On the origin of turbulence in Giant Molecular Clouds in the Milky Way
Norman Murray (CITA) - September 12, 2016 at 12:10 pm
The first detection of CO line emission in space by Wilson, Jefferts, and Penzias revealed a line width of 6.2 km/s, far broader than the thermal line width. A decade later, Larson noticed that the line width varied with the size of the emitting region, interpreting his result in the context of a turbulent cascade. This is still the accepted interpretation, which raises a puzzle. Turbulence is believed to decay on an eddy turnover time, about 5-10 Myrs in Milky Way GMCs, so that a source of energy to power the steady state turbulence would seem to be required. Two types of energy sources have been proposed, gravitational potential energy, or nuclear energy from stars. Gravitational sources can be further broken down to accretion of gas from the Milky Way's halo onto the gas disk, accretion through the gas disk (triggered either by the magneto-rotational stability or by gravitational instability), accretion of atomic gas onto GMCs, contraction of GMCs, or proto-stellar jets and their associated outflows. Nuclear energy can be expressed in the form of O star winds, ionizing radiation, radiation pressure, or supernovae.
Using a new GMC catalog (Mivilles-Deschenes, Murray, and Lee 2016), we show that in the outer disk (beyond the solar circle) the turbulence in CO emitting gas can be powered by accretion through the disk, or by accretion onto GMCs. But the dissipation rate in inner Galaxy GMCs is factor of 100 larger than that in the outer disk. We show that most of the GMCs lack sufficient young stellar mass to provide this much energy, and that the accretion rate through the disk or onto the GMCs cannot supply enough power either. It appears that the turbulence is powered by contraction of the GMCs.
The seminar will be held in 131A Campbell Hall.
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