Kristen Garofali

My research interests lie in probing massive star and binary evolution using resolved X-ray source populations in nearby galaxies. In particular, I study the high-mass X-ray binary (HMXB) and supernova remnant (SNR) populations in nearby galaxies (latest SNR paper and HMXB paper) using a combination of Chandra, XMM-Newton, and the Hubble Space Telescope (HST) in order to learn more about the life and death cycles of massive stars, the effects of binaries on massive star evolution, and the influence of host galaxy properties such as metallicity, star formation history (SFH), and star formation rate on the observed populations. Massive stars are key drivers in the cycle of star formation, feedback, galactic chemical evolution, and compact object formation so understanding their evolution, in particular the end stages of their lifetimes and the effects of binarity, is of great importance to a broad range of astrophysics. Below I summarize some of my recent work on the HMXB population in M33.

This animation first displays the deep Chandra coverage available for M33 from Tullmann et al. (2011), and next the Local Group Galaxy Survey (Massey et al. 2006) of M33 overlaid with the archival HST fields pertinent to this work (magenta). Cyan crosses are X-ray sources from Tullmann et al. (2011) that fall within the current archival HST coverage, and the white crosses are X- ray sources contained in the upcoming M33 HST legacy survey (white regions; PI: J. Dalcanton). There are 286 X-ray sources covered by 91 HST fields, which allows us to identify optical counterparts for a large number of X-ray sources. The next two frames are zoom-ins on one portion of an RGB (F814W,F606W, blue equivalent) rendered HST ACS field from the disk of M33 with the X-ray error circle (0.7") shown in white, and the position of the potential optical counterpart (bright, blue star indicated with white arrow) to an X-ray source; the optical counterpart in this case makes this source a good candidate HMXB.


HMXBs provide an exciting window into the impact of mass transfer on massive star evolution, and can be used as fossil records to unlock information about the lives and deaths of their massive progenitors. They are also likely progenitors of gamma-ray bursts and gravitational wave events, and may be an important source of feedback in the early universe. My recent work has focused on the identification and characterization of the HMXB population in M33. After careful astrometric alignment, we leverage archival data from both Chandra and the Hubble Space Telescope to identify HMXB candidates and their potential optical counterparts (above gif). We can then measure ages for these candidate HMXBs using the surrounding stellar population as a clock. Modeling the color-magnitude diagram of the stellar population in the vicinity of a young source, such as an HMXB, yields a star formation history (SFH) for that region and thus that young source. This SFH can be viewed as a probability distribution function for the age of the source, where the bin with the highest star formation rate denotes one measure of the most likely age for the source. An age measurement such as this is useful as it also yields information about progenitor mass and energetics, unlocking information about the evolution and history of a given source. The distribution of measured ages for a population of HMXBs puts new constraints on prescriptions used by binary population synthesis models which attempt to map the characteristics of the parent population to the resulting HMXBs. Below is an example of the HMXB age recovery method for M33 X-7, and known eclipsing X-ray binary hosting a 90 solar mass star in a 3.5 day orbit around a 15 solar mass black hole. We recover an age of < 7 Myr for this system, in agreement with the systems characteristics, and models of binary evolution.

Left: Cumulative star formation history for stars younger than 100 Myr from CMD fitting using MATCH (Dolphin 2002), with the most likely age denoted with the dashed lines (errors in red), and the Monte Carlo derived errors on the cumulative star formation history in grey. Right: The SFH for the 50 pc region surrounding M33 X-7. The most likely age for this HMXB candidate is < 6 Myr. Inset is an image of the optical counterpart from HST (F439W), with the X-ray error circle in cyan.