Comparing limits

Figures 19 and 20 compare the full-settling and full-dusty limits for ${\rm T}_{\rm eff}= 2000$ and 1500K respectively. In the 2000K case, the presence of dust opacities simply has the effect to veil the optical to red spectral region, while the additional heat causes flux redistribution to the infrared. This ${\rm T}_{\rm eff}$ and gravity is typical of dusty red and brown dwarfs. In the 1500K case however, the dust opacity profiles are blocking nearly all flux bluewards of 1.0 $\mu$m, and only the water vapor bands are still distinguishable in the AMES-Dusty model. The AMES-Cond models, on the other hand, are very transparent since all trace of TiO, VO, CaH, MgH and FeH have vanished through condensation to dust grains. And since more flux can escape from short wavelengths, the upper atmosphere is cooler and the water bands stronger.

As mentioned earlier, one of the most important effects of dust extinction in the photospheres of red and brown dwarfs is the resulting heating of the outer atmospheric layers. In Figure 7, we compare the thermal structures of our two limiting cases to NextGen models for two ${\rm T}_{\rm eff}$: one typical of M dwarfs and young/massive brown dwarfs (2800K), the other typical of the reddest L dwarfs (1800K). In the former case, the cloud layers form above the photosphere, as can be seen from Figure 1, such that heating takes place above the line forming region, leaving the thermal structure little affected by the dust. At 1800K, on the other hand, the clouds form deep into the photosphere (see Figure 2). The line forming region is therefore heated up by as much as 500K, while the internal layers only warm up by less than 80K. This effect is enough to dissociate water vapor by nearly 50% in the outer layers, leaving a far shallower structure over most of the photosphere. It is interesting to note that Cond models cooler than about ${\rm T}_{\rm eff}\ = 1800$K, have similar thermal structures than corresponding NextGen models.

This similarity of the thermal structures translates into similar synthetic spectra. We compare the present AMES-Cond models to our earlier brown dwarfs models (NextGen) as of in Figure for the ${\rm T}_{\rm eff}= 1000$K case. Although the NextGen models used in our 1996 publication were constructed without dust condensation, the molecular bands of TiO, VO and FeH were completely crushed by the wings of the alkali lines and the results quasi independent of condensation. The models remain sensitive to the physics mainly in the inter-band regions at 1.0, 1.25, 1.6 and 2.2 $\mu$m which probe, by their relative brightness, the thermal structure of the atmosphere at various depths.