THE SPARSEST CLUSTERS WITH O STARS
- Department of Astronomy, University of Michigan, 500 Church St., Ann Arbor, MI 48109 (United States)
There is much debate on how high-mass star formation varies with environment, and whether the sparsest star-forming environments are capable of forming massive stars. To address this issue, we have observed eight apparently isolated OB stars in the Small Magellanic Cloud (SMC) using Hubble Space Telescope's Advanced Camera for Surveys. Five of these objects appear as isolated stars, two of which are confirmed to be runaways. The remaining three objects are found to exist in sparse clusters, with {approx}<10 companion stars revealed, having masses of 1-4 M{sub sun}. Stochastic effects dominate in these sparse clusters, so we perform Monte Carlo simulations to explore how our observations fit within the framework of empirical, galactic cluster properties. We generate clusters using a simplistic -2 power-law distribution for either the number of stars per cluster (N{sub *}) or cluster mass (M{sub cl}). These clusters are then populated with stars randomly chosen from a Kroupa initial mass function (IMF). We find that simulations with cluster lower-mass limits of M{sub cl,lo} {>=} 20 M{sub sun} and N{sub *,lo} {>=} 40 match best with observations of SMC and Galactic OB star populations. We examine the mass ratio of the second most massive and most massive stars m{sub max,2}/m{sub max}, finding that our observations all exist below the 20th percentile of our simulated clusters. However, all of our observed clusters lie within the parameter space spanned by the simulated clusters, although some are in the lowest 5th percentile frequency. These results suggest that clusters are built stochastically by randomly sampling stars from a universal IMF with a fixed stellar upper-mass limit. In particular, we see no evidence to suggest an m{sub max}-M{sub cl} relation. Our results may be more consistent with core accretion models of star formation than with competitive accretion models, and they are inconsistent with the proposed steepening of the integrated galactic stellar initial mass function (IGIMF).
- OSTI ID:
- 21476676
- Journal Information:
- Astrophysical Journal, Vol. 725, Issue 2; Other Information: DOI: 10.1088/0004-637X/725/2/1886; ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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