Brown Dwarfs

Brown dwarfs are stars that never become hot enough in their core to maintain nuclear fusion. Lacking the energy source that powers stars, they slowly dim and fade as their initial heat from gravitational contraction is radiated away into space. In a sense, they are intermediate between true stars and gas giant planets. Their upper mass limit, set by physics of stellar structure and nuclear fusion, is about 0.074 times the mass of the Sun. Their lower mass limit, perhaps set by the physics of 'star' formation, is as low as 0.005 times the mass of the Sun, and may be even less. Such masses overlap the extrasolar giant planet distribution (Jupiter is 0.001 times the mass of the Sun), and indeed the distinction between the lowest mass brown dwarfs and the highest mass planets is matter of debate.

Because they are so dim and cool, the search for brown dwarfs is difficult. Only in 1995 were brown dwarfs discovered that were widely accepted as genuine by the astronomical community. My research, starting with the Rare Objects Team of the Two Micron All-Sky Survey (2MASS), focuses on using near-infrared images to identify brown dwarfs and to study the characteristics of brown dwarfs using these discoveries.

Figure 1. A brown dwarf (shown by arrow) visible in a 2MASS image is invisible in an optical POSS image. The blue color in the 2MASS images is due to methane absorption. M dwarf.

Identifying brown dwarfs requires comparing the 2MASS near-infrared data to optical data such as the Palomar Sky Survey (POSS). Figure 1 shows the example of a very cool brown dwarf discovered with 2MASS data. In the 2MASS image, the 1000K brown dwarf appears as a faint, blue source; in the POSS image, the brown dwarf is invisible. USing 2MASS, very-low-mass stars and brown dwarfs between 2500K and 800K can be detected. The numbers seen suggest that the numbers of brown dwarfs is comparable to the number of stars. The search for field brown dwarfs is ongoing --- furthermore, we are using 2MASS data to search for brown dwarfs in nearby clusters such as the TW Hya Association (10 million years old) and the Hyades (600 million years old).

Figure 2. New discoveries of cool dwarfs have led to the definition of the new L Dwarf spectral class shown here. From Kirkpatrick et al. (1999)

Hundreds of brown dwarfs have been discovered using 2MASS and other surveys. Since their temperatures are often lower than the previously known coolest stars (M9 dwarfs), their spectra appear quite different. We have worked on developing a new classification systems for "L Dwarfs" -- correspoding to temperatures between 2200K and 1400K -- and "T Dwarfs" -- corresponding to temperatures below 1400K. These temperature ranges are uncertain and are a subject of further research. Figure 2 shows the L dwarf spectroscopic sequence. Note that the TiO molecular bands which charcterize the classical M class disappear.

Figure 3. Two double L dwarf systems resolved by Hubble Space Telescope.

If brown dwarfs form like stars, we expect them to sometimes form mutiple systems just as stars are known to. Radial velocity searches for extrasolar planets have shown that brown dwarfs are extremely rare as close (within 3-5 A.U.) companions to solar-type stars. This "brown dwarf desert" is convenient since it prevents the confusion of brown dwarfs and planetary companions, but it is surprising since brown dwarfs ar very common as isolated objects. Do brown dwarfs form other types of binary systems? We have found the answer is yes. At least some 2MASS brown dwarfs are orbiting solar-type (G dwarf) stars, but at distances of 1000 A.U. or more. Furthermore, Hubble Space Telescope images of isolated 2MASS brown dwarfs, such as the one shown above, reveal that many are actually double systems with separations of 1-10 A.U. Improving our knowledge of the frequency and characteristics of brown dwarf binary systems is a priority in research.

Figure 4. The strength of the chromospheric hydrogen emission line drops rapidly for cooler dwarfs. From Gizis et al. (2000)

Do brown dwarfs have magnetic fields? Rotating, convective stars generate strong magnetic fields which result in activity such as chromospheres, coronae, and flares. Brown dwarfs rotate and are convective, so the characterization of their magnetic activity is a an important research project. We have found that chromospheric activity as measured by emission lines becomes both weaker and less common for spectral types cooler than M8 (see figure above). Other researchers have found that the X ray corona is weak, but flares are present in both X rays and the radio. We have begun a study with the Chandra X-ray telescope to study more brown dwarfs.

Nearby Stars

Most stars are cool, main-sequence M dwarfs with masses between 0.6 and 0.08 times the mass of the Sun. Because they are relatively ordinary, they are neglectec by most astronomers, yet they are a promising tool for fields as diverse as Galactic structure, extrasolar planets, and magnetic activity. In the Palomar/Michigan State Spectroscopic Survey (PMSU), Neill Reid, Suzanne Hawley, and I obtained spectra of more than 2000 M dwarfs within 25 parsecs -- nearly all of those known. The resulting database is a resource for additional scientific studies as well as the basic reference for the numbers and activity of M dwarfs.

Nevertheless, many M dwarfs within 25 parsecs are as yet unidentified. As part of a NASA/NSF NSTARS project, we are searching for unknown M dwarf stars using the 2MASS database.

The Coolest Metal-Poor Stars

As the results of the PMSU survey indicate, 99% of low-mass stars are ordinary M dwarfs characeterized by TiO molecular bands. A few low-mass stars, however, formed in the earliest years of the Galaxy and can still be found. These stars have less metals than the Sun and as a result have unusual spectra. I have developed a spectral classification system for these "M subdwarfs" and shown that the composition of M subdwarfs may be reliably estimated. The figure below shows a partial spectroscopic sequence for extreme M subdwarfs (esdM) with approximately 1/100 the metallicity of the Sun.

Figure 5. The extreme M subdwarf sequence compared to an ordinary M dwarf. From Gizis (1997)