Friday, 20 February 2009

A role for self-gravity at multiple length scales in the process of star formation

Alyssa A. Goodman1,2, Erik W. Rosolowsky2,3, Michelle A. Borkin1,5, Jonathan B. Foster2, Michael Halle1,4, Jens Kauffmann1,2 & Jaime E. Pineda2

Nature 457, 63-66 (1 January 2009)

Fig: Observation and simulation of molecular cloud L1448. Most of the emission in the L1448 region is contained with large-scale self-gravitating structures, but only a low fraction of small-scale objects show signs of self-gravitation. In the L1448 observations, gravity is significant on all scales, but not in all regions. In contrast, the simulated map implies that nearly all scales, and all regions, should be influenced by gravity (which was ignored in the simulation).

Self-gravity plays a decisive role in the final stages of star formation, where dense cores (size 0.1 parsecs) inside molecular clouds collapse to form star-plus-disk systems1. But self-gravity's role at earlier times (and on larger length scales, such as 1 parsec) is unclear; some molecular cloud simulations that do not include self-gravity suggest that 'turbulent fragmentation' alone is sufficient to create a mass distribution of dense cores that resembles, and sets, the stellar initial mass function2. Here we report a 'dendrogram' (hierarchical tree-diagram) analysis that reveals that self-gravity plays a significant role over the full range of possible scales traced by 13CO observations in the L1448 molecular cloud, but not everywhere in the observed region. In particular, more than 90 per cent of the compact 'pre-stellar cores' traced by peaks of dust emission3 are projected on the sky within one of the dendrogram's self-gravitating 'leaves'. As these peaks mark the locations of already-forming stars, or of those probably about to form, a self-gravitating cocoon seems a critical condition for their existence. Turbulent fragmentation simulations without self-gravity—even of unmagnetized isothermal material—can yield mass and velocity power spectra very similar to what is observed in clouds like L1448. But a dendrogram of such a simulation4 shows that nearly all the gas in it (much more than in the observations) appears to be self-gravitating. A potentially significant role for gravity in 'non-self-gravitating' simulations suggests inconsistency in simulation assumptions and output, and that it is necessary to include self-gravity in any realistic simulation of the star-formation process on subparsec scales.

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