by Robert Dolm-Palmer
Under the supervision of Professor Evan D. Skillman
I have explored the star formation histories of the dwarf irregular galaxies Sextans A and GR 8. I measured photometry of individual stars from images taken by the Wide Field and Planetary Camera 2 aboard the Hubble Space Telescope. With the photometry I constructed color-magnitude diagrams (CMDs) in the $B$, $V$, and $I$. I investigated the errors in the photometry extraction, and conducted artificial star tests to measure the photometric limits. The high resolution of the Hubble Space Telescope allowed photometric measurements that were far more accurate than ground-based observations. For galaxies at these distances (1-2 Mpc), the accuracy of stellar photometry from ground-based observations is limited by crowding of stellar images.
The high accuracy photometry showed a clear separation of the main sequence from the massive, blue, core He-burning stars (HeB). These are stars in the bluest extent of the so-called ``blue-loop'' phase of stellar evolution. This is the first time this phase of evolution has been clearly identified in a low metallicity system. The distributions of stars in the CMDs agreed very well with stellar evolution model predictions.
I have used the CMDs to calculate the recent star formation histories of both galaxies. The main sequence luminosity function provided the star formation rate (SFR) over the past $\sim 50$ Myr. I developed a new technique for calculating the SFR from the blue HeB luminosity function. Furthermore, the blue HeB evolutionary phase has a one-to-one relation between age and magnitude. This allowed me to calculate the position, as well as the strength of star formation over the past $\sim 500$ Myr.
The star formation was found in concentrated regions. These regions are
of order 100 pc across and last of order 100 Myr. The regions were found
near the highest density HI gas. I estimated the gas-to-star conversion
efficiency to be 5-10\%. The results from GR 8 suggest that the star forming
gas clouds may be self-gravitating, and that each cloud can support several
star forming episodes. There is an age progression among the star forming
regions which suggests that the star formation is propagating through the
by Robert Dolm-Palmer
Under the supervision of Professor Tom Jones
- We investigate the dynamics of young supernova remnants in the presence of a non-uniform density such as might be present at the edge of a molecular cloud. Using a cylindrical, two dimensional, TVD, hydrodynamic code we perform three high resolution (1024 zones in r, 2048 zones in z) simulations of explosions in the vicinity of a smooth transition from a high density region to a low density region, and a comparison simulation in a uniform density. These calculations improve upon past calculations by including the inertia and kinetic energy of the stellar ejecta. We find that the presence of the density transition has important dynamical effects on the remnant. When the remnant is smaller that the density transition width, the apparent center shifts toward the low density region while maintaining a nearly circular shape. As the remnant size exceeds the density transition width, it expands as distinct lobes into the high and low density regions. Each lob approach the self-similar Sedov-Taylor expansion rate appropriate for the surrounding gas density. The non-uniform density also creates a pressure imbalance in the interior of the SNR that drives gas flow from the high density side to the low density side. This sets up asymmetric, non-radial flows in the remnant interior.