Research

The Most Massive Stars

    I am currently studying massive (i.e. > 30 Mo) stars in young clusters in our Galaxy and in nearby galaxies. Specifically, I'm observing eclipsing binaries containing massive stars, for which one can determine their fundamental parameters (mass, radius, luminosity) accurately. The goal is to constrain stellar formation and evolution models and understand the progenitors of x-ray binaries, supernovae and gamma-ray bursts. I targeted the young massive Westerlund 1 super star cluster in search of massive eclipsing binaries and discovered 4, possibly 5, eclipsing binary systems that are cluster members. Read more here.

    One of the questions I'm trying to answer is what is the upper limit on the mass of a star? The most massive binary ever weighed accurately is WR 20a, located in the Westerlund 2 open cluster in Carina. WR 20a is a short period (3.7 days) eclipsing Wolf-Rayet system, with component masses of 82 Mo and 83 Mo, respectively. In May 2004, we obtained I-band photometry with the OGLE 1.3 m telescope at Las Campanas Observatory, confirming and refining the large masses of this system. Our paper was published in ApJL. Here is the press release issued by the CfA on May 26th, 2004. There were also articles written by Reuters, New Scientist magazine, the Boston Globe, Sky & Telescope (November 2004, p. 16) and the Harvard Gazette on WR 20a. Astronomy Now also wrote up a story in their December 2004 issue.

Hypervelocity Stars

    With collaborators Mercedes Lopez-Morales, Ian Hunter and Robert Ryans, we analyzed the spectrum of the hypervelocity star HE 0437-5439 and found its chemical abundance to be half-solar, establishing its origin in the Large Magellanic Cloud. Our result implies that a massive black hole exists somewhere in this galaxy! Check out the paper (published in the Astrophysical Journal Letters) and press release issued by Carnegie and Space.com. It even made it to Time Magazine!

Distances to Nearby Galaxies

    I completed my PhD thesis in June 2005. My thesis advisor was Kris Stanek at the CfA and the title of my thesis was "Determining Accurate Distances to Nearby Galaxies". The last paper in my thesis presents the first DIRECT distance to a detached eclipsing binary in M33. It is the result of 10 years of work, surveying M31 and M33 for detached eclipsing binaries and following up the brightest candidate to obtain the distance. You can read the paper here: astro-ph/0606279. The goal of the DIRECT Project is to use M33 and M31 to calibrate the Cepheid distance scale and replace the current anchor of the extragalactic distance scale, the Large Magellanic Cloud, with more robust anchor galaxies. We obtained BV photometry with the KPNO 2.1 m, JHK photometry with Gemini/NIRI and spectroscopy with the Keck 10 meter telescope and Gemini 8 meter telescope on Mauna Kea. We measured a distance to M33 15% longer than the currently accepted value, which has implications for the value of the Hubble constant. A press release was issued on August 3rd, 2006. CNN, New Scientist, Science Now and USA Today published articles on it.

    The first project involved searching for variable stars in a region of the spiral arms in the Andromeda Galaxy M31, by analyzing data taken by the DIRECT project with the 48" telescope at the FLWO on Mount Hopkins in Arizona. We found 264 variables in the 22x22 arcmin field: 41 eclipsing binaries (EBs), 126 Cepheids, and 97 other periodic or nonperiodic varibles, including several interesting objects (a LBV candidate, a nova and a galactic CV). The paper was published in the Astronomical Journal (in July 2003) and can be found here.

   Another project was re-analyzing some excellent VLT data of the spiral galaxy M83 with image subtraction. We found 112 Cepheids and ~60 other variables in the field and proposed a hybrid method for measuring distances to nearby galaxies. The paper was published in ApJL (July 2003) and can be found here. Full versions of the figures can be found here.

    Our paper "The RR Lyrae Distance to the Draco Dwarf Spheroidal Galaxy" was published in the February 2004 issue of The Astronomical Journal. It is the first CCD variability survey of this galaxy, since Baade & Swope's (1961) photographic survey. We found 163 variable stars, 146 of which were RR Lyrae: 123 RRab, 16 RRc, 6 RRd and one RR12. The other variables include a SX Phe star, four anomalous Cepheids and a field eclipsing binary. Using the short distance scale statistical parallax calibration of Gould & Popowski and 94 RRab stars from our field, we obtained a distance modulus of (m-M)_0=19.40 +/- 0.02 (stat) +/- 0.15 (syst) mag for Draco, corresponding to a distance of 75.7 kpc +/- 0.7 (stat) +/- 5.4 (syst) kpc. Data are available via this ftp URL and this http URL.

Undergraduate Research

   I did some research as an undergraduate at Wellesley College as well. I spent two summers (1997 & 1998) at Wellesley's Whitin Observatory working for Priscilla Benson. I observed variable stars and supernovae with the 24-inch telescope and more specifically found periods for the eclipsing binaries BH Cassiopeia and BG Geminorum. It turns out BG Gem may contain a black hole! Read more about it on the ApJ paper (Benson et al. 2000) we published on it.

   I participated in the summer REU program at the CfA in 1999 and had the opportunity to work with Peter Garnavich. I analyzed the images and spectra for the subluminous Type Ia supernova 1999by. We presented a poster at the AAS in January 2000 and our paper has been published in ApJ ( ADS Abstract, astro-ph/0105490).

   This is a picture of Saturn and the Moon on November 11, 1997. It was taken with Wellesley's 24-inch telescope. Okay it has nothing to do with my research, but it's pretty.

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