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Brown Dwarfs
Daniel W. Kittell
Stellar Astrophysics II: Stellar Interiors
September 9, 2005
Summary
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Basic Properties
Very Low Mass Stars (VLMs)
Spectral Features of M,L,T Dwarfs
 Convection
 H2 Dissociation
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Recognizing Brown Dwarfs
Degeneracy & the Minimum Mass for HBurning
 Lithium Depletion
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Basic Properties
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Spectral Classification System:
One
Boy
And
Five
Giant
Killer
Monkeys
Left
Toledo
Old *
Boring *
Astronomers *
Feel *
Greatly *
Knowledgeable *
Making *
Ludicrous *
Tests *
* The Statements in this presentation do not necessarily reflect
those of the presenter. Any resemblance of, or likeness to, any
person, real or imagined, is purely coincidental.
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Basic Properties
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Brown Dwarf (BD) Characteristics:
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Not defined by spectral type, but includes late M,L,and T spectral
types.
No central, stable H fusion.
Convection is the dominant form of energy transport.
(M < 0.3M)
Defined by mass (but hard to directly determine):
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15MJup < MBD < 0.08 M
No central stable H fusion: not a star
Burns deuterium: not a planet
Teff < 2800 K (M6 spectral type), the presence of lithium proves
that they are substellar.
Short luminous lifetimes.
Candidate for baryonic DM.
LBD < 10-4 L .
RBD ~ RJup (Degeneracy).
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Basic Properties
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Basic Properties
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Substellar-mass objects were first theorized by Kumar,S.S. 1963,
ApJ, 137, 1121.
Objects renamed BDs by Tarter J.C., 1974, PhD Thesis, CalBerkeley.
Very few potential BDs few observed prior to 2MASS, SDSS, &
other surveys in the mid 1990’s. Wien’s Displacement Law:
max(m) = 2898 / TBB(K)
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Very Low Mass Stars
L: Weakening bands of
metallic oxides - TiO &
VO (these are dominant
in M dwarfs)
L: Strengthening bands of
metallic hydrides-CrH &
FeH; and alkali metals-Na
I&KI
T: Exhibit H20 & CH4
absorption bands
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Very Low Mass Stars
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Very Low Mass Stars
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T Dwarfs:
Teff < 1200 K
 Methane
absorption similar
to Jupiter: causes
a bluer color- see
next slide…
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QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
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Very Low Mass Stars
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Expect dwarfs to get redder
for late spectral types.
This trend is seen, except for
the T class.
Methane absorption bands
lower observed flux in the Ks
NIR band.
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Very Low Mass Stars
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In very low mass (VLM) objects (stars & BDs), convection, not radiation, is
the dominant form of energy transport.
dT(r)
3

F(r)
3
dr
4acT
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dT(r)
1 T dP
 (1 )
dr
 P dr
For VLMs below 0.3 M, the objects are fully convective.
Convective stability occurs when the adiabatic temp. gradient is less than that for
radiation:


radiation > adiabatic
adiabatic = ( - 1)/ 
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 = 1 + Rg/Cv
Temperatures are low enough such that H2 can form. A larger amount of energy
is needed to raise the temperature due to subsequent H2 dissociation: The
specific heat rises significantly.
A small temp gradient allows the pressure to increase. In the case of convection,
this allows for more efficient energy transport to the envelope thus increasing
luminosity and Teff. Change in slope of HR?
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Very Low Mass Stars
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More H2 dissoc. Means larger Teff means
bluer color?
Note change
in slope – H2
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Recognizing a Brown Dwarf
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As a protostar collapses, core temperature rises.
Low mass stars must collapse to higher densities: Without fusion to
support the collapse, the low mass stars attain higher densities.
As density increases, core becomes partially degenerate: An increasing
fraction of energy from collapse goes into compressing degenerate gas.
(Mass X Volume = Constant)
Degeneracy stops star from collapsing below 0.1 R (and the core
temperature can’t get any higher than this).
Smaller Mass  Smaller Radius
Smaller Mass  Larger Radius
At 0.1 M, Electron Degeneracy
becomes the dominant source of
pressure support.
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Recognizing a Brown Dwarf
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Solid: boundaries
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Dotted: .5 M
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Dash: .085 M
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Dot-dash: 0.05 M
Degeneracy
becomes
increasingly
important for
decreasing mass!!

QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
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Recognizing a Brown Dwarf
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VLM objects evolve along 4 possible paths: H fuses at 3x106K.
1. H fusion begins and is sustained: M > 0.09 M. No degeneracy.
2.
3.
4.
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Stable low mass star is produced.
Degeneracy reduces the temperature, but still sustains fusion: M > 0.08
M. Stable low mass star is produced.
Fusion begins, but degeneracy lowers temperature: transition object
M ~ 0.075 M
Fusion never becomes a significant source of energy: M < 0.07 M. BD
is produced.
Stellar mass limit somewhere between transition object and brown
dwarf. This is arbitrarily placed at a mass where Lnuc/Ltot never
exceeds 50%.
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Recognizing a Brown Dwarf
MHBL = 0.073 M
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Recognizing a Brown Dwarf
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Deuterium
burning
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Hydrogen
burning
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At a given
luminosity,
it is hard to
distinguish
between
young
brown
dwarfs and
older stars
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Recognizing a Brown Dwarf
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Lithium (Li) is a Big Bang nucleosynthetic product, so every VLM
object contains Li at the beginning of its life.
As mentioned, objects below ~ 0.3 M are fully convective: all of the
material is exposed to the hottest temperatures at the core of the
object.
The minimum core temperature for Li to burn is Tcrit = 3x106 K,
corresponding to a minimum mass of 0.06 M. Thus, Li is quickly
destroyed in dwarfs whose mass exceeds 0.06 M.
Therefore, VLM objects with detectable Li absorption must NOT be
undergoing fusion, and are therefore classified as Brown Dwarfs.
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Recognizing a Brown Dwarf
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
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Recognizing a Brown Dwarf
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Presence of Li
absorption (6708Å
lithium doublet) means
the mass is below ~
0.06 M. Thus, no
fusion.
For spectral types later
than M6, this is
conclusive evidence of
a BD.
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References
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New Light of Dark Stars, I. Neill Reid and
Suzanne L. Hawley, Springer 2000. (and
references therein)
Kirkpatrick, Reid, Liebert, ApJ 519, 802, 1999
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