Stellar Abundances and Winds of A-Type Supergiant Stars in M33: First Results from the Keck HIRES Spectrograph^*

In this Letter, we report the first results of detailed analyses of A-type supergiants in M33 from high-quality Keck HIRES echelle spectra. The two stars, designated 117-A and B-324, now constitute the most distant stars for which detailed abundances have been measured. We find 117-A is metal-poor (roughly 1/10 solar), resembling an early A supergiant in the SMC. B-324 has P Cygni-like profiles for most of the metal lines, but fitting only the photospheric component yields metal abundances similar to solar. These two stars are located at distinctly different galactocentric distances in M33; comparing their abundances shows that the metallicity gradient of this galaxy, previously reported only from H II-region surveys, is also qualitatively apparent in these stars.

Estimates of the stellar-wind parameters for these stars yield mass-loss rates of ~2.5 × 10^-8 M_☉ yr^-1 for 117-A and ~1.2 × 10^-5 M_☉ yr^-1 for B-324 from the Hα line profiles. When we examine the location of these stars in the wind momentum-luminosity plane, we find that they are in excellent agreement with the loci of luminous blue stars in the Galaxy, LMC, and SMC, taking into account differences in metallicity.