Spectroscopy of bright blue objects in the field of M 81

We obtained spectra of the seven brightest blue supergiant candidates in the SbI-II spiral galaxy M 81 which so far had no spectroscopic identification. As members of M 81 the candidates would be about as bright as the blue supergiants in the ScIII spiral M 33 and the LMC, but ≈ 1^m brighter than in the morphologically similar galaxy M 31. Five objects were observed at the 3.5m telescope on Calar Alto, Spain, in Jan. 1992 with the TWIN spectrograph. Spectra of two additional objects were obtained with the 2.1m telescope at the Guillermo Haro Observatory, Mexico, in Feb. 1993 and Jun. 1995. Four of the objects turned out to be compact HII regions, two are foreground dwarfs. One object, M 81-642, shows a spectrum with an A to F spectral type. However, the absorption lines appear too broad for a supergiant. It is most likely a foreground star or star cluster. Assuming the generally accepted values for the reddening and the true distance modulus of M 81 of A_v = 1^m and m - M = 27.6 we conclude that the visually brightest early-type supergiant in M 81 is less bright than M_v ≈ -9.5. The next brightest candidate would have M_v ≈ -9.1 which is about as bright as the brightest blue supergiant in M 31 for which M_v ≈ -9.2 was found in recent studies. It seems therefore that the population of massive stars in the SbI-II spiral M 81 resembles that of the morphologically similar galaxy M 31 which shows a deficiency of the most massive stars.