The initial mass function for massive stars in the Galaxy and the Magellanic Clouds

Abstract

We have determined the “present-day” initial mass function for stars greater than 15 M0 in our Galaxy and the Large and Small Magellanic Clouds. Our data set includes O type stars, supergiants of all types, and the less luminous B stars for a total of over 5000 stars in our Galaxy and nearly 1300 stars in the LMC and almost 500 stars in the SMC. More than 2300 of the galactic stars are in 91 associations and young clusters. We find that incompleteness may be a serious problem for determining the IMF for massive stars. We present evidence that the data for the less luminous stars (MBol < — 8 mag) are incomplete in our Galaxy and the Magellanic Clouds. When we apply a simple estimate for the incompleteness, we derive an IMF for our region of the Galaxy (r < 3 kpc) with a slope of —2.4. The IMFs for the stars interior and exterior to the solar orbit also have similar slopes. They are parallel, although more of the massive stars are found interior to the Sun. The number of stars per unit mass interval is greater in the inner region, but the relative number in each mass interval is the same in the two regions. The IMFs for the outer region of our Galaxy, the LMC, and SMC are all consistent with the Miller-Scalo IMF for the , solar neighborhood. We find no observational evidence that the slope of the massive star IMF differs from the normal IMF for the solar neighborhood or that it varies with galactocentric distance.