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Astron. Astrophys. 350, 970-984 (1999)

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

R.P. Kudritzki 1,2,3, J. Puls 1, D.J. Lennon 1,4, K.A. Venn 5, J. Reetz 1, F. Najarro 9, J.K. McCarthy 6 and A. Herrero 7,8

1 Institut für Astronomie und Astrophysik der Universität München, Scheinerstrasse 1, 81679 München, Germany
2 Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Strasse 1, 85740 Garching, Germany
3 University of Arizona, Steward Observatory, 933 N. Cherry Av., Tucson, AZ 85721, USA
4 Isaac Newton Group of Telescopes, Apartado de Correos 368, 38700 Santa Cruz de La Palma, Spain
5 Department of Physics and Astronomy, Macalaster College, St.Paul, MN 55101, USA
6 Palomar Observatory, California Institute of Technology, Pasadena, CA 91125, USA
7 Instituto de Astrofísica de Canarias, c/Vía Lactea s/n, 38200 La Laguna, Tenerife, Spain
8 Departamento de Astrofisica, Universidad de La Laguna, Avda. Astrof. Francisco Sanchez s/n, 38071 La Laguna, Spain
9 Instituto de Estructura de la Materia, CSIC, Serrano 121, 28006 Madrid, Spain

Received 28 April 1999 / Accepted 27 August 1999


The Balmer lines of four A Ia-supergiants (spectral type A0 to A3) and fourteen B Ia and Ib-supergiants (spectral type B0 to B3) in the solar neighbourhood 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 between stellar wind momentum and luminosity ("WLR") is found. However, the WLR varies as function of spectral type. Wind momenta are strongest for O-supergiants, then decrease from early B (B0 and B1) to mid B (B1.5 to B3) spectral types and become stronger again for A-supergiants. The slope of the WLR appears to be steeper for A- and mid B-supergiants than for O-supergiants. The spectral type dependence is interpreted as an effect of ionization changing the effective number and the line strength distribution function of spectral lines absorbing photon momentum around the stellar flux maximum. This interpretation needs to be confirmed by theoretical calculations for radiation driven winds.

The "Pistol-Star" in the Galactic Centre, an extreme mid B-hypergiant recently identified as one of the most luminous stars (Figer et al. 1998) is found to coincide with the extrapolation of the mid B-supergiant WLR towards higher 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 higher than the WLR of the "normal" supergiants. This significant difference is explained in terms of the well-known stellar wind bi-stability of supergiants very close to the Eddinton-limit in this particular range of effective temperatures. A-supergiants in M31 observed with HIRES at the Keck telescope have wind momenta compatible with their galactic counterparts.

The potential of the WLR as a new, independent extragalactic distance indicator is discussed. It is concluded that with ten to twenty objects, photometry with HST and medium resolution spectroscopy with 8m-telescopes from the ground distance moduli can be obtained with an accuracy of about [FORMULA] out to the Virgo and Fornax clusters of galaxies.

Key words: stars: atmospheres – stars: early-type – stars: mass-loss – galaxies: distances and redshifts – cosmology: distance scale

Send offprint requests to: R.P. Kudritzki (kudritzki@usm.uni-muenchen.de)

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© European Southern Observatory (ESO) 1999

Online publication: October 14, 1999