4. The synthetic continuum spectra in the infrared region
We present the synthetic continuum spectra of Gl 229B (Fig. 1) in the infra-red region where the signature of methane is clear and dominant. The monochromatic radiative transfer equations are solved numerically by using discrete space theory (Peraiah and Grant 1973). We have adopted two sets of model parameters, model (a): =940 K, log g=5.0 (g in cm s-2) and [M/H]=-0.3 (K band) and model (b): =1030 K, log g=5.5 and [M/H]=-0.1 (K band). The temperature and pressure profiles for both the models are obtained from M. Marley (private communication). The values of and the surface gravity g are constrained by the bolometric luminosity of Gl 229B and the evolutionary sequence of Saumon et al. (1996). These synthetic spectra fit the entire observed spectrum of Gl 229B except in the near infra-red region. It should be mentioned that a good fit with the observational data for the entire wavelength region is not possible with the same value of the metallicity and the surface gravity. The above sets of model parameters are two of many optimal sets of parameters that can produce good fit. In order to match the observed flux shortward of 1.1 that declines very rapidly, either dust particulates or alkali metals have to be incorporated. However, the size of the grain that is needed to explain the observed spectrum in the optical region is too small to play any role in the infra-red region we are interested in the present work. Hence in this spectral region the law of Mie scattering that describes the angular distribution of photons in a dusty atmosphere reduces to that of Rayleigh scattering. Therefore, the synthetic spectrum in this region matches well with the observed flux when Rayleigh's law of scattering is used. Recent ovservational evidence (Liebert et al. 2000) implies that there is no compelling reason to introduce dust or additional opacity source in the atmosphere of methane dwarf. Therefore in the present work we have not incorporated dust opacity.
Fig. 1 shows that there is no difference in the calculated continuum flux with the two sets of model parameters. Both the models fit the observed data very well.
© European Southern Observatory (ESO) 2000
Online publication: June 8, 2000