Exoplanets: Discovery and characterization

There are now over 500 extrasolar planets known, but only ~35 orbit stars more massive than 1.5 solar masses.  Our understanding of planet formation in this stellar mass regime pales in comparison to what we know about planet formation around Sun-like stars.  I am collaborating with John Johnson and members of the California Planet Search team to better understand the frequency and properties of giant planets around intermediate-mass stars.  Intriguingly, the frequency and masses of planets appears to increase with increasing stellar mass, and there also appears to be a dearth of planets within 0.6 AU around intermediate-mass stars.  What is it about doubling the mass of a star that so dramatically changes the planet formation process?  The answer probably lies in the early evolution of massive protoplanetary disks around these stars; the inner disk regions may photoevaporate before migration can shepherd giant planets to small radii.

My dissertation research is a direct imaging search for giant planets and brown dwarfs around nearby, young, low-mass stars.   The goals are to determine the frequency of giant planets at large separations around low-mass stars for the first time, which provides critical information about the planet formation process, and to identify young giant planets and brown dwarfs, which are amenable to follow-up spectroscopic observations. 

The image at left shows the residuals after building a model of a stellar point spread function and subtracting it from the image of the star, revealing any faint companions.  My survey is sensitive to planets down to a few Jupiter masses.

Some of the fundamental goals of exoplanetary science are to understand the formation and evolution of planets and their atmospheres.  High contrast spectroscopy of faint companions is possible with existing instruments, especially integral field spectrographs, enabling detailed studies of planetary atmospheres outside our solar system for the first time.

The image at right shows the halo of a bright star (orange-green) and a small region of the halo subtracted out (blue-purple), revealing the known planet HR 8799 b.  The image is in fact a data cube with each pixel representing a spectrum in the third dimension.

The analysis of this spectrum was published in 2010 in collaboration with Michael Liu, Trent Dupuy, and Michael Cushing.  The press release from Keck Observatory can be found here.

Atmospheric Properies of

Low-Mass Stars and Brown Dwarfs

Most of the stars in our galaxy are low-mass M dwarfs. Theoretical atmospheric and evolutionary models do a reasonable job of reproducing the observed properties of solar-metallicity M dwarfs, but low-metallicity M dwarf models have yet to be tested.  Together with Michael Liu and Michael Cushing, I have used the first benchmark ultracool subdwarf -  a low-mass M9 companion to a metal-poor star - to test low-metallicity theoretical models for the first time. The current generation of atmospheric and evolutionary models generally do a good job at reproducing the spectra and colors of this benchmark object, but we discovered several important caveats which limit their use to specific spectral regions.  This work will help theoreticians focus their attention on the most prominent discrepancies between theory and observation.