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 (2002) finds such a deficit in
samples of the field population in the Small and Large Magellanic Clouds, with
an IMF slope of {\Gamma} ~ 4. These IMF measurements represent some of the
largest known deviations from the standard Salpeter IMF slope of {\Gamma}=1.35.
Here, we carry out a comprehensive investigation of the mass function above 20
solar masses for the entire field population of the Small Magellanic Cloud,
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 solar masses is {\Gamma}=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 {\Gamma}=2.3+/-0.6 for
stellar masses from 7-20 solar masses. The discovery and removal of ten
runaways in our RIOTS4 sample steepens the field IMF slope to
{\Gamma}=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.
