THE INITIAL MASS FUNCTION OF FIELD OB STARS IN THE SMALL MAGELLANIC CLOUD

Some theories of star formation suggest massive stars may only form in clustered environments, which would create a deficit of massive stars in low-density environments. Observationally, Massey finds such a deficit in samples of the field population in the Small and Large Magellanic Clouds, with an initial mass function (IMF) slope of ΓIMF ∼ 4. These IMF measurements represent some of the largest known deviations from the standard Salpeter IMF slope of ΓIMF = 1.35. Here, we carry out a comprehensive investigation of the mass function above 20 M ☉ for the entire field population of the Small Magellanic Cloud (SMC), based on data from the Runaways and Isolated O Type Star Spectroscopic Survey of the SMC (RIOTS4). This is a spatially complete census of the entire field OB star population of the SMC obtained with the IMACS multi-object spectrograph and MIKE echelle spectrograph on the Magellan telescopes. Based on Monte Carlo simulations of the evolved present-day mass function, we find the slope of the field IMF above 20 M ☉ is ΓIMF = 2.3 ± 0.4. We extend our IMF measurement to lower masses using BV photometry from the OGLE II survey. We use a statistical approach to generate a probability distribution for the mass of each star from the OGLE photometry, and we again find ΓIMF = 2.3 ± 0.6 for stellar masses from 7 M ☉ to 20 M ☉. The discovery and removal of ten runaways in our RIOTS4 sample steepens the field IMF slope to ΓIMF = 2.8 ± 0.5. We discuss the possible effects of binarity and star formation history on our results, and conclude that the steep field massive star IMF is most likely a real effect.