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The universe's most massive stars can form in near isolation, new study finds

New observations by University of Michigan astronomers add weight to the theory that the most massive stars in the universe could form essentially anywhere, including in near isolation; they don't need a large stellar cluster nursery.
This is the most detailed observational study to date of massive stars that appear (from the ground) to be alone. The scientists used the Hubble Space Telescope to zoom in on eight of these giants, which range from 20 to 150 times as massive as the Sun. The stars they looked at are in the Small Magellanic Cloud, a dwarf galaxy that's one of the Milky Way's nearest neighbours.
Their results, published in the Dec. 20 edition of the Astrophysical Journal, show that five of the stars had no neighbours large enough for Hubble to discern. The remaining three appeared to be in tiny clusters of ten or fewer stars.
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Blobrana


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Title: The Sparsest Clusters With O Stars
Authors: J. B. Lamb, M. S. Oey, J. K. Werk, L. D. Ingleby

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 SMC using HST'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 <10 companion stars revealed, having masses of 1-4 solar mass. 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_*) or cluster mass (M_cl). These clusters are then populated with stars randomly chosen from a Kroupa IMF. We find that simulations with cluster lower-mass limits of M_cl,lo >20 solar mass and N_*,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_max,2/m_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 a m_max - M_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 galaxy IMF (IGIMF).

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