No abstract

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).

We report 24 and/or 70 µm measurements of ∼160 A-type main-sequence stars using the Multiband Imaging Photometer for Spitzer (MIPS). Their ages range from 5 to 850 Myr based on estimates from the literature (cluster or moving group associations) or from the H-R diagram and isochrones. The thermal infrared excess is identified by comparing the deviation (∼3% and ∼15% at the 1-σ level at 24 and 70 µm, respectively) between the measurements and the synthetic Kurucz photospheric predictions. Stars showing excess infrared emission due to strong emission lines or extended nebulosity seen at 24 µm are excluded from our sample; therefore, the remaining infrared excesses are likely to arise from circumstellar debris disks. At the 3-σ confidence level, the excess rate at 24 and 70 µm is 32% and ≥33% (with an uncertainty of 5%), considerably higher than has been found for old solar analogs and M dwarfs. Our measurements place constraints on the fractional dust luminosities and temperatures in the disks. We find that older stars tend to have lower fractional dust luminosity than younger ones. While the fractional luminosity from the excess infrared emission follows a general 1/t relationship, the values at a given stellar age vary by at least two orders of magnitude. We also find that (1) older stars possess a narrow range of temperature distribution peaking at colder temperatures, and (2) the disk emission at 70 µm persists longer than that at 24 µm. Both results suggest that the debris-disk clearing process is more effective in the inner regions.