CGS4 Spectroscopy of Brown Dwarf Candidates

Brown dwarfs are objects with a mass below the minimum mass for stable hydrogen burning; they are less massive than about 0.08 solar masses and after a very brief period (<0.1 Gyr) of deuterium burning, they will fade. Their luminosity decreases by a factor of 100 in about 1 Gyr. Astronomers have been searching for these faint objects for decades, in particular as an explanation of missing mass.

In 1995, just when the community was beginning to think that somehow the process of star formation did not allow brown dwarfs to be formed, the first non-controversial brown dwarf was discovered during a search for low-mass companions to young M-dwarfs (Nakajima et al 1995). The definitive infrared spectra of this object, Gliese 229B, were obtained by Tom Geballe and collaborators using CGS4 on UKIRT (Geballe et al 1996). The spectra look planet-like, with strong absorption bands of methane and water (steam). The presence of methane implies an effective temperature cooler than around 1200K, and this necessarily implies a sub-stellar mass. Recent data (Leggett et al. 1999) suggest Gliese 229B is aged 0.5-1.5 Gyr, has an effective temperature ~900K and mass ~0.02-0.04 solar masses (or about 20-40 times the mass of Jupiter).

Since 1995 the community have been searching very hard for another example of a methane-dwarf. However, although objects with temperatures ~2000K have been found, 1000K objects were elusive - until a few months ago. On April 21st 1999 astronomers from the Sloan Digital Sky Survey contacted Sandy Leggett (UKIRT) and Tom Geballe (Gemini) and asked them to get infrared data of a candidate brown dwarf. The target was the reddest confirmed object in the Sloan preliminary survey area, and an optical spectrum had shown no flux blueward of 0.8 microns, as well as water and CS absorption features. Subsequent spectroscopy and photometry obtained by Sandy and Tom showed this object, SDSS1624+00, to be extremely similar to Gliese 229B.

A few weeks later the Sloan group contacted Sandy with a second candidate, and CGS4 spectra showed this to be another methane dwarf. Furthermore, the then three known examples of this type of object had amazingly similar energy distributions, as shown in the above figure.

Although we do not yet know the distance to the Sloan dwarfs, and so cannot determine luminosity and constrain age, the presence of methane in the spectra implies an effective temperature less than 1200K, and the presence of cesium and potassium implies an effective temperature greater than about 700K. Assuming an effective temperature similar to that for Gliese 229B, the Sloan dwarfs will have a mass between 15-60 Jupiters for an age 0.3-6 Gyr.

Report and data courtesy of Sandy Leggett