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Straddling the traditional realms of the planets and the stars, objects below the edge of the main sequence have such unique properties, and are being discovered in such quantities, that one can rightly claim that a new field at the interface of planetary science and astronomy is being born. This article extends the previous review of Burrows and Liebert (1993) and describes the essential elements of the theory of brown dwarfs and giant planets. It discusses their evolution, atmospheric composition, and spectra, including the new spectroscopic classes L and T. Particular topics which are important for an understanding of the spectral properties include the effects of condensates, clouds, molecular abundances, and atomic opacities. Moreover, it discusses the distinctive features of these extrasolar giant planets that are irradiated by a central primary, in particular, their reflection spectra, albedos, and transits. Overall, the theory explains the basic systematics of substellar-mass objects over three orders of magnitude in mass and age, and a factor of 30 in temperature. CONTENTS

Before the 2-Micron All-Sky Survey (2MASS) began, only six objects were known with spectral types later than M9.5 V. In the Ðrst 371 deg2 of actual 2MASS survey data, we have identiÐed another 20 such objects spectroscopically conÐrmed using the Low Resolution Imaging Spectrograph (LRIS) at the W. M. Keck Observatory. Because the TiO and VO bands, which dominate the far-optical portions of late-M spectra, disappear in these cooler dwarfs, we deÐne a new spectral class "" L ÏÏ in which metallic oxides are replaced by metallic hydrides and neutral alkali metals as the major spectroscopic signatures. We establish classiÐcation indices and type all 25 L dwarfs. The 26th "" post-M9.5 ÏÏ objectÈGl 229BÈis the prototype of a methane-dominated spectral class, which we propose as class "" T.ÏÏ At least Ðve of the 20 2MASS L dwarfs show the 6708 lithium doublet at low resolution, the strongest having an equiva-A lent width of 18.5For objects this cool, the presence of lithium proves that they are substellar. Two A . other 2MASS objects appear to have lithium lines at the limit of our detectability, which if veriÐed means that at least one-third of our L dwarfs are bona Ðde brown dwarfs. All of the 2MASS brown dwarfs discovered so far haveWe have not yet, despite deliberately searching for them, J[K s [1.30. found any brown dwarfs with colors resembling Gl 229B (J[K s B[0.1).

Spectrophotometric observations at high signal-to-noise ratio were obtained of a complete sample of 347 DA white dwarfs from the Palomar Green (PG) Survey. Fits of observed Balmer lines to synthetic spectra calculated from pure-hydrogen model atmospheres were used to obtain robust values of T eff , log g, masses, radii, and cooling ages. The luminosity function of the sample, weighted by 1/V max , was obtained and compared with other determinations. Incompleteness of the sample due to selection by photographic m u -m b color and magnitude limits was

We have combined 2MASS and POSS II data in a search for nearby ultracool (later than M6.5) dwarfs with Spectroscopic follow-up observations identify 53 M7ÈM9.5 dwarfs and seven L K s \ 12. dwarfs. The observed space density is 0.0045^0.0008 M8ÈM9.5 dwarfs per cubic parsec, without accounting for biases, consistent with a mass function that is smooth across the stellar/substellar limit. We show the observed frequency of Ha emission peaks at D100% for M7 dwarfs and then decreases for cooler dwarfs. In absolute terms, however, as measured by the ratio of Ha to bolometric luminosity, none of the ultracool M dwarfs can be considered very active compared to earlier M dwarfs, and we show that the decrease that begins at spectral type M6 continues to the latest L dwarfs. We Ðnd that Ñaring is common among the coolest M dwarfs and estimate the frequency of Ñares at 7% or higher. We show that the kinematics of relatively active (EW [ 6 ultracool M dwarfs are consistent with an ordi-A ) nary old disk stellar population, while the kinematics of inactive ultracool M dwarfs are more typical of a 0.5 Gyr old population. The early L dwarfs in the sample have kinematics consistent with old ages, suggesting that the hydrogen-burning limit is near spectral types L2ÈL4. We use the available data on M and L dwarfs to show that chromospheric activity drops with decreasing mass and temperature and that at a given (M8 or later) spectral type, the younger Ðeld (brown) dwarfs are less active than many of the older, more massive Ðeld stellar dwarfs. Thus, contrary to the well-known stellar age-activity relationship, low activity in Ðeld ultracool dwarfs can be an indication of comparative youth and substellar mass.