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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).

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The horizontal branch (HB) morphology of globular clusters (GCs) is most strongly influenced by metallicity. The second parameter phenomenon, first described in the 1960's, acknowledges that metallicity alone is not enough to describe the HB morphology of all GCs. In particular, astronomers noticed that the outer Galactic halo contains GCs with redder HBs at a given metallicity than are found inside the Solar circle. Thus, at least a second parameter was required to characterize HB morphology. While the term 'second parameter' has since come to be used in a broader context, its identity with respect to the original problem has not been conclusively determined. Here we analyze the median color difference between the HB and the red giant branch (RGB), hereafter denoted ∆(V − I), measured from Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) photometry of 60 GCs within ∼20 kpc of the Galactic Center.

We present the first results of a large Advanced Camera for Surveys (ACS) survey of Galactic globular clusters. This Hubble Space Telescope (HST ) Treasury project is designed to obtain photometry with S/N (signal-to-noise ratio) k10 for main-sequence stars with masses k0.2 M in a sample of globulars using the ACS Wide Field Channel. Here we focus on clusters without previous HST imaging data. These include NGC 5466, NGC 6779, NGC 5053, NGC 6144, Palomar 2, E3, Lyngå 7, Palomar 1, and NGC 6366. Our color-magnitude diagrams (CMDs) extend reliably from the horizontal branch to as much as 7 mag fainter than the main-sequence turnoff and represent the deepest CMDs published to date for these clusters. Using fiducial sequences for three standard clusters (M92, NGC 6752, and 47 Tuc) with well-known metallicities and distances, we perform main-sequence fitting on the target clusters in order to obtain estimates of their distances and reddenings. These comparisons, along with fitting the cluster main sequences to theoretical isochrones, yield ages for the target clusters. We find that the majority of the clusters have ages that are consistent with the standard clusters at their metallicities. The exceptions are E3, which appears $2 Gyr younger than 47 Tuc, and Pal 1, which could be as much as 8 Gyr younger than 47 Tuc.

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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.

We present JHK s photometry, far red spectra, and spectral classifications for an additional 67 L dwarfs discovered by the Two Micron All Sky Survey. One of the goals of this new search was to locate more examples of the latest L dwarfs. Of the 67 new discoveries, 17 have types of L6 or later. Analysis of these new discoveries shows that Hα emission has yet to be convincingly detected in any L dwarf later than type L4.5, indicating a decline or absence of chromospheric activity in the latest L dwarfs. Further analysis shows that 16 (and possibly 4 more) of the new L dwarfs are lithium brown dwarfs and that the average line strength for those L dwarfs showing lithium increases until type ∼L6.5 V then declines for later types. This disappearance may be the first sign of depletion of atomic lithium as it begins to form into lithium-bearing molecules. Another goal of the search was to locate nearer, brighter L dwarfs of all subtypes. Using absolute magnitudes for 17 L dwarf systems with trigonometric parallax measurements, we develop spectrophotometric relations to estimate distances to the other L dwarfs. Of the 67 new discoveries, 21 have photometric distances placing them within 25 parsecs of the Sun. A table of all known L and T dwarfs believed to lie within 25 parsecs -53 in total -is also presented. Using the distance measurement -2of the coolest L dwarf known, we calculate that the gap in temperature between L8 and the warmest known T dwarfs is less than 350K and probably much less. If the transition region between the two classes spans a very small temperature interval, this would explain why no transition objects have yet been uncovered. This evidence, combined with model fits to low-resolution spectra of late-M and early-L dwarfs, indicates that L-class objects span the range 1300K ∼ < T ef f ∼ < 2000K. The near-infrared color-color diagram shows that L dwarfs fall along a natural, redder extension of the well known M dwarf track. These near-infrared colors get progressively redder for later spectral types, with the L dwarf sequence abruptly ending near (J − H, H − K s , J − K s ) ≈ (1.3, 0.8, 2.1).