International audienceWe use moderate-resolution optical spectrophotometry and the new MARCS stellar atmosphere models to determine the effective temperatures of 74 Galactic red supergiants (RSGs). The stars are mostly members of OB associations or clusters with known distances, allowing a critical comparison with modern stellar evolutionary tracks. We find we can achieve excellent matches between the observations and the reddened model fluxes and molecular transitions, although the atomic lines Ca I ?4226 and Ca II H and K are found to be unrealistically strong in the models. Our new effective temperature scale is significantly warmer than those in the literature, with the differences amounting to 400 K for the latest type M supergiants (i.e., M5 I). We show that the newly derived temperatures and bolometric corrections give much better agreement with stellar evolutionary tracks. This agreement provides a completely independent verification of our new temperature scale. The combination of effective temperature and bolometric luminosities allows us to calculate stellar radii; the coolest and most luminous stars (KW Sgr, Case 75, KY Cyg, HD 206936=? Cep) have radii of roughly 1500 Rsolar (7 AU), in excellent accordance with the largest stellar radii predicted from current evolutionary theory, although smaller than that found by others for the binary VV Cep and for the peculiar star VY CMa. We find that similar results are obtained for the effective temperatures and bolometric luminosities using only the dereddened V-K colors, providing a powerful demonstration of the self-consistency of the MARCS models

The R136 cluster in 30 Doradus is the prototype "" super star cluster,ÏÏ and the only example sufficiently close that its massive star content can be studied directly. We have used the Hubble Space Telescope to obtain spectra of 65 of the bluest, most luminous stars in R136 and Ðnd that the majority of these stars are of type O3, the hottest, most luminous, and most massive stars known. The total number of O3 stars in this one cluster exceeds the total number known elsewhere in the Milky Way or Magellanic Clouds. The highest luminosity stars found are O3 If*, O4 If], O3 If/WN6-A, and H-rich WN stars, with masses in excess of 120 the highest masses for which appropriate evolutionary tracks are cur-M _ , rently available. In accord with de Koter, Heap, & Hubeny, we conclude that these WN stars must be core H-burning stars whose spectra are WR-like because of high luminosity, and we Ðnd that their individual luminosities are a factor of 10 higher than is normal for WN stars of similar type but are like those found in the Galactic cluster NGC 3603, which they also resemble spectroscopically. Our spectroscopy does include stars as late as B0 V and samples most stars in the core of the R136 cluster with masses [50 The spectroscopy has been combined with HST photometry to study the star forma-M _. tion history and initial mass function of the R136 cluster. The young age (\1È2 Myr) for the highest mass stars, combined with what was previously known for the intermediate-mass populations, suggests that the lower mass stars began forming 4È5 Myr ago and continued forming until the high mass stars formed, consistent with the paradigm in which the formation of massive stars shuts down further star formation in the molecular cloud. Despite the unique preponderance of the highest mass and luminosity stars ever seen, the initial mass function (IMF) is found to be completely normal, with a slope ! \ [1.3 to [1.4. The number of high-mass stars is in good accord with that predicted by the IMF of the intermediate-mass stars, suggesting that a Salpeter-like IMF holds over the mass range 2.8È120 M _ within the R136 cluster. The fact that the IMF slope in R136 is indistinguishable from those of Galactic and Magellanic Cloud OB associations suggests that star formation produces the same distribution of masses over a range of D200 times in stellar density, from that of sparse OB associations to that typical of globular clusters. The large number of O3 stars in R136 is then simply a consequence of its youth (\1È2 Myr) and its richness, suggesting that the upper mass "" cuto † ÏÏ to the IMF seen in OB associations may simply be the result of their sparcity.

We present UBVRI photometry obtained from Mosaic images of M31 and M33 using the KPNO 4-m telescope. The survey covers 2.2 sq degrees of M31, and 0.8 sq degrees of M33, chosen so as to include all of the regions currently active in forming massive stars. The catalog contains 371,781 and 146,622 stars in M31 and M33, respectively, where every star has a counterpart (at least) in B, V, and R. We compare our photometry to previous studies. We provide cross references to the stars confirmed as members by spectroscopy, and compare the location of these to the complete set in color-magnitude diagrams. While follow-up spectroscopy is needed for many projects, we demonstrate the success of our photometry in being able to distinguish M31/M33 members from foreground Galactic stars. We also present the results of newly obtained spectroscopy, which identifies 34 newly confirmed members, including B-A supergiants, the earliest O star known in M31, and two new Luminous Blue Variable candidates whose spectra are similar to that of P Cygni.Comment: Accepted by the Astronomical Journal. A version with higher resolution figures can be found at: http://www.lowell.edu/users/massey/M3133.pdf.g

We present photometry and a preliminary interpretation of a UBVR survey of the Large and Small Magellanic Clouds, which covers 14.5 deg2 and 7.2 deg2, respectively. This study is aimed at obtaining well-calibrated data on the brighter, massive stars, complementing recent, deeper CCD surveys. Our catalog contains 179,655 LMC and 84,995 SMC stars brighter than V ~ 18.0, and is photometrically complete to U ~ B ~ V ~ 15.7 and R ~ 15.2, although stars in crowded regions are selectively missed. We compare our photometry to that of others, and describe the need for gravity-dependent corrections to our CCD U-band photometry. We provide a preliminary cross-reference between our catalog stars and the stars with existing spectroscopy. We discuss the population of stars seen toward the two Clouds, identifying the features in the color-magnitude diagram, and using existing spectroscopy to help construct H-R diagrams. We derive improved values for the blue-to-red star ratios in the two Clouds, emphasizing the uncertainties involved in this before additional spectroscopy. We compare the relative number of red supergiants and Wolf-Rayet stars in the LMC and SMC with that of other galaxies in the Local Group, demonstrating a very strong, tight trend with metallicity, with the ratio changing by a factor of 160 from the SMC to M31. We also reinvestigate the initial mass function of the massive stars found outside of the OB associations. With the newer data, we find that the initial mass function (IMF) slope of this field population is very steep, with Γ ~ -4 ± 0.5, in agreement with our earlier work. This is in sharp contrast to the IMF slope found for the massive stars with OB associations (Γ ~ -1.3). Although much more spectroscopy is needed to make this result firm, incompleteness can no longer be invoked as an explanation.