Jason is interested in a wide range of scientific problems, from improving our understanding of how stars evolve and change as they age, to studying the structure of galaxies to infer their formation mechanisms.  His research program takes advantage of many professional telescopes on Earth and in space, including the Canada-France-Hawaii telescope, Subaru, Gemini, Keck, MMT, Kitt Peak National Observatory, Hubble, GALEX, and XMM.  Jason has published more than 70 papers in peer-reviewed journals.  His research program is supported both through NASA grants and a multi-year National Science Foundation grant at the Johns Hopkins University (2013-2016).

Jason's professional CV is available here.

The Initial-Final Mass Relation

Stars spend most of their lifetimes in quiescent phases.  Most stars burn hydrogen for many billions of years, and then cool as white dwarfs for many billions of years.  However, between these two phases a stars life cycle is very dramatic.  It swells up and reaches a very high luminosity, and rapidly loses a large fraction of its mass.  Jason's primary research aims to understand this phase of stellar evolution, by directly linking the properties of stars that are at the end of their evolution to those that are still burning hydrogen.  This, the initial-final mass relation, is one of the most important astrophysical relations and impacts many interesting scientific questions.

Jason's continuing work in this field involves several steps, from imaging star clusters and their white dwarf populations, to measuring spectroscopic parameters for the white dwarfs (e.g., their masses), to establishing the connection between the mass of the star that is presently evolving in the star cluster to the white dwarf mass.  The difference between these two values informs the amount of mass that is lost through stellar evolution.  Over the past decade, Jason's work on the initial-final mass relation has provided numerous new discoveries, including the only measurements of the relation at the low mass end.  For example, his work suggests that our own Sun will lose 45% of its mass through stellar evolution (not for several billion years, however).  Jason's relation has been cited by other astronomers more than 100 times, for studies ranging from chemical evolution in galaxies, enrichment of the interstellar medium (and therefore the efficiency of star formation), the birth rates of neutron stars and type II supernovae, the ages of stars, and the characterization of exoplanet host stars.

Star Clusters as Tools to Study Stellar Evolution

Star clusters in the Milky Way have served as the most important laboratory for our understanding of the theory of stellar evolution.  The constituent stars in a cluster share incredible similarities, they all have the same age and chemical composition, and are located at the same distance from us.  The primary difference among the "family" of stars in any given cluster are their masses, and mass is the single most important parameter that influences stellar evolution. 

Jason is leading a large survey of rich star clusters with the Canada-France-Hawaii Telescope.  The resulting photometry from this survey is the deepest ever obtained for these stellar systems, and provides a unique testbed for the theories of stellar evolution.  The target sample includes clusters over a range of age and metallicity, each one therefore provides a snapshot of how stellar evolution has shaped that population at fixed a input.

In addition to ground-based telescope studies, Jason has worked with Harvey Richer to characterize the complete stellar populations in the oldest star clusters of the Milky Way, the globulars, with the Hubble Space Telescope.  Their team's observations are among the deepest images ever taken in astronomy, and provide a census of all of the hydrogen-burning stars and the remnant stars of previous generation hydrogen-burning stars (i.e., the white dwarfs).  As these white dwarfs contain no nuclear fuel, they cool simply as time passes.  The team uses these remnants as cosmic clocks to establish ultra-precise ages for the star clusters.  The Hubble observations also provide a wealth of information on the dynamics of star clusters, the hydrogen-burning limit, the stellar mass function, and much more.

The Initial Mass Function of Stars

The distribution of stellar masses that follows from the process of star formation is one of the most sought after astrophysical quantities.  This "initial mass function" is a key input to derive the total mass budget of the Milky Way galaxy, to determine how much nucleosynthesis occurs from the mass that is lost through stellar evolution, and aids in our general interpretation of light from distant galaxies.  Most studies of the initial mass function are based on counting stars either in the nearby Milky Way or in star clusters, and then using stellar models to convert photometry to individual masses.  In a recent paper, Jason showed that studying the initial mass function in dwarf galaxies offers several advantages over other approaches.  Through ultra-deep Hubble Space Telescope observations, his work demonstrates that the initial mass function is a power law form with exponent -1.90.  

The Halos of the Milky Way and Andromeda Galaxies

Jason's recent research has provided important insights on the formation epoch of our own Milky Way galaxy.  The galaxy contains a central bulge, a spiral disk where the Sun is located, and a vast halo of old stars extending out to 100's of light years.  This halo is believed to contain the Galaxy's pristine population of stars, however it is difficult to determine precisely when it formed.  Most techniques that astronomers have developed to age-date stars do not work well for low mass objects (i.e., those with mass less than the Sun), because the observed properties of these stars (e.g., their luminosities and colors) do not change appreciably with age.  In a recent paper, published in the journal Nature, Jason developed a new technique to age date the stellar halo of our own Galaxy by using dead stars, white dwarfs.  These are the remnants of low mass hydrogen-burning stars, and Jason's work uses them as a proxy to set the mass (and therefore age) of stars that are evolving in the halo today.   This research demonstrates that the inner halo of our galaxy, at approximately the same galactocentric distance as our Sun, is 11.5 billion years old.  This result indicates that this part of the stellar halo is younger than the oldest star clusters in the Milky Way.  The remote outer parts of the Milky Way halo may be several billion years older, and Jason hopes to measure that directly in the near future.  

In addition to his work on our own Milky Way galaxy, Jason is a part of the SPLASH collaboration, a large international imaging and spectroscopic survey of the Andromeda Spiral galaxy.  The "Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo" (or SPLASH) is led by Raja Guhathakurta at the University of California at Santa Cruz.  The Andromeda Spiral galaxy represents the nearest large, spiral system that is similar to the Milky Way. Given our vantage point, studies of Andromeda are in many ways superior to those in the Milky Way for contributing to our understanding of the processes that shape the formation of galaxies and their evolution. Recently, the SPLASH team published a series of papers addressing the detailed properties of Andromeda, as measured using thousands of spectroscopically confirmed individual stars.  Jason and his collaborators showed, for the first time, that Andromeda does contain an extended, metal-poor, sparse stellar halo, and that this halo dominates the more metal-rich inner spheroid of the galaxy at a distance beyond ~100 light years. Jason's collaborators are now improving our detailed understanding of Andromeda, and using these powerful observations to confront modern day theoretical models and simulations of galaxy formation in exquisite detail.

Dark Matter

In the currently accepted picture of galaxy formation, small "dwarf" galaxies are the basic building blocks of large galaxies.  These dwarfs are gravitationally pulled in by more massive galaxies, and then their stars are shredded and accreted from the smaller system into the larger system.  However, some of the dwarfs survive this process.  Jason has led studies with the Keck telescope to establish the properties of these surviving satellites, such as their chemical composition and masses.  His work takes advantage of a technique called "Multi Object Spectroscopy", where over a hundred stars in a single system can be spectroscopically observed simultaneously using a "mask".  This mask is simply a sheet of metal mounted to the telescope's camera,  and contains tiny slit lets milled into it according to the sky positions of the target stars.  Jason's work is the first to establish the detailed properties of the family of such dwarfs that belong to the Andromeda Spiral galaxy.  The motions of individual stars in these dwarfs suggest that there is a large amount of missing mass (i.e., "dark matter) among the stellar distribution, and the nature of this matter can be tested by analyzing the velocity signatures in detail.  Additionally, a comparison of these satellites, which belong to Andromeda, with those that belong to the Milky Way, provides important clues on how the host galaxy environment can shape its dwarf galaxy population.