The Initial Mass Function and Massive Star Evolution in the OB Associations of the Northern Milky Way
We investigate the massive star content of Milky Way clusters and OB associations in order to answer three questions: (1) How coeval is star formation? (2) How constant is the initial mass function (IMF)? (3) What is the progenitor mass of Wolf-Rayet stars? Our sample includes NGC 6823/Vul OB1, NGC 6871/Cyg OB3, Berkeley 86/Cyg OB1, NGC 6913/Cyg OB1, NGC 7235, NGC 7380/Cep OB1, Cep OB5, IC 1805/Cas OB6, NGC 1893/Aug OB2, and NGC 2244/Mon OB2. Large-field CCD imaging and multiobject, fiber spectroscopy has resulted in UBV photometry for >10,000 stars and new spectral types for ≍200 stars. These data are used to redetermine distances and reddenings for these regions and to help exclude probable nonmembers in constructing the H-R diagrams. We reanalyze comparable data previously published on Cyg OB2, Tr 14/16, and NGC 6611 and use all of these to paint a picture of star formation and to measure the IMFs. We find the following: (1) Most of the massive stars are born during a period ∆τ <3 Myr in each association. Some star formation has clearly preceded this event, as evidenced by the occasional presence of evolved (τ ≍ 10 Myr) 15 Msun stars despite a typical age τ ≍ 2 Myr for the more massive population. However, all these regions also show evidence of 5-10 Msun pre-main-sequence stars (τ < 1 Myr), demonstrating that some star formation at lower masses does continue for at least 1 Myr after the formation of high-mass stars. (2) There is no statistically significant difference in IMF slopes among these clusters, and the average value is found to be Γ = -1.1±0.1 for stars with masses >7 Msun. A comparison with similarly studied OB associations in the Magellanic Clouds reveals no difference in IMF slope, and hence we conclude that star formation of massive stars in clusters proceeds independently of metallicity, at least between z = 0.02 and z = 0.002. The masses of the highest mass stars are approximately equal in the Milky Way, LMC, and SMC associations, contrary to the expectation that this value should vary by a factor of 3 over this metallicity range. We conclude that radiation pressure on grains must not limit the mass of the highest mass star that can form, in accord with the suggestion of Wolfire & Cassinelli that the mere existence of massive stars suggests that shocks or other mechanisms have disrupted grains in star-forming events. (3) The four Wolf- Rayet stars in our sample have come from stars more massive than 40 Msun; one WC star and one late-type WN star each appear to have come from very massive (≍100 Msun) progenitors.