Thesis Topic

Thesis Topic: The dynamics and characterization of Infrared Dark Clouds, the precursers of massive star and star cluster formation.

Advisor: Jonathan Tan

Recent Abstracts


Stars To Planets Meeting, Gainesville, Florida, USA, April 2007

Kinematics of IRDCs

Massive stars and star clusters are important throughout astrophysics due to their effects on many systems, from galaxy evolution to planetary formation. Galactic massive stars and star clusters form from the densest gas clumps within Giant Molecular Clouds. These clumps reveal themselves by IR extinction and are known as Infrared Dark Clouds (IRDCs). The McKee \& Tan (2003) and Tan et al. (2006) models for massive star and star cluster formation assume that IRDCs are near virial equilibrium. To test this, we use 13CO Galactic Ring Survey (GRS) data to determine kinematic distances and dynamical masses of a sample of 9 IRDCs and their embedded cores. First we employ a simple virial relation, ignoring external pressure, to derive cloud masses and compare those with the extinction mass results of Butler et al. (2007). We find that the cloud masses derived from the average 13CO line width are within a factor of about three of the extinction masses. Cloud masses derived from the dispersion of the peak velocities of embedded cores are much smaller. This may be explained by the spatial concentration of dense cores in the centers of the clouds or by optical depth effects in the 13CO spectra so that they fail to accurately trace the dense core velocity structure. We have also derived pressure-bounded virial masses of cores by assuming the density and pressure are power-laws in radius, and the external pressure is related to the mass surface density of the surrounding cloud material. These core masses are also compared with the core extinction masses of Butler et al. (2007).

Massive Star Formation Meeting, Heidelberg, Germany, Sept. 2007

Dynamics of IRDCs

Massive stars and star clusters are thought to form from the densest gas clumps within giant molecular clouds. These reveal themselves by absorption of the diffuse Galactic infrared background, appearing as Infrared Dark Clouds (IRDCs). Studies of IRDCs thus provide information on the initial conditions of massive star and star cluster formation. We study IRDC kinematics using the Galactic Ring Survey of 13CO(1-0) (Jackson et al. 2006). Here we first present properties of a small (nine) cloud sample, including maps of 8 micron intensity, 13CO-derived surface density, average line of sight velocity and momentum. In the dense central regions of the clouds, masses derived from 13CO are about a factor of 3 smaller than those derived from extinction, perhaps because of optical depth effects. We study the radial profiles of enclosed mass, projected rotation axis position angle and ratio of rotational to gravitational energy, beta. We also study a larger sample of 285 IRDCs identified by Simon et al. (2006).  We measure the projected rotation axis position angle for each cloud at various radii and compare our results with those of Galactic GMCs (Phillips 1999) and M33 GMCs (Rosolowsky et al. 2003). We also measure beta for these clouds. We find a significant number of clouds rotating in a retrograde sense to the Galactic rotation, perhaps indicating that IRDC formation is driven by turbulent or cloud collision velocities that have been decoupled from Galactic shear, as in the model of Gammie et al. (1991) and Tan (2000).