiants. T he spectral type dependence is interpreted as an effect of ionization chang ing the effective number and the line strength distribution function of spe ctral lines absorbing photon momentum around the stellar flux maximum. This interpretation needs to be confirmed by theoretical calculations for radia tion driven winds. The "Pistol-Star" in the Galactic Centre, an extreme mid B-hypegiant recent ly identified as one of the most luminous stars (Figer et al. 1998) is foun d to coincide with the extrapolation of the mid B-supergiant WLR towards hi gher luminosities. However, the wind momentum of the Luminous Blue Variable P Cygni, a mid B-supergiant with extremely strong mass-loss, is 1.2 dex hi gher than the WLR of the "normal" supergiants. This significant difference is explained in terms of the well-known stellar wind bi-stability of superg iants very close to the Eddinton-limit in this particular range of effectiv e temperatures. a-supergiants in M31 observed with HIRES at the Keck telesc ope have wind moments compatible with their galactic counterparts. The potential of the WLR as a new, independent extragalactic distance indic ator is discussed. It is concluded that with ten to twenty objects, photome try with FIST and medium resolution spectroscopy with 8m-telescopes from th e ground distance moduli can be obtained with an accuracy of about 0.(m)1 o ut to the Virgo and Fornax clusters of galaxies.