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Astron. Astrophys. 358, L33-L36 (2000)

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1. Introduction

The discovery of methane bands in the spectrum of Gl 229B (Nakajima et al. 1995, Oppenheimer, Kulkarni, Matthews & Nakajima 1995, Geballe, Kulkarni, Woodward, & Sloan 1996) has not only helped to identify Gl 229B as a Brown Dwarf but also prompted the creation of the new spectral class of T dwarfs (Kirkpatrick et al. 1999). The synthetic continuum spectra of the object has been obtained with and without dust particles (Marley et al. 1996, Griffith, Yelle & Marley 1998, for a review see Allard et al. 1997). Although the incorporation of condensates can explain the very rapid decline of the observed continuum flux in the optical region, it is shown (Tsuji, Ohnaka, & Aoki 1999, Burrows, Marley, & Sharp 1999) that the pressure broadened red wing of the 0.77 [FORMULA] K I doublet could also account for the observed features of the continuum flux shortward of 1.1 [FORMULA]. The spectrum of the first field methane T dwarf SDSS 1624+0029 shows a broad band absorption feature centered at 7700 [FORMULA] which is interpreted (Liebert et al. 2000) as the K I 7665/7699 resonance doublet. Hence, it is most likely that the shape of the red spectrum is due to the broad wings of the K I and the Na I doublets and not due to the presence of condensates. All these model spectra together with the bolometric luminosity of the object (Leggett et al. 1999) and the evolutionary sequences (Saumon et al. 1996) can constrain the effective temperature of the object very tightly, however the surface gravity is still poorly constrained. It is shown (Saumon et al. 2000) that a multi-parameter fit of the observed spectrum both for the K-band and for the red end is possible with different metallicities. Therefore, in order to determine the physical properties of the atmosphere of Gl 229B and the other T dwarfs uniquely, more comprehensive theoretical modeling of the observed spectrum is required. One of the most important studies is the individual line formation by the most abundant molecules, eg. [FORMULA], [FORMULA], [FORMULA] etc.

In this paper, we for the first time, attempt to model the line formed by methane at 2.3 [FORMULA] and show that if individual lines of abundant molecules, in particular that of methane can be resolved observationally, then stringent constraint on the surface gravity, on the metallicity and on the temperature at the bottom of the atmosphere, where the optical depth is very high, can be obtained.

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© European Southern Observatory (ESO) 2000

Online publication: June 8, 2000