![[GC age image]](./hist_FINAL.png)
My research is in the general area of stellar evolution and galaxy formation. I'm particulary interested in the ages of globular clusters (see a picture of their locations ) and the formation of the Milky Way.
A paper I wrote determined the age, extinction and distance of a very old, metal-rich open cluster, NGC 6791. This cluster may be used as a template for understanding the evolution of old, metal-rich stellar systems found in the bulges of spirals and in elliptical galaxies. The isochrones I used in this paper are available for public use, along with further details of this work.
To determine the best estimate for the age of the universe, I performed Monte Carlo study in which the various uncertainties in the age determination process were taken into account. This involved the production of over 4 million stellar models and a detailed study of the globular cluster distance scale. The results are summarized in the histogram below, which shows my best estimate for the mean age of the oldest globular clusters, 12.6 Gyr, with a one-sided 95% confidence lower limit of 10.4 Gyr. This paper (PDF) has been published in Science .
The discovery of Jupiter sized planets around other stars naturally leads one to wonder how common terrestial planets are in the Milky Way. Current search techniques cannot find low mass objects like the Earth around other stars, so one must look for indirect evidence for terrestial planets around other stars. In our work , we argue that the formation of the solar system likely led to ~1/2 an Earth mass of iron being accreted onto the Sun after its convection zone is thin. Combining stellar evolution calculations with existing observational databases of solar type stars, I will show that there is a correlation between stellar mass and iron abundance which is best explained by a model in which nearly all stars are polluted by ~1/2 an Earth mass of iron falling onto their thin convection zones. These findings suggest that terrestrial-type material is common around solar type stars.
Our study of stars with radial velocity planets confirm recent work indicating that the stars-with-planet sample as a whole is iron rich. However, the lowest mass stars tend to be iron poor, with several having [Fe/H] <-0.2, demonstrating that high metallicity is not required for the formation of short period Jupiter-mass planets. We show that the average [Fe/H] increases with increasing stellar mass (for masses below 1.25 solar masses) in both samples, but that the increase is much more rapid in the stars-with-planet sample. We use Monte Carlo models to show that adding an average of 6.5 earth masses of iron to each star can explain both the mass-metallicity and the age-metallicity relations of the stars-with-planets sample. However, for at least one star, HD 38529, there is good evidence that the bulk metallicity is high. We conclude that the observed metallicities and metallicity trends are the result of the interaction of three effects; accretion of ~ 6 earth mass of iron rich material, selection effects, and in some cases, high intrinsic metallicity.