Michael C. Cushing, Ph.D.

What is the lower mass limit to the process of star formation? What is the space density of brown dwarfs in the solar neighborhood? What can brown dwarfs tell us about the diversity of planetary systems outside our solar system? My research aims to answer these (and other) questions using both ground- and spaced-based observations and advanced theoretical models.

Hunting for Cool Brown Dwarfs

[WISE 1828]

The study of cool brown dwarfs (Teff < 800 K) can offer important insights into the atmospheres of gas giant exoplanets, constrain theories of star formation, and directly probe the low-mass limit of star formation. The WISE brown dwarf science team (of which I am a member), has been searching the WISE database for cool brown dwarfs using various color constraints. To date, we have identified roughly twenty brown dwarfs that populate a new spectral class, 'Y', with Teff < 500 K. Followup observations of additional candidates are underway as well as observations to obtain distances of the entire sample of late-type T and Y dwarfs.

Studying Weather on Y Dwarfs

[WISE 1828]

Y dwarfs are the coolest class of brown dwarfs known with effective temperatures less than about 500 K. Variability, both photometric and spectroscopic, is commonly observed in the hotter L and T dwarfs so we fully expect Y dwarfs to be variable as well. The most common explanation for this variability is some kind of patchy cloudy cover, although variations in the temperature/pressure profiles can also produce variability. We are currently executing a large Spitzer Space Telescope program to search for photometric variability at 3.6 and 4.5 microns. Combined with the latest atmospheric models from Marley and collaborators, graduate students Kevin Hardegree-Ullman and Jesica Trucks and I hope to provide the first constraints on Y dwarf weather.

Infrared Proper Motion Searches

[Luhman 16]

Although searching for cool brown dwarfs by color has proven fruitful, we can also search for these brown dwarfs via their motion on the sky. Indeed the second and fourth closest star systems to the Sun (actually brown dwarfs) were discovered in just this way. Postdoc Adam Schneider, Davy Kirkpatrick and I are searching the NEOWISE-R database for high proper motion stars and brown dwarfs in the hope that other nearby systems await discovery.

Infrared Spectral Libraries

[MLT Sequence]

Observational spectral libraries are important for both spectral classification of unknown sources and for population synthesis. John Rayner, William Vacca, and myself are currently constructing a near-infrared spectral library using SpeX on the NASA IRTF. The spectra, which sample the entire HR diagram (including WR stars and L and T dwarfs) have R=2000 and cover from 0.8 to 2.5 microns, with a large subset of the spectra extending to ~5 microns. The library can be found here.