Optical Spectroscopy of T dwarfs

Optical spectra are presented for a sample of T dwarfs, obtained using the Keck 10m Low Resolution Imaging Spectrograph. Dominant features are discussed, including possible temperature diagnostics. We have used the Low Resolution Imaging Spectrograph (LRIS; Oke et al. 1995), mounted on the Keck 10m Telescope, to obtain red optical spectra of nine T dwarfs selected from Burgasser et al. (1999, 2000a, 2000c, 2001), Strauss et al. (1999), and Tsvetanov et al. (2000). Observations of 2MASS 1237+65, SDSS 1346-00, and SDSS 1624+00 are presented in Burgasser et al. (2000b), which discusses instrumental setup and data reduction; the remaining targets, discussed here, were observed on 5 March 2000 (UT). Reduced spectra are shown in Figure 1a, normalized at 9200 Å and vertically offset by 0.5. The rapid rise in flux from 8000 to 10000 Å is likely due to the extremely pressure-broadened K I doublet at 7665 and 7699 Å, as noted in SDSS 1624+00 by Liebert et al. (2000). A slight rise in flux between 6500 and 7800 Å is seen in a few objects, most clearly in 2MASS 0559-14 and SDSS 1624+00 (Figure 1b), and is due to an opacity window between the K I doublet and the similarly broadened Na I D lines (5890 and 5896 Å; Reid et al. 2000). We note that the increase in flux on the red side of the K I doublet is greater in Gl 570D (Teff ∼ 750 K) than in 2MASS 0559-14 (Teff ∼ 1200 K). H2O absorption at 9200 Å similarly increases from 2MASS 0559-14 to Gl 570D. The behavior of the Cs I lines (8521 and 8943 Å) is unclear due to poor signal-tonoise in these faint dwarfs, as well as the influence of CH4 at 8900 Å; similarly, Rb I (7948 Å) is detected only in the high signal-to-noise spectrum of 2MASS 0559-14. These alkali features are seen to strengthen throughout the L dwarf sequence (Kirkpatrick et al. 1999). FeH (9896 Å), which weakens in midto lateL dwarfs, is seen in 2MASS 0559-14, SDSS 1624+00, and 2MASS 1225-27, but is not readily apparent in the other objects. Finally, the Hα emission reported in 2MASS 1237+65 (Burgasser et al. 2000b) is not seen in any of the other T dwarfs, in agreement with activity relations observed in late-M and L dwarfs (Gizis et al. 2000), and further evidence that 2MASS 1237+65 may have an unusual activity mechanism. We propose that FeH, H2O, and the red slope of the K I doublet provide potential temperature diagnostics in the optical regime.