Brown Dwarfs Daniel W. Kittell Stellar Astrophysics II: Stellar Interiors September 9, 2005

Brown DwarfsBrown Dwarfs

Daniel W. KittellDaniel W. Kittell

Stellar Astrophysics II: Stellar Stellar Astrophysics II: Stellar InteriorsInteriors

September 9, 2005September 9, 2005

9 Sept 2005 Stellar Astro II : Brown Dwarfs.ppt 2

SummarySummary Basic PropertiesBasic Properties Very Low Mass Stars (VLMs)Very Low Mass Stars (VLMs)

Spectral Features of M,L,T DwarfsSpectral Features of M,L,T Dwarfs ConvectionConvection HH22 Dissociation Dissociation

Recognizing Brown DwarfsRecognizing Brown Dwarfs Degeneracy & the Minimum Mass for Degeneracy & the Minimum Mass for

H-BurningH-Burning Lithium DepletionLithium Depletion


Basic PropertiesBasic Properties Spectral Classification System:Spectral Classification System:

OOne ne

BBoy oy

AAnd nd

FFive ive

GGiantiant

KKilleriller

MMonkeys onkeys

LLeft eft

TToledooledo

OOld ld **

BBoring oring **

AAstronomers stronomers **

FFeel eel **

GGreatly reatly **

KKnowledgeable nowledgeable **

MMakingaking **

LLudicrous udicrous **

TTests ests **

** The Statements in this presentation do not necessarily The Statements in this presentation do not necessarily reflect those of the presenter. Any resemblance of, or reflect those of the presenter. Any resemblance of, or likeness to, any person, real or imagined, is purely likeness to, any person, real or imagined, is purely coincidental.coincidental.


Basic PropertiesBasic Properties Brown Dwarf (BD) Characteristics: Brown Dwarf (BD) Characteristics:

Not defined by spectral type, but includes late M,L,and T Not defined by spectral type, but includes late M,L,and T spectral types.spectral types.

No central, stable H fusion.No central, stable H fusion. Convection is the dominant form of energy transport. Convection is the dominant form of energy transport.

(M < 0.3M(M < 0.3M) ) Defined by mass (but hard to directly determine):Defined by mass (but hard to directly determine):

15M15MJupJup < M < MBDBD < 0.08 M < 0.08 M

No central stable H fusion: not a starNo central stable H fusion: not a star Burns deuterium: not a planetBurns deuterium: not a planet

TTeffeff < 2800 K (M6 spectral type), < 2800 K (M6 spectral type), the presence of lithium the presence of lithium proves that they are substellar.proves that they are substellar.

Short luminous lifetimes.Short luminous lifetimes. Candidate for baryonic DM.Candidate for baryonic DM. LLBDBD < 10 < 10-4-4 L L . . RRBDBD ~ R ~ RJupJup (Degeneracy). (Degeneracy).


Basic PropertiesBasic Properties


Basic PropertiesBasic Properties Substellar-mass objects were first theorized by Kumar,S.S. Substellar-mass objects were first theorized by Kumar,S.S.

1963, ApJ, 1963, ApJ, 137137, 1121., 1121. Objects renamed BDs by Tarter J.C., 1974, PhD Thesis, Objects renamed BDs by Tarter J.C., 1974, PhD Thesis,

Cal-Berkeley.Cal-Berkeley. Very few potential BDs few observed prior to 2MASS, Very few potential BDs few observed prior to 2MASS,

SDSS, & other surveys in the mid 1990’s. Wien’s SDSS, & other surveys in the mid 1990’s. Wien’s Displacement Law:Displacement Law:

maxmax((m) = 2898 / Tm) = 2898 / TBBBB(K)(K)


Very Low Mass StarsVery Low Mass StarsL: Weakening bands of L: Weakening bands of metallic oxides - TiO & metallic oxides - TiO & VO (these are dominant VO (these are dominant in M dwarfs)in M dwarfs)

L: Strengthening bands of L: Strengthening bands of metallic hydrides-CrH & metallic hydrides-CrH & FeH; and alkali metals-Na FeH; and alkali metals-Na I & K II & K I

T: Exhibit HT: Exhibit H220 & 0 & CHCH44

absorption bandsabsorption bands


Very Low Mass StarsVery Low Mass Stars


Very Low Mass StarsVery Low Mass Stars

T Dwarfs:T Dwarfs: TTeffeff < 1200 K < 1200 K Methane Methane

absorption absorption similar to similar to Jupiter: causes Jupiter: causes a bluer color- a bluer color- see next slide…see next slide…

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.


Very Low Mass StarsVery Low Mass Stars Expect dwarfs to get Expect dwarfs to get

redder for late spectral redder for late spectral types.types.

This trend is seen, except This trend is seen, except for the T class.for the T class.

Methane absorption Methane absorption bands lower observed bands lower observed flux in the Kflux in the Kss NIR band. NIR band.


Very Low Mass StarsVery Low Mass Stars In very low mass (VLM) objects (stars & BDs), In very low mass (VLM) objects (stars & BDs), convectionconvection, not , not

radiation, is the dominant form of energy transport.radiation, is the dominant form of energy transport.

For VLMs below 0.3 MFor VLMs below 0.3 M, the objects are fully convective., the objects are fully convective. Convective stability occurs when the adiabatic temp. gradient is less Convective stability occurs when the adiabatic temp. gradient is less

than that for radiation:than that for radiation:

radiationradiation > > adiabaticadiabatic

adiabatic adiabatic = (= ( - 1)/ - 1)/ = 1 + R = 1 + Rgg/C/Cvv

Temperatures are low enough such that HTemperatures are low enough such that H22 can form. A larger amount can form. A larger amount of energy is needed to raise the temperature due to subsequent Hof energy is needed to raise the temperature due to subsequent H22 dissociation: The specific heat rises significantly.dissociation: The specific heat rises significantly.

A small temp gradient allows the pressure to increase. In the case of A small temp gradient allows the pressure to increase. In the case of convection, this allows for more efficient energy transport to the convection, this allows for more efficient energy transport to the envelope thus increasing luminosity and Tenvelope thus increasing luminosity and Teffeff. Change in slope of HR?. Change in slope of HR?

€

dT(r)

dr=

−3κ

4acT 3F(r)

€

dT(r)

dr= (1−

1

γ)T

P

dP

dr


Very Low Mass StarsVery Low Mass Stars More HMore H22 dissoc. Means larger T dissoc. Means larger Teffeff

means bluer color?means bluer color?

Note change in slope – H2


Recognizing a Brown Recognizing a Brown DwarfDwarf As a protostar collapses, core temperature rises.As a protostar collapses, core temperature rises.

Low mass stars must collapse to higher densities: Without Low mass stars must collapse to higher densities: Without fusion to support the collapse, the low mass stars attain higher fusion to support the collapse, the low mass stars attain higher densities.densities.

As density increases, core becomes partially degenerate: An As density increases, core becomes partially degenerate: An increasing fraction of energy from collapse goes into increasing fraction of energy from collapse goes into compressing degenerate gas. (Mass X Volume = Constant)compressing degenerate gas. (Mass X Volume = Constant)

Degeneracy stops star from collapsing below 0.1 RDegeneracy stops star from collapsing below 0.1 R (and the (and the core temperature can’t get any higher than this).core temperature can’t get any higher than this).

Smaller Mass Smaller Mass Smaller Radius Smaller Radius

Smaller Mass Smaller Mass Larger Radius Larger Radius

At 0.1 MAt 0.1 M, Electron Degeneracy , Electron Degeneracy becomes the dominant source of becomes the dominant source of pressure support.pressure support.


Recognizing a Brown Recognizing a Brown DwarfDwarf

Solid: boundariesSolid: boundaries Dotted: .5 Dotted: .5 MM

Dash: .085 Dash: .085 MM

Dot-dash: 0.05 Dot-dash: 0.05 MM

Degeneracy Degeneracy becomes becomes increasingly increasingly important for important for decreasing decreasing mass!!mass!!




Recognizing a Brown Recognizing a Brown DwarfDwarf VLM objects VLM objects evolveevolve along 4 possible paths: H fuses at along 4 possible paths: H fuses at

3x103x1066K.K.

1.1. H fusion begins and is sustained: M > 0.09 H fusion begins and is sustained: M > 0.09 MM. No . No degeneracy. degeneracy. Stable low mass star is produced.Stable low mass star is produced.

2.2. Degeneracy reduces the temperature, but still sustains fusion: Degeneracy reduces the temperature, but still sustains fusion: M > 0.08 M > 0.08 MM. . Stable low mass star is produced.Stable low mass star is produced.

3.3. Fusion begins, but degeneracy lowers temperature: Fusion begins, but degeneracy lowers temperature: transition transition objectobject M ~ 0.075 M ~ 0.075 MM

4.4. Fusion never becomes a significant source of energy: M < Fusion never becomes a significant source of energy: M < 0.07 0.07 MM. . BD is produced.BD is produced.

Stellar mass limit somewhere between transition object Stellar mass limit somewhere between transition object and brown dwarf. This is arbitrarily placed at a mass and brown dwarf. This is arbitrarily placed at a mass where where LLnucnuc/L/Ltottot never exceeds 50%.never exceeds 50%.



MMHBLHBL = 0.073 = 0.073 MM



Deuterium Deuterium burningburning

Hydrogen Hydrogen burningburning

At a given At a given luminosity, luminosity, it is hard to it is hard to distinguish distinguish between between young young brown brown dwarfs and dwarfs and older starsolder stars


Recognizing a Brown Recognizing a Brown DwarfDwarf Lithium (Li) is a Big Bang nucleosynthetic product, so Lithium (Li) is a Big Bang nucleosynthetic product, so

every VLM object contains Li at the beginning of its life.every VLM object contains Li at the beginning of its life. As mentioned, objects below ~ 0.3 MAs mentioned, objects below ~ 0.3 M are fully convective: are fully convective:

all of the material is exposed to the hottest temperatures all of the material is exposed to the hottest temperatures at the core of the object.at the core of the object.

The minimum core temperature for Li to burn is TThe minimum core temperature for Li to burn is Tcritcrit = = 3x103x1066 K, corresponding to a minimum mass of 0.06 M K, corresponding to a minimum mass of 0.06 M. . Thus, Li is quickly destroyed in dwarfs whose mass Thus, Li is quickly destroyed in dwarfs whose mass exceeds 0.06 Mexceeds 0.06 M..

Therefore, VLM objects with detectable Li absorption must Therefore, VLM objects with detectable Li absorption must NOT be undergoing fusion, and are therefore classified as NOT be undergoing fusion, and are therefore classified as Brown Dwarfs.Brown Dwarfs.







Presence of Li Presence of Li absorption (absorption (67086708ÅÅ lithium doublet) lithium doublet) means the mass is means the mass is below ~ 0.06 Mbelow ~ 0.06 M. . Thus, no fusion.Thus, no fusion.

For spectral types For spectral types later than M6, this later than M6, this is conclusive is conclusive evidence of a BD.evidence of a BD.


ReferencesReferences New Light of Dark StarsNew Light of Dark Stars, I. Neill Reid and , I. Neill Reid and

Suzanne L. Hawley, Springer 2000. (and Suzanne L. Hawley, Springer 2000. (and references therein)references therein)

Kirkpatrick, Reid, Liebert, ApJ Kirkpatrick, Reid, Liebert, ApJ 519519, 802, , 802, 19991999

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Brown Dwarfs Daniel W. Kittell Stellar Astrophysics II: Stellar Interiors September 9, 2005