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 a ssociations in order to answer three questions: (1) How coeval is star formation? (2) How constant is the initial mass function (IMF)? (3) W hat is the progenitor mass of Wolf-Rayet stars? Our sample includes NG C 6823/Vul OB1, NGC 6871/Cyg OB3, Berkeley 86/Cyg OB1, NGC 6983/Cyg OB 1, NGC 7235, NGC 7380/Cep OB1, Cep OB5, IC 1805/Gas OB6, NGC 1893/Aug OB2, and NGC 2244/Mon OB2. Large-field CCD imaging and multiobject, fi ber spectroscopy has resulted in UBV photometry for >14000 stars and n ew spectral types for approximate to 200 stars. These data are used to redetermine distances and reddenings for these regions and to help ex clude probable nonmembers in constructing the H-R diagrams. We reanaly ze 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 Delta tau < 3 Myr in each association. Some star formation has dearly preceded this event, as evidenced by the occ asional presence of evolved (tau approximate to 10 Myr) 15 M. stars de spite a typical age tau approximate to 2 Myr for the more massive popu lation. However, all these regions also show evidence of 5-10 M. pre-m ain-sequence stars (tau < 1 Myr), demonstrating that some star formati on at lower masses does continue for at least 1 Myr after the formatio n of high-mass stars. (2) There is no statistically significant differ ence in IMF slopes among these clusters, and the average value is foun d to be Gamma = -1.1 +/- 0.1 for stars with masses > 7 M. A comparison with similarly studied OB associations in the Magellanic Clouds revea ls no difference in IMF slope, and hence we conclude that starformatio n of massive stars in clusters proceeds independently of metallicity, at least between z = 0.02 and z = 0.002. The masses of the highest mas s stars are approximately equal in the Milky Way, LMC, and SMC associa tions, contrary to the expectation that this value should vary by a fa ctor of 3 over this metallicity range. We conclude that radiation pres sure on grains must not limit the mass of the highest mass star that c an form, in accord with the suggestion of Wolfire & Cassinelli that th e mere existence of massive stars suggests that shocks or other mechan isms have disrupted grains in star-forming events. (3) The four Wolf-R ayet stars in our sample have come from stars more massive than 40 M.; one WC star and one late-type WN star each appear to have come from v ery massive (approximate to 100 M.) progenitors.