MASSIVE STARS IN THE FIELD AND ASSOCIATIONS OF THE MAGELLANIC CLOUDS: THE UPPER MASS LIMIT, THE INITIAL MASS FUNCTION, AND A CRITICAL TEST OF MAIN-SEQUENCE STELLAR EVOLUTIONARY THEORY

摘要

We investigate the massive star population of the Magellanic Clouds with an emphasis on the field population, which we define as stars located further from any OB association than massive stars are likely to travel during their short lifetimes. The field stars must have been born as part of more modest star-forming events than those that have populated the large OB associations found throughout the Clouds. We use new and existing data to answer the following questions: Does the field produce stars as massive as those found in associations? Is the initial mass function (IMF) of these field massive stars the same as those of large OB complexes? How well do the Geneva low-metallicity evolutionary models reproduce what is seen in the field population, with its mixed ages? To address these issues we begin by updating existing catalogs of LMC and SMC members with our own new spectral types and derive H-R diagrams (HRDs) of 1584 LMC and 512 SMC stars. We use new photometry and spectroscopy of selected regions in order to determine the incompleteness corrections of the catalogs as a function of mass and find that we can reliably correct the number of stars in our HRDs down to 25 solar mass. Using these data, we derive distance moduli for the Clouds via spectroscopic parallax, finding values of 18.4 ± 0.1 and 19.1 ± 0.3 for the LMC and SMC. The average reddening of the field stars is small: E(B-V) = 0.13 (LMC) and 0.09 (SMC), with little spread.