Astrophys. J., 763, 101 (2013/February-1)
The initial mass function of field OB stars in the Small Magellanic Cloud.
LAMB J.B., OEY M.S., GRAUS A.S., ADAMS F.C. and SEGURA-COX D.M.
Abstract (from CDS):
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.
galaxies: stellar content - Magellanic Clouds - stars: early-type - stars: formation - stars: fundamental parameters - stars: luminosity function, mass function
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<Available at CDS (J/ApJ/763/101): table1.dat>
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