The Detailed Chemical Properties of M31
We have applied our new method for obtaining detailed element abundances to obtain the first abundances for 5 globular clusters in M31. These 5 clusters are the first in an ongoing project to study the stellar populations and star formation history in M31. We have obtained high resolution (R~25,000) spectra with the HIRES spectrograph on the Keck I telescope, and have measured equivalent widths of over 200 resolved spectral lines in each globular cluster. Thus far, we have measured abundances of 14 elements in old stellar populations in M31: Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Y and Ba. We use our new method to put constraints on the ages of the globular clusters, as well as reddening toward M31, and use the high resolution spectra to measure internal velocity dispersions of the clusters. We find our first sample of globular clusters in M31 to be similar to typical Milky Way globular clusters in many respects (e.g. [Fe/H], [alpha/Fe], Fe-peak and neutron-capture abundances).
Our ongoing work with M31 globular clusters is focusing on extending the sample to cover the available range in properties of the M31 globular cluster system, including areas in which the M31 cluster system is thought to be different from the Milky Way globular cluster system. We have now extended our pilot sample of 5 globular clusters to a sample of 25 clusters.
Chemical Abundances in the LMC with High Resolution, Integrated Light Spectra
We have measured detailed chemical abundances of 22 elements in 8 resolved clusters in the Large Magellanic Cloud (LMC). These clusters have ages of 0.05 to 12 Gyr, which provides a comprehensive picture of the chemical enrichment and star formation history of the LMC. The abundances were obtained using our original technique for the analysis of high resolution, integrated light spectra of star clusters. This LMC sample was used to demonstrate the accuracy (+/- 0.1 dex in [X/Fe]) of our new method on clusters with ages younger than ~10 Gyr. We find a spread of [alpha/Fe] at constant [Fe/H], as well as evolution of [alpha/Fe] with [Fe/H] and age. Fe-peak abundance ratios are similar to those in the Milky Way, with the exception of [Cu/Fe] and [Mn/Fe], which are sub-solar at high metallicities. The heavy elements Ba, La, Nd, Sm, and Eu are significantly enhanced in the youngest clusters. Also, the heavy to light s-process ratio is elevated relative to the Milky Way ([Ba/Y] >+0.5) and increases with decreasing age, indicating a strong contribution of low-metallicity AGB star ejecta to the inter-stellar medium throughout the later history of the LMC.
Integrated Light Spectra of
PhD Thesis Advisor : Dr. Rebecca Bernstein, UCO/Lick Observatory, University of California, Santa Cruz
We are studying the properties of unresolved extragalactic globular clusters using high resolution (R > 25,000) spectra of their integrated light. We have a new method for obtaining the first detailed chemical abundances for unresolved clusters, which makes it possible to determine detailed abundances beyond the Milky Way and its closest satellites for the first time. Detailed abundances in extragalactic clusters can help constrain galaxy formation theories by providing information on star formation rates and durations, chemical enrichment history, and supernovae yields for galaxies beyond the Milky Way.
The high signal-to-noise ratio, high resolution spectra we obtain for abundance analyses also contains a wealth of other information about globular cluster stellar populations. We are able to measure internal velocity dispersions of extragalactic globular clusters to high precision, providing constraints on their total masses and mass-to-light ratios. Additionally, we are investigating methods for constraining horizontal branch morphology of extragalactic clusters using their integrated Balmer line profiles. In conjunction with the ages and metallicities we determine with our abundance analysis, we can also use the observed and predicted integrated colors of globular clusters to probe the reddening in galaxies.
This new method has been developed and refined on a "training set" of globular clusters in the Milky Way and Large Magellanic Cloud with known properties. Details can be found in the following:
Colucci, J. E. & Bernstein, R. A., 2012, ApJ, 749, 124
Colucci, J. E. , Bernstein, R. A., Cameron, S., & McWilliam, A. 2012, ApJ, 746, 29
Colucci, J. E. , Bernstein, R. A., Cameron, S., & McWilliam, A. 2011, ApJ, 735, 55
Colucci, J. E. , Bernstein, R. A., Cameron, S., McWilliam, A. & Cohen, J.G. 2009, ApJ, 704, 385
McWilliam, A. & Bernstein, R. A. 2008, ApJ, 684, 326
Measurements of the Submillimeter-wave Atmospheric Transmission at South Pole
Undergraduate Honors Thesis Advisor : Dr. Giles Novak, Northwestern University, Evanston, IL
An analysis of atmospheric transmission at 450 microns at the South Pole using data from the Submillimeter Polarimeter for Antarctic Remote Observations (SPARO). (Unpublished.)