Research

My advisor at UW Madison, Prof. Robert Mathieu, has been part of the ongoing collaboration the WIYN Open Cluster Study since its inception over 20 years ago. The goal has been to provide a catalog of photometry, astrometry, and spectroscopy for a selection of open cluster spanning a range in age and metallicity and derive quality kinematic memberships for these clusters, including their binary populations. My graduate work included a WOCS membership study of the young open cluster M35 that included orbital solutions for the binary population (Leiner et al. 2015), and many nights obtaining spectra on the WIYN 3.5 meter telescope at Kitt Peak.

The bulk of my PhD was spent analyzing the non-standard stellar population of the star cluster M67, a well-studied 4 Gyr open cluster. Here is a color-magnitude diagram of the cluster that uses WOCS proper motions, WOCS radial velocities, and Gaia parallaxes to determine high quality memberships. More than 30 years of radial velocity data from WOCS and the CfA allows us to identify binaries in this cluster out to 10,000 day orbital periods.

M67 Color-magnitude diagram showing members of the cluster down to the WOCS limiting magnitude of V= 16.5. Populations of non-standard stars including blue stragglers, yellow stragglers, and sub-subgiant stars are all present. These stars are not simply field contaminants in the CMD. They areĀ  cluster members whose existence cannot be explained using standard single-star evolutionary models. Head here for an interactive version of this CMD.

 

Thanks to the long baseline of the WOCS RV observations, we know that 80% of the blue stragglers, yellow stragglers, and sub-subgiants in this cluster are binary systems, most in long period orbits of ~1000 days. Their origins likely lie in episodes of mass transfer, stellar collisions, or binary mergers, all evolutionary scenarios commonly experienced by binary stars.

My work has produced the first demographic study of sub-subgiant stars in open and globular clusters, as well as putting forth the first detailed formation models to explain their origins (Leiner, Geller, & Mathieu 2017, Geller et al. 2017a, Geller et al. 2017b). I have also conducted the first asteroseismic analysis of a yellow straggler in M67 (Leiner et al. 2016), demonstrating that it is an evolved blue straggler star that likely formed in a merger or collision.

Most recently, my work has been focused on the angular momentum evolution of post-mass-transfer stars, and the possibility of using rotation to identify and age-date recent mass-transfer products like the blue stragglers.