## 5. Results and discussionThe continuum opacity and the temperature-pressure profile of the atmosphere are set by matching the synthetic continuum spectrum with the observed spectrum for the entire wavelength region. The modeling of individual lines needs the values of additional parameters. In the absence of observational data for individual lines, we set the value of these parameters by matching the calculated flux in the continuum with the observed continuum flux at 2.3 . Since the line is very weak in intensity, we choose the profile function as (Mihalas 1978) where is the thermal Doppler width and is the line center. It should be worth mentioning that individual molecular lines are usually not saturated enough so that pressure broadening is less important for them as compared to strong atomic lines. Moreover, molecular lines often overlap so strongly that their wings are completely masked (Schweitzer et al. 1996) and only the Gaussian line cores of the strongest molecular transitions are observed. The atmosphere of a brown dwarf is therefore only weakly sensitive to the Van der Walls damping constant. Nothing is known, at present, about the rotation of the brown dwarf Gl 229B around its own axis of rotation. If the projected velocity `' of the object is greater than 2 to 5 then rotational broadening could be significant. However, in the present work we have neglected rotational broadening in order to make the results consistent with the calculation of the evolutionary sequences by Saumon et al. (1996) that constrains the surface gravity and the effective temperature of the object. The whole purpose of the present work is to show that with different values of the surface gravity and the metallicity, the flux at the line core is significantly different although it is the same in the continuum. Since rotational broadening would affect the spectrum equally for both the models, it is not important in the context of the present work. We assume complete frequency redistribution and use Rayleigh phase function for the angular redistribution. The parameters that are to be set in order to model the
line at
2.3 are
,
Fig. 2 shows that a spectral resolution as high as 200,000 at is needed in order to investigate the individual molecular lines. This may be possible with an appropriate combination of the telescope and the instrument. For example, the Cooled Grating Spectrometer 4 (CGS4) available in UKIRT (United Kingdome Infra-Red Telescope) has a spectral resolution upto 40,000. If observation of Gl 229B is possible at present by UKIRT with the maximum resolution power of CGS4 then keeping the signal to noise ratio (which is proportional to the diameter of the telescope and to the square root of the integration time of exposure) unaltered, a 10 m telescope (such as Keck I) can obtain the desired resolution by using a similar type of spectrometer provided the resolution of the instrument is increased by about five times and the integration time of exposure is increased by about 2.5 times. It is found that the numerical values of
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