Assistant Professor of Astronomy & AstrophysicsB.A., B.S., The University of Texas at Austin, 1993
Kevin Luhman studies the formation of stars, brown dwarfs, and planets through optical and infrared observations with ground- and space-based telescopes, including IRTF, KPNO, CTIO, MMT, Magellan, Gemini, Keck, HST, Spitzer Space Telescope, and Penn State's Hobby-Eberly Telescope. One of his primary projects has involved measuring the initial mass function (IMF) of low-mass stars and brown dwarfs. The presence of a flattening or turnover in the IMF, the value of such a characteristic mass, the shape of the IMF into the substellar regime, the minimum mass at which objects form in isolation, and the variation of these properties with environmental conditions provide discriminating tests of the wide range of theories for the formation of stars and brown dwarfs. Through deep photometric and spectroscopic surveys of nearby star-forming regions, Dr. Luhman has measured IMFs down to masses of 0.01 solar masses (or 10 Jupiter masses) and compared these data to the predictions of theoretical models for the formation of stars and brown dwarfs. To continue to test those theories more critically, he is is attempting to extend these measurements of the IMF to the mass of Jupiter, and in this way determine the lowest mass at which star-like bodies can form.
The large, unbiased samples of young brown dwarfs discovered during Dr. Luhman's IMF measurements represent a unique opportunity to extend knowledge of the star formation process into an unexplored mass regime. The various methods developed over the years for studying the birth of stars can now be applied to these young substellar sources. For instance, with the recent deployment of the Spitzer Space Telescope, the structure and evolution of circumstellar material around young brown dwarfs can be constrained through modeling of their mid-IR SEDs and spectral features, allowing one to address a variety of fundamental questions, such as: How does the structure of disks depend on mass from a solar mass across the hydrogen and deuterium burning limits? Is there an indication in these results of fundamentally different formation mechanisms for stars and brown dwarfs? Can planets form around brown dwarfs? Dr. Luhman has recently begun a longterm program to address these questions using current observatories (Hubble, Spitzer, Keck, Gemini, HET) and upcoming facilities (JWST, ALMA). In an early result from this work, he and his collaborators on the Spitzer IRAC GTO team have discovered the two least massive known brown dwarfs with circumstellar disks. The presence of disks around these extremely low-mass objects (8 and 15 Jupiter masses) demonstrates that the formation of free-floating bodies via disks extends down to planetary masses and raises the possibility of planet formation around objects that themselves have planetary masses.