Northwestern University

Apr
4
Tue 4:00 PM

Norman Murray: On the Origin of Turbulence in Giant Molecular Clouds in the Milky Way

When: Tuesday, April 4, 2017
4:00 PM - 5:00 PM  

Where: Technological Institute, F160, 2145 Sheridan Road, Evanston, IL 60208 map it

Audience: Faculty/Staff - Student - Public - Post Docs/Docs - Graduate Students

Contact: Pamela Villalovoz   847.491.3644

Group: Physics and Astronomy Astrophysics Seminars

Category: Academic

More Info

Description:

Title: On the Origin of Turbulence in Giant Molecular Clouds in the Milky Way


Speaker: Norman Murray, CITA

Host: Alex Richings

Abstract: 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.

 

Keywords: Physics, Astronomy, Astrophysics

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