On the interpretation of the optical spectra of L-type dwarfs

We present synthetic optical spectra in the red and far-red (640-930 nm) of a sample of field L dwarfs suitably selected to cover this new spectral cl ass, and the brown dwarf GL 229B. We have used the recent "dusty" atmospher es by Tsuji (2000) and by Allard(1999), and a synthesis code (Pavlenko et a l. 1995) working under LTE conditions which considers the chemical equilibr ium of more than 100 molecular species and the detailed opacities for the m ost relevant bands. Our computations show that the alkali elements Li, Na, K, Rb, and Cs govern the optical spectra of the objects in our sample, with Na and K contributing significantly to block the optical emergent radiatio n. Molecular absorption bands of oxides (TiO and VO) and hydrides (CrH, Fel l and CaH) also dominate at these wavelengths in the early L-types showing a strength that progressively decreases for later types. We find that the d ensities of these molecules in the atmospheres of our objects are considera bly smaller by larger factors than those predicted by chemical equilibrium considerations. This is consistent with Ti and V atoms being depleted into grains of dust. In order to reproduce the overall shape of the optical spectra of our obser vations an additional opacity is required to be implemented in the computat ions. We have modelled it with a simple law of the form a(o) (nu/nu(o))(N), With N = 4, and found that this provides a sufficiently good fit to the da ta. This additional opacity could be due to molecular/dust absorption or to dust scattering. We remark that the equivalent widths and intensities of t he alkali lines are highly affected by this opacity. In particular, the lit hium resonance line at 670.8 nm, which is widely used as a substellarity di scriminator, is more affected by the additional opacity than by the natural depletion of neutral lithium atoms into molecular species. Our theoretical spectra displays a rather strong resonance feature even at very cool effec tive temperatures (similar to 1000 K); depending on the effective temperatu re and on the amount of dust in the atmospheres of very cool dwarfs, it mig ht be possible to achieve the detection of lithium even at temperatures thi s cool. Changes in the physical conditions governing dust formation in L-ty pe objects will cause variability of the alkali lines, particularly of the shorter wavelength lines.