Massive Star Formation in Sparse Environments.

[摘要] Sparse massive star formation provides a critical test for star formation theories.To investigate the extreme case, we probe the immediate stellar environment around eight seemingly isolated massive stars in the Small Magellanic Cloud using the Hubble Space Telescope;;s Advanced Camera for Surveys.Our observations reveal sparse clusters of 8-10 stars ranging between 1-4 solar masses around three of our targets, while three other stars remain candidates for in situ field massive star formation.Stochastic effects dominate in these sparse clusters, so we compare our observations to a simulated cluster population generated from a -2 power-law distribution in star count or cluster mass and populated randomly following a Kroupa IMF.Our results suggest that clusters are built stochastically by randomly sampling stars from a universal IMF with a fixed stellar upper-mass limit and are inconsistent with a fixed relationship between cluster mass and maximum stellar mass.To investigate field massive stars as a population, we carry out a comprehensive survey, the Runaways and Isolated O Type Star Spectroscopic Survey of the SMC (RIOTS4).RIOTS4 is a spatially complete census of the field massive star population of the SMC undertaken with the IMACS multi-object spectrograph and MIKE echelle spectrograph on the Magellan telescopes.We find the field IMF slope above 20 solar masses is Gamma=2.3, which is steeper than the canonical Salpeter slope of Gamma=1.35.We extend our IMF measurement to lower masses using photometry from the OGLE survey.We develop a statistical approach to generate a mass probability distribution for each star to measure the IMF with OGLE photometry, from which we again find Gamma=2.3 above 7 solar masses.We rule out the possibility that a unique star formation history or high binary fraction could make a Salpeter IMF consistent with the steep observed present day mass function.Interestingly, the steep field IMF does not support our earlier results, indicating that massive stars are depleted in the field.We may reconcile these findings if sparse O star clusters are remnants of larger clusters that rapidly dissolve or if the mechanism for sparse massive star formation is different from clustered formation.

[发布机构] University of Michigan

[效力级别] Stellar Populations

[关键词] Star Formation;Stellar Populations;Massive Stars;Field Stars;Magellanic Clouds;Astronomy;Science;Astronomy and Astrophysics