Since beginning my PhD thesis in 1995, my research has focused on the formation of low-mass stars and brown dwarfs, and the formation of planets around these objects. Why are these topics important? One of the highest priorities for research in the astronomical community is the study of star and planet formation (NRC, 2010). By bridging the gap between them, brown dwarfs can provide insight into the formation of both stars and planets. In addition, low-mass stars and brown dwarfs are valuable laboratories for planet formation because the former are the most common stars in the Galaxy while the latter offer an opportunity to test theories of planet formation in an extreme environment.
I have studied the formation of stars, brown dwarfs, and planets primarily through optical and infrared observations with ground- and space-based telescopes. The central component of my research has consisted of systematic searches for low-mass stars and brown dwarfs in the nearest star-forming regions (age<10 Myr, d=150-450 pc). I have used these surveys to measure the low-mass initial mass function (IMF) and to provide targets for followup observations of various aspects of young low-mass stars and brown dwarfs, such as their circumstellar disks and multiplicity. In addition to work on young objects, I have searched for very cool brown dwarfs in the solar neighborhood, both free floating and as companions to nearby stars.
Initial Mass Function
The presence of a flattening or turnover in the IMF, the value of this 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 can act as observational tests of theories for the formation of stars and brown dwarfs. Through my surveys of star-forming regions, I have found that:
- The number ratio of brown dwarfs to stars is ~1/6. Large variations in this ratio are not present among the nearest star-forming regions.
- The IMF extends down to at least ~0.005-10 MSun (5-10 MJup) in Taurus and Chamaeleon, which is lower than the values predicted by some theories of star formation. Definitive measurement of the minimum mass of the IMF will require deeper observations, which are ongoing by myself and others.
Much of our knowledge of planet formation has been derived from observations of circumstellar disks around newborn stars. To study disks around stars and brown dwarfs, I have performed mid-IR photometric and spectroscopic surveys of the stellar and substellar populations in nearby star-forming regions using the Spitzer Space Telescope. Results from this work include:
- A complete census of disks in Taurus, Chamaeleon, IC 348, sigma Ori, and Upper Sco.
- Disks exist around brown dwarfs with masses as low as ~5 MJup, indicating that planets may form around objects that have the masses of giant planets.
- Disks are as common around brown dwarfs as around stars. Hence, the same may be true for planetary systems.
- Grain growth in disks -- one of first steps in planet formation -- may occur faster for brown dwarfs than for stars.
- The first known edge-on disk around a brown dwarf has provided the first constraints on the diameter of a brown dwarf disk. This disk is somewhat larger than expected from embryo ejection models for the formation of brown dwarfs.
- Identification of a relatively large sample of stars that have disks in advanced stages of evolution, such as disks with inner holes (due to forming planets?) and candidates for the youngest known debris disks (~1 Myr).
- The timescale for clearing of optically thick inner disks around stars is short compared to the lifetimes of those disks.
The multiplicity of stars is influenced by the specific characteristics of the star formation process. Thus, theories for the formation of brown dwarfs can be tested with multiplicity measurements. I have used ground-based telescopes and the Hubble Space Telescope to obtain data of this kind in star-forming regions, which has produced the following constraints on the origin of brown dwarfs:
- The discovery of the first known wide binary brown dwarf has demonstrated that brown dwarfs can form without the involvement of ejection or other dynamical processes.
- The components of a second wide binary in Taurus are the first known brown dwarfs that have been born in isolation rather than in a stellar cluster. The isolated nature of this system provides additional evidence that dynamical interactions among stars are not required for the formation of brown dwarfs.
- A Hubble survey in Taurus has uncovered a 5-10 MJup companion to a 25 MJup brown dwarf (0.1" = 15 AU). This pair appears to be part of a quadruple system, and the hierarchical configuration of that system suggests that the fragmentation of molecular cloud cores can produce companions below 10 MJup.
Nearby Cold Brown Dwarfs
Older, very cool brown dwarfs represent valuable laboratories for studying planetary atmospheres. I have searched for cool brown dwarfs in the neighborhood of the Sun with the Spitzer Space Telescope and the Wide-field Infrared Survey Explorer, resulting in:
- The discovery of the first substellar companion directly imaged around a star that is known to harbor a close-in planet from radial velocity surveys.
- The discovery of one of the youngest known T dwarfs.
- The discovery and confirmation of the coldest companion imaged outside of the solar system, which has a temperature similar to that of Earth.
- The discovery of a binary brown dwarf that is the 3rd closest star system to the Sun and the closest system found in nearly a century.
- The demonstration that a brown dwarf or large gas giant planet ("Nemesis", "Planet X") probably does not exist in the outer solar system.
- The discovery of a brown dwarf that is the 4th closest neighbor of the Sun and the coldest object seen outside the solar system.