I am an associate research scientist at the Center for Astrophysical Sciences at Johns Hopkins University. Before coming here I had a 2-year postdoctoral position at the Universidad de Concepcion in Chile working with Douglas Geisler. I earned my Bachelors of Science in Physics & Astronomy at the University of Rochester and my Ph.D. in Astronomy at Indiana University Bloomington working with Constantine Deliyannis.

My research interests have focused on studying the fundamental characteristics of stars and their evolution, primarily through using star clusters as a tool. I am the Co-PI of a recently completed NSF funded project to use white dwarfs to study stellar evolution and the mass at which stars begin to undergo core-collapse supernovae. I am also the PI of a current HST project to analyze light element abundances in cluster stars to trace the effects of stellar rotation on their interiors.

Tracing the Evolution of Stars

White Dwarfs are the final stage of stellar evolution for 98% of stars in the universe. They are the hot and compact cinders that remain of a star's core after they expel their outer shells. White dwarfs are valuable tracers of our Galaxy's past, but they also provide direct constraint on the evolution of a star's core during its life.

NGC 6397 White Dwarfs

Light Element Abundances (lithium, beryllium, and boron) are valuable tracers of surface convection and other mixing processes in a star's interior. This is because each of these elements are fragile and are destroyed in the hot and dense stellar interiors. Analyzing how each element's abundance at a star's surface changes over time constrains how much mixing occurs between the surface and differing layers of their interiors.

Stellar Mixing
Credit: www.sun.org, released under CC-BY-SA 3.0

Using Star Clusters as a Tool

Star clusters are one of the most powerful tools for analyzing how stars form, evolve, and die. They provide a dense environment with a rich sample of stars that formed together from the same material yet span a broad range of masses.

Cluster Parameter Analysis is crucial for giving context for the characteristics of the stars that compose each cluster. I have worked to continue development and refinement of cluster analysis tools and methods for selecting out peculiar stars and measuring cluster age, composition, distance, reddening, etc., which are still foundational pieces of using star clusters as a tool. Below, photometric analysis of NGC 2516 selects turnoff stars with normal ultravoilet flux (red) and selects out peculiar stars, including Be stars (cyan) and blue stragglers (blue).

NGC 2516 Cluter Parameter Fits

The Initial-Final Mass Relation is a direct comparison of the initial mass that a star forms with to its final mass as a white dwarf. This initial mass can be determined by analyzing white dwarfs in star clusters and using the cluster parameters to calculate each white dwarf's progenitor evolutionary timescale, and hence initial mass. This both constrains a star's core evolution and mass loss across a wide range of initial masses. Note the offset of the model and observed mean trend. The data scatter is also larger than can be explained by the observational errors.


Stellar Rotation and its broad effects on a star's evolution and observable characteristics remain poorly constrained. For example, rotation both extends a star's evolutionary lifetime and changes its color and luminosity. The depletion of light elements, which are depleted further by the rotational mixing processes, provide valuable constraints of rotation. Additionally, the initial-final mass relation gives a novel approach to constraining the effects of rotational mixing on a star's core-mass evolution, which becomes the resulting white dwarf. We have created a synthetic IFMR (shown below in red) that produces higher-mass white dwarfs using the ATON models (Ventura+ 2018) with more convective-core overshoot and an intrinsic scatter due to different progenitor rotation rates (Choi+ 2016).

Synthetic IFMR

Contact Information

3400 N. Charles Street
Baltimore, MD 21218
United States
Office: Bloomberg 519
Email: jcummi19@jhu.edu