Mass-Loss Rates and Stellar Wind Momenta of A-Supergiants in M31: First Results from the Keck HIRES Spectrograph^*

High-quality Keck HIRES echelle spectra of two A-supergiants in M31, designated 41-3654 (spectral type A2 Ia-O) and 41-3712 (A3 Ia-O), at 10.2 kpc galactocentric distance are analyzed by means of NLTE unified model atmospheres to determine the properties of their stellar winds, in particular the wind momenta. We obtain mass-loss rates of 1.9 × 10^-6 and 1.1 × 10^-6 M_☉ yr^-1 for 41-3654 and 41-3712, respectively, with an accuracy of 15% from the fit of the Hα profiles. The terminal velocities are 200 ± 25 km s^-1. In addition, the exponents of the wind velocity fields are determined (β = 2 and 3, respectively) and microturbulent velocities of the order of 20 km s^-1 are found.

The wind momenta of the two targets are in excellent agreement with the wind momentum-luminosity relation (WLR) for galactic A- and B-supergiants. We discuss the potential of the WLR for extragalactic distance determinations and conclude that it may allow independent distance moduli to be obtained with an accuracy of 10%.