Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011aas...21741602g&link_type=abstract
American Astronomical Society, AAS Meeting #217, #416.02; Bulletin of the American Astronomical Society, Vol. 43, 2011
Astronomy and Astrophysics
Astronomy
Scientific paper
The 'really' massive stars (those with more than 15 to 20 solar masses) likely start to ionize their surroundings before they reach their final mass. How can they accrete in spite of the presence of over-pressurized gas? We present results of Submillimeter Array (SMA) and Very Large Array (VLA) studies of massive-star formation regions in the early stages of ionization: Molecular-line observations at resolutions from a few arcsec to 0.3 arcsec reveal the presence of rotation, infall, and/or outflow from parsec scales to <0.05 pc. Small groups of massive (proto)stars are found at different evolutionary stages. The hypercompact HII regions with positive cm-to-mm spectral indices tend to be the smallest and most embedded, and they appear to have density gradients and/or clumpiness within the ionized gas. In the innermost few thousand AU, the ionized gas is sometimes found to have organized motions, probably in the form of outflow and/or rotation. Multiepoch observations of the free-free continuum reveal significant flux variations in timescales of years, attributable to interactions with the surrounding molecular gas. These observations, as well as recent models and numerical simulations of HII region evolution in star-forming accretion flows, favor a picture in which: i) Stars with M > 15 Msun form by accretion processes similar to those of lower-mass stars, but with significant ionization. ii) The masses of the cores from which these stars form are set until relatively late times in the evolution of the cluster. iii) Accretion can continue past the onset of an HII region. iv) The HII region is kept partially confined by its own molecular accretion flow for a period of time.
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