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Opacities and Chemical Equilibria for
Brown Dwarf and Extra-Solar Giant
Planet Models
Christopher M. Sharp
June 9, 2004
Definitions:
A brown dwarf is a substellar object below the sustained hydrogenburning 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
CH4/CO
NH3/N2
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.
Log absorption in cm2/molecule
H2O Opacity at 2000 K and 1 Atm
Wavenumber in cm-1
TiH Opacity at 2000 K and 1 Atm
Log absorption in cm2/molecule
A-X
Wavenumber in cm-1
B-X
KI
Na-D
Burgasser et al. 2001
Kirkpatrick et al. 1999
The Five Classes of EGPs
Class 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