The Missing Link: Early Methane ("T") Dwarfs in the Sloan Digital Sky Survey.

We report the discovery of three cool brown dwarfs that fall in the effective temperature gap between the latest L dwarfs currently known, with no methane absorption bands in the 1-2.5 µm range, and the previously known methane (T) dwarfs, whose spectra are dominated by methane and water. The newly discovered objects were detected as very red objects in the Sloan Digital Sky Survey imaging data and have JHK colors between the red L dwarfs and the blue Gl 229B-like T dwarfs. They show both CO and CH(4) absorption in their near-infrared spectra in addition to H(2)O, with weaker CH(4) absorption features in the H and K bands than those in all other methane dwarfs reported to date. Due to the presence of CH(4) in these bands, we propose that these objects are early T dwarfs. The three form part of the brown dwarf spectral sequence and fill in the large gap in the overall spectral sequence from the hottest main-sequence stars to the coolest methane dwarfs currently known.

Ultracompact X-Ray Binaries in Globular Clusters: Variability of the Optical Counterpart of X1832-330 in NGC 6652.

Evidence is emerging that the luminous X-ray sources in the cores of globular clusters may often consist of, or perhaps even as a class be dominated by, ultracompact (P less, similar1 hr) binary stars. To the two such systems already known, in NGC 6624 and NGC 6712, we now add evidence for two more. We detect large-amplitude variability in the candidate optical counterpart for the X-ray source in the core of NGC 6652. Although the available observations are relatively brief, the existing Hubble Space Telescope data indicate a strong 43.6 minute periodic modulation of the visible flux of semiamplitude 30%. Further, although the orbital period of the source in NGC 1851 is not yet explicitly measured, we demonstrate that previous correlations of optical luminosity with X-ray luminosity and accretion disk size, strengthened by recent data, strongly imply that the period of that system is also less than 1 hr. Thus, currently there is evidence that four of the seven globular cluster X-ray sources with constrained periods are ultracompact, a fraction far greater than that found in X-ray binaries the field.