AAS 196th Meeting, June 2000
Session 20. Stellar Populations
Invited, Monday, June 5, 2000, 3:40-5:10pm, Lilac Ballroom

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[20.02] What We Have Learned about L and T Dwarfs

J. D. Kirkpatrick (IPAC/Caltech)

In 1901, Annie Jump Cannon gave us the OBAFGKM spectral sequence with which we are all familiar. One hundred years later, a change of the main sequence types to OBAFGKMLT is now proposed so that objects even cooler than M dwarfs can be included.

Just a few short years ago - in 1997, in fact - the lower end of the main sequence was populated by a handful of late-M dwarfs and two other objects, GD 165B and Gl 229B. Dismissed at the time by many researchers as a freak unrelated to normal stars or brown dwarfs, GD 165B has instead proven to be the prototype of a new spectral class, called ``L dwarfs'', which is just cooler than class M and encompasses both low-mass stars and warm brown dwarfs. Likewise the methane dwarf Gl 229B, heralded as the first bona fide brown dwarf discovery in the immediate vicinity of the Sun, has proven to be the prototype of a spectral class just cooler than L, a class referred to as ``T dwarfs''.

In three short years, the number of known L dwarfs has jumped from 1 to over 120, with the number of T dwarfs jumping from 1 to 20. L dwarfs are characterized spectroscopically in the far red by weak or absent bands of TiO and VO, strong bands of FeH and CrH, and strong lines of neutral Na, K, Cs, Rb, and sometimes Li. In the near-infrared they show strong bands of H2O along with bands of FeH and CO and lines of neutral Na and K. T dwarfs are characterized by methane absorption notably at H- and K-bands and show strong H2O bands throughout the far red and near-infrared regions. The known examples also show neutral K and Cs lines along with weak or absent bands of FeH and CO.

In this talk the L0-L8 dwarf spectral sequence, along with a T dwarf sequence still under development, will be described. Using available data, it will be shown that the L dwarf sequence spans the temperature range from ~1300 to ~2000 K and that T dwarfs fall at temperatures below ~1300 K. Using lithium and H-alpha data on the nearby sample, the fraction of stars to brown dwarfs as a function of spectral subclass can be deduced - with the fraction being near one at mid-M and reaching zero roughly around type L4. The implied space densities show that a given slice of the Solar Neighborhood should have a roughly equal number of L and T dwarfs as it does main sequence stars of types O, B, A, F, G, K, and M. However, L and T dwarfs make up only ~15% of the mass that stars do.


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