Optical Spectroscopy of T dwarfs
Adam J.\ Burgasser
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.
Burgasser_poster.tex
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\begin{document}
\title{Optical Spectroscopy of T dwarfs}
\author{Adam J.\ Burgasser}
\affil{California Institute of Technology, MSC 103-33, Pasadena, CA 91125}
\author{J.\ Davy Kirkpatrick}
\affil{Infrared Processing and Analysis Center, MSC 100-22, Pasadena, CA 91125}

\begin{abstract}
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.  
\end{abstract}

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 {\AA} and 
vertically offset by 0.5.  
The rapid rise in flux from
8000 to 10000 {\AA} is likely due to the extremely pressure-broadened K I
doublet at 7665 and 7699 {\AA}, as noted in SDSS 1624+00 by 
Liebert et al.\ (2000).  
A slight rise in flux between 6500 and 7800 {\AA}
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 {\AA};
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 (T$_{eff}$ $\sim$ 750 K) than in
2MASS 0559-14 (T$_{eff}$ $\sim$ 1200 K).  
H$_2$O absorption at 9200 {\AA}
similarly increases from 2MASS 0559-14 to Gl 570D.  
The behavior of the Cs I
lines (8521 and 8943 {\AA})
is unclear due to poor signal-to-noise in these faint dwarfs, as well as the
influence of CH$_4$ at 8900 {\AA}; similarly,
Rb I (7948 {\AA}) 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 {\AA}), which weakens in mid- to late-L 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 H$\alpha$ 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, H$_2$O, and the red slope of the K I doublet provide
potential temperature diagnostics in the optical regime.


\begin{figure}
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\caption{Optical spectra of T dwarfs. (a) Full 6300--10100 {\AA} spectra,
with dominant features indicated.  
(b) 6300--8000 {\AA} region, magnified 20$\times$
to show flux between Na I and K I doublets.}
\end{figure}
 
\begin{references}
\reference Burgasser, A.\ J., et al.\ 1999, \apj, 522, L65
\reference Burgasser, A.\ J., et al.\ 2000a, \apj, 531, L57
\reference Burgasser, A.\ J., Kirkpatrick, J.\ D., Reid, I.\ N., 
Liebert, J., Gizis, J.\ E., \& Brown, M.\ E.\ 2000b, \aj, 120, 473
\reference Burgasser, A.\ J., et al.\ 2000c, \aj, 120, 1100
\reference Burgasser, A.\ J., et al.\ 2001, \apj, in prep.\
\reference Gizis, J.\ E., Monet, D.\ G., Reid, I.\ N., Kirkpatrick, J.\ D.,
Liebert, J., \& Williams, R.\ J.\ 2000, \aj, 120, 1085
\reference Kirkpatrick, J.\ D., et al.\ 1999, \apj, 519, 802
\reference Liebert, J., Reid, I.\ N., Burrows, A., Burgasser, A.\ J., 
Kirkpatrick, J.\ D., \& Gizis, J.\ E.\ 2000, \apj, 533, L155
\reference Oke, J.\ B., et al.\ 1995, \pasp, 107, 375
\reference Reid, I.\ N., Kirkpatrick, J.\ D., Gizis, J.\ E., Dahn, C.\ C., 
Monet, D.\ G., Williams, R.\ J., Liebert, J., \& Burgasser, A.\ J.\
2000, \aj, 119, 369
\reference Strauss, M.\ A., et al.\ 1999, \apj, 522, L61
\reference Tsvetanov, Z.\ I., et al.\ 2000, \apj, 531, L61
\end{references}


\end{document}
