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A minimum column density of 1 g cm-2 for massive star formation

Nature volume 451, pages 1082–1084 (2008)Cite this article

Abstract

Massive stars are very rare, but their extreme luminosities make them both the only type of young star we can observe in distant galaxies and the dominant energy sources in the Universe today. They form rarely because efficient radiative cooling keeps most star-forming gas clouds close to isothermal as they collapse, and this favours fragmentation into stars of one solar mass or lower1,2,3. Heating of a cloud by accreting low-mass stars within it can prevent fragmentation and allow formation of massive stars4,5, but the necessary properties for a cloud to form massive stars—and therefore where massive stars form in a galaxy—have not yet been determined. Here we show that only clouds with column densities of at least 1 g cm-2 can avoid fragmentation and form massive stars. This threshold, and the environmental variation of the stellar initial mass function that it implies, naturally explain the characteristic column densities associated with massive star clusters6,7,8,9 and the difference between the radial profiles of Hα and ultraviolet emission in galactic disks10,11. The existence of a threshold also implies that the initial mass function should show detectable variation with environment within the Galaxy, that the characteristic column densities of clusters containing massive stars should vary between galaxies, and that star formation rates in some galactic environments may have been systematically underestimated.

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Figure 1: Critical and equilibrium light-to-mass ratios versus cloud column density.
Figure 2: Energy per unit mass radiated and formation time versus protostellar mass.
Figure 3: Threshold column density versus stellar mass.

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Acknowledgements

We acknowledge S. Boissier, I. Bonnell, B. Elmegreen, E. Feigelson and C. Martin for discussions. We thank A. Urban, N. Evans and S. Doty for providing a copy of their grain–gas coupling code. This work was supported by NASA through the Hubble Fellowship program and by the NSF. Parts of this work were performed while the authors were in residence at the Kavli Institute for Theoretical Physics at UCSB.

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Authors and Affiliations

  1. Astrophysics Department, Princeton University, Princeton, New Jersey 08544, USA,

    Mark R. Krumholz

  2. Astrophysics Department, University of California Santa Cruz, Santa Cruz, California 95064, USA,

    Mark R. Krumholz

  3. Physics and Astronomy Departments, University of California Berkeley, Berkeley, California 94720, USA,

    Christopher F. McKee

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Correspondence to Mark R. Krumholz.

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Supplementary Information

The file contains Supplementary Discussion in which several of the results stated in the main text are derived in more detail and Supplementary Figure 1 with Legend. (PDF 279 kb)

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Krumholz, M., McKee, C. A minimum column density of 1 g cm-2 for massive star formation. Nature 451, 1082–1084 (2008). https://doi.org/10.1038/nature06620

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