Opacities and Chemical Equilibria forBrown Dwarf and Extra-Solar Giant

Planet Models

Christopher M. SharpJune 9, 2004

Definitions:A brown dwarf is a substellar object below the sustained hydrogen-burning limit of about 7.5% to 8.0% solar masses, and forms in a manner similar to stars by fragmentation of collapsing gas clouds.

An extra-solar giant planet is a giant planet like Jupiter in orbit around a star other than the sun, and forms in a protoplanetary disk around its parent star.

However, according to these definitions, a giant planet can be more massive than a brown dwarf. An alternative definition of a giant planet is an object below the deuterium burning limit of about 13 Jupiter masses, i.e. 1.3% solar masses.

1995 - A Watershed Year in Brown Dwarf and Extra-Solar Giant Planet Research

In that year the first brown dwarf was unambiguously confirmed - Gliese 229B.

In that year the first planet in orbit around a star similar to the sun was also unambiguously detected – 51 Peg b. Note that in 1992 planets were found in orbit around the pulsar PSR B1257+12.

In that year a whole new branch of astrophysics was opened up, as theoretical models could be tested against observations.

The Chemistry of BD and EGP Atmospheres

Because the temperatures are usually much lower than the coolest stellar atmospheres, the chemistry is much more complicated. A large number of polyatomic molecules are present, and condensates play a very important rôle.

In our calculations we use the Anders-Grevesse (1989) solar composition for the following 27 elements: H, He, Li, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Rb and Cs, resulting in about 300 gas-phase molecular species, and over 100 condensates.

M Dwarfs L Dwarfs T Dwarfs Jupiter

The CO/CH4 and N2/NH3 Equilibria

CO + 3H2 H2 O + CH4

N2 + 3H2 2NH3

NH3/N2

CH4/CO

The Opacity of BD and EGP Atmospheres

As a consequence of the chemistry being much more complicated than cool stellar atmospheres, the opacities are also much more complicated. A large number of diatomic and polyatomic species that may not be particularly important species in cool stellar atmospheres, or completely absent, can become very important sources of absorption, depending on the temperature and pressure.

Such species include H2O, CH4, NH3, various hydrides and chlorides. For many species data are poorly known or not available. Additionally, the condensates are an important source of scattering and absorption.

Surprisingly, the alkali elements, Li, Na, K, Rb and Cs in their monatomic gaseous phase can also be important and show very broad lines, especially Na and K.

This a consequence that for a range of temperatures and pressures, the atmosphere is relatively transparent, and these lines are formed deep in the atmosphere.

H2O Opacity at 2000 K and 1 Atm

Wavenumber in cm-1

Log

abs

orpt

ion

in c

m2/m

olec

ule

A-XB-X

TiH Opacity at 2000 K and 1 Atm

Wavenumber in cm-1

Log

abs

orpt

ion

in c

m2/m

olec

ule

Burgasser et al. 2001

Na-D

K I

Kirkpatrick et al. 1999

The Five Classes of EGPsClass I: The coolest with Teff <= 150 K, with NH3 clouds and

strong CH4 absorption. Examples: Jupiter and Saturn.

Class II: H2O class with H2O condensation and H2O and CH4 opacity. Examples: υ And d, 55 Cancri d and ε Eri b.

Class III: Too hot for H2O condensation so the outer atmosphere is transparent. Rotation-vibration molecular opacities are important. Examples: GJ 876 b and c.

Class IV: T >= 900 K and are at small orbital distances. Strong pressure broadened lines of Na and K. Example: 55 Cancri b.

Class V: Very hot “roasters” with distances of about 0.05 AU from the star. Clouds of Fe and silcates are present. Example: HD 209458b

Sudarsky, Burrows and Hubeny 2003.

Irradiated EGPs

Sudarsky, Burrows, and Hubeny 2003

Collaborators

Adam Burrows

David Sudarsky

Ivan Hubeny

William B. Hubbard

Johnathan I. Lunine

Richard Freedman