Gravity

In Figure 15 and 16, we explore the effects of surface gravity on AMES-Cond models with ${\rm T}_{\rm eff}= 2500$ and 500K. At 2500K, gravity sensitivity is essentially noticeable in hydride bands (CaH at 0.624 and 0.639 and FeH at 0.98 $\mu$m) while the oxide bands (TiO and VO) form too high in the atmosphere ( $\tau_{\rm std}=10^{-4}$) to be affected except in interband pseudo-continuum regions. Effects of gravity are however more extensive in the strength of atomic lines (essentially K I and Ti I) at the peak of the spectral distribution (1.05 to 1.3 $\mu$m), and in the red wing of the water vapor bands as well as in the CO bands at 2.3 to 2.4 $\mu$m. In the 500K case, H₂O bands are only moderately affected by the gravity change, while the optical continuum opacity, provided by the van der Waals wings of the Na I D and K I resonance doublets, is reduced by nearly a factor of 10 in the 500K in the low gravity case in response to the drop in pressure. The most interesting feature is the enhanced sensitivity of the K-band flux at 2.2 $\mu$m to gravity. As already pointed out by , this feature can be used to disentangle temperature, age and mass of a brown dwarf or planet independently. This trend is observed in the entire regime from 1500K to 300K, and provides a useful tool in the analysis of free floating methane dwarfs such as discovered recently by and . The CH₄ and CO bands are also sensitive to gravity in this regime.

One more gravity indicator should be the slope and height of the Z-band flux between the core of the K I resonance doublet at $\lambda$7687,7701Å to 1.1 $\mu$m compared to the height of the J-band flux peak. However, for the reasons mentioned above, it is difficult to quantify this effect on the basis of the present models.

Surface gravity effects have also been explored for the fully dusty case (AMES-Dusty models) at ${\rm T}_{\rm eff}= 2000$ and 1500K (see Figures and 18). In the 2000K case, the pseudo-continuum formed by saturated bands of TiO bluewards of 0.75 $\mu$m is fainter and flatter at reduce gravity. This is a result of the cooler temperatures prevailing in the outskirts of the photosphere at reduced gravity. This explains and supports the conclusions of who noticed a similar trend comparing young red dwarfs of the Pleiades cluster to presumably older field M dwarfs. Previous M dwarf model atmospheres did not show such a sensitivity due to the overestimated blocking caused by straight mean and JOLA opacities. To longer wavelengths an important veiling provided by the dust covers the 0.7 to 1.3 $\mu$m region in the high gravity model. Atomic lines and molecular bands are generally quite sensitive to gravity change in this range, while the near-infrared water vapor bands are also affected markedly (much more than in the grainless 2500K case discussed above). Especially, collision induced H₂ absorption cutting the flux in a negative slope at 2.2-2.3 $\mu$m, as well as the CO bands redwards of this, makes again the shape of the K band spectrum an excellent gravity indicator. A similar behavior is observed at 1500K (Figure ), where however the dust is now so strong that most features, except H₂O band troughs, are no longer seen.