Sept 28

"Precursors to Stellar Clusters and their Evolution"
Cara Battersby, University of Colorado, Boulder

The light from massive stars dominates our picture of the universe around us, yet our understanding of the conditions under which massive stars and stellar clusters form and how they evolve is far from complete. I report on the physical properties of dense molecular gas clumps in our Galaxy, derived from Herschel, Spitzer, and the EVLA, and the transformation of these clumps into massive stars and stellar clusters. I present the structure, properties, and dynamics of a massive star-forming Infrared Dark Cloud embedded within a Massive Molecular Filament at high-resolution using NH3 on the EVLA and then expand this to a global view of massive star formation using Herschel and Spitzer. We derive temperatures and column densities from Herschel data using a unique method that separates the dense clumps from the diffuse ISM. Comparing these temperatures and densities with star formation tracers reveals that warmer, more diffuse gas traces active star formation while colder, denser gas remains in a pre- or embedded star-forming phase. We estimate the duration of evolutionary phases of massive star forming clumps from the fraction of Herschel sources exhibiting each star formation tracer. I will discuss the search for pre-cursors to stellar clusters in the Milky Way and what recent results of global dense gas kinematics in star-forming filaments tells us about the mode and duration of star formation in a clustered environment.

Oct 3

"The Nature of Interstellar Hydrocarbons: PAHs, HAC, and Fullerene"
Gregory C. Sloan, Cornell University

The discovery of the unidentified infrared (UIR) emission features in 1973 opened a long-unsettled debate. What is the carrier of the UIR features? Currently, agreement is widespread that they arise from polycyclic aromatic hydrocarbons (PAHs), and for good reasons. However, interstellar hydrocarbon grains may have a different composition when they are outside of the harsh radiative fields where UIR emission is usually observed. Shifts in the spectral structure of PAH-like spectra in cooler environments can be explained by an increase in the ratio of aliphatic/aromatic bonds. Aliphatic bonds are more fragile and the first to be destroyed as hydrocarbon grains are photo-processed. Their presence in cooler environments is consistent with the structure of hydrogenated amorphous carbon (HAC), which remains the leading alternative to the PAH model. Each model appears to be valid within the appropriate domain. Finally, fullerenes appear to be the last gasp of interstellar hydrocarbons before they are completely destroyed.

Nov 16

"Active Galactic Nuclei and their Hosts"
Jonathan Stern, Israel Institute of Technology (Technion)

We study the properties of low redshift broad line AGN, and their relation to their host galaxies, based on a new sample derived from the SDSS survey. The sample is supplemented by data from the GALEX, ROSAT, and 2MASS surveys. We find the following. The average AGN hosts are regular non emission line galaxies (NEG), which become bluer with increasing AGN luminosity, suggesting a correlation of the AGN luminosity and the host star formation rate. The observed AGN optical-UV emission is subject to some reddening, and the intrinsic emission is blue, consistent with accretion disk model predictions. The narrow emission lines reveal that the covering factor of circumnuclear gas (10s - 100s pc) decreases with increasing AGN luminosity, and the gas metallicity follows the host mass, similar to the mass - metallicity relation of normal galaxies. The metallicity of the broad line region (0.01s - 0.1s pc) also appears to be related to the host mass.