Andrew McWilliam
Andrew McWilliam's research focuses on nucleogenesis and galactic chemical evolution. His goal is to understand where and how the elements were produced by studying the composition of very old stars and stars born in different environments.
Stars produce most of the elements in the universe; supernovae (SNe) are particularly important sources. Theoretical models of SNe and other astrophysical enrichment events are very crude and require observational constraint. Stars of low mass are very long-lived and span the age of our galaxy. Such fossil stars can be used to trace the history of chemical evolution. In general, the oldest stars are the most metal-poor because fewer supernovae had occurred before the very early epoch when the stars formed.
Most stars contain the chemical signatures of many supernovae (over 100 million SNe have occurred in our galaxy), so the amplitude of abundance variations from individual SNe are usually insignificant. The abundance variations of individual SNe are more evident in stars composed of ejecta from only one or a few supernovae. These stars are very metal poor and very rare. They show large abundance variations, some with detectable amounts of the radioactive element thorium, which can be used to measure the age of the galaxy. In 1998 and 1999, there were two searches for extremely metal-poor stars. Staff Members George Preston, Ian Thompson, and McWilliam conducted the first search toward the galactic bulge; the second was done by McWilliam in metal-poor dwarf spheroidal galaxies (dSph). At present, stars with metal abundances near 10-3 those of the Sun are confirmed, and two candidates near abundances of 10-4 solar have been identified.
McWilliam and Director Emeritus Leonard Searle have produced a model of stochastic chemical evolution to understand the 300-fold dispersion of strontium abundance in extremely metal-poor stars. The model assumes a distribution of supernovae Sr/Fe ratios from the most metal-poor stars; the ratios are selected randomly from this distribution and then mixed. Figure 7 shows predicted and observed abundances; note the excellent fit. The unusual position of two stars with metal abundances near 10-4 solar constrain the model, and suggest that stars composed of material from individual supernovae have metal abundances near 10-3.2 of the Sun.
The compositions of other environments permit tests of the chemical evolution paradigm. To this end, McWilliam and Tammy Smecker-Hane of UCI used the Keck telescope to acquire spectra of red giant stars in the Sagittarius dSph. The results show a large spread in metallicity and unusual abundances of O, Mg, Si, Ca, Al, and Na. One explanation for this spread is that the Sagittarius dSph experienced an extended quiescent period of several billion years followed by a burst of star formation, with the composition of the metal-rich population coming from metal-poor (10-2 of solar) type Ia supernovae.
McWilliam acquired CTIO spectra of 70 galactic-bulge red giants, and 12 spectra using the Keck I with Mike Rich of UCLA. The scientists found unusual enhancements of O and Eu at solar metallicity. These elements are mostly made by type II SNe (with short progenitor lifetimes), which indicates a rapid formation timescale for the galactic bulge on the order of one billion years. It is remarkable that the bulge reached solar metallicity so quickly.
Fig. 7. The figure shows the observed abundances of Sr/Fe (crosses) compared with predictions. The region bounded by the dotted lines indicates the stars used to set the initial distribution in Sr/Fe ratio, while solid lines show the predicted evolution (5, 15, 85 and 95 percentiles) of Sr/Fe. Filled circles indicate the predicted evolution of the median Sr/Fe ratio.