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R. C. Dohm-Palmer (University of Minnesota)
I have explored the star formation histories of the dwarf irregular galaxies Sextans A and GR 8. Photometry of individual stars in B, V, and I was measured from images taken with WFPC2 aboard HST. I investigated the errors in the photometry extraction, and conducted artificial star tests to measure the photometric limits. The high resolution of HST allowed photometric measurements that were far more accurate than previous ground-based observations.
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 used the CMDs to calculate the recent star formation histories of both galaxies. A new technique was developed for calculating the SFR from the blue HeB luminosity function. Since the blue HeB evolutionary phase has a nearly one-to-one relation between age and magnitude, both the position and strength of star formation over the past ~ 500 Myr could be calculated.
The star formation was found in concentrated regions of order 100 pc across which 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 galaxies.
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