An outline of the paper: The Brown Dwarf-Exoplanet Connection by Adam J. Burgasser
In my last post, I talked about how brown dwarfs and hot-Jupiters can have similar photospheric temperatures and that a cautious approach should be taken when comparing their atmospheres, due to how the hot Jupiters and brown dwarfs are heated differently. Even when the photospheric temperatures are comparable, their photospheric gas pressures can be quite different. The atmospheric pressure at the photosphere of hot Jupiters can be 1-2 orders of magnitude lower than that of brown dwarfs (not taking opacity effects into account). The different gravity explains most of this difference. Since brown dwarfs and Jupiters have a nearly constant radius, the surface gravity depends mainly on the total mass. Where exoplanets typically have surface gravities in the 10-30 m/s2 regime, it is not uncommon for brown dwarfs to have a surface gravity 10-100 times that.
The only time where we have an overlap in both temperature and pressure is between the youngest, lowest mass brown dwarfs and the dense hot-Jupiters such as HAT-P-2b and CoRoT-3b. This does not mean that the atmospheric composition and structure is similar however. Highly irradiated hot Jupiters have deep radiative envelopes, whilst brown dwarfs are fully convective throughout their photosphere. In the case of hot Jupiters, the external heating from the host star has the effect of flattening the temperature-pressure profile and can give rise to temperature inversions.
The different gas pressure has an effect on the atmospheric chemistry, as shown in the temperature-pressure diagram show below (from Lodders & Fegley (2006)):
After hydrogen, which is the most abundant element in the atmospheres of both exoplanets and brown dwarfs, we have C, N, and O each playing an important role as the most chemically reactive elements. Take for instance the equilibrium reaction:
CO + 3H2 ↔ CH4+H2O
The ratio of methane to CO in the atmosphere strongly affects the observed photospheric spectrum and also has a fundamental effect on the stability of other gases and condensates in the photosphere. To give an example, the destruction of CO, which happens at the L/T transition for brown dwarfs, causes the abundance of CH4 to increase, thereby changing the observed spectra in the infra-red due to the fact that CH4 is the second strongest absober after H2O. In fact, the L/T type brown dwarf transition is determined by the appearance of abundant methane bands which appear in the spectrum at temperatures below ~1400 K for typical brown dwarf photospheric pressures of 1-10 bars. The T7 brown dwarf Gl229 B for instance, shown in the figure above, has a methane-dominated photosphere at an effective temperature of 960K. Incidently, Gl229B was one of the first brown dwarfs discovered and the first T-dwarf to be discovered (Nakajima et al. 1995).
L-type brown dwarfs typically show weaker TiO and VO bands, compared to M-dwarfs with the alkali, metal hydride and H2O aboption features becoming more prominent. Globally they display a spectral energy distribution which peaks more towards the red optical/NIR spectral, compared to M-dwarfs. The spectra of T-type brown dwarfs are distinguished by the presence of CH4 absoprtion in the near infrared, as well as strong H2O bands and collision induced H2 absorption. For the subdivisions of the spectral classes, spectral standards are used i.e. Burgasser et. al 2006.