We investigate the formation of methane line at 2.3 { \mu m } in Brown Dwarf Gliese 229B .
Two sets of model parameters with ( a ) T _ { eff } = 940 K and \log ( g ) = 5.0 , ( b ) T _ { eff } = 1030 K and \log ( g ) = 5.5 are adopted both of which provide excellent fit for the synthetic continuum spectra with the observed flux at a wide range of wavelengths .
In the absence of observational data for individual molecular lines , we set the additional parameters that are needed in order to model the individual lines by fitting the calculated flux with the observed flux at the continuum .
A significant difference in the amount of flux at the core of the line is found with the two different models although the flux at the continuum remains the same .
Hence , we show that if spectroscopic observation at 2.3 { \mu m } with a resolution as high as R \simeq 200 , 000 is possible then a much better constraint on the surface gravity and on the metallicity of the object could be obtained by fitting the theoretical model of individual molecular line with the observed data .