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

9 Sept 2005 Stellar Astro II : Brown Dwarfs.ppt2Summary Basic Properties Basic Properties Very Low Mass Stars (VLMs) Very Low Mass Stars (VLMs) Spectral Features of M,L,T Dwarfs Spectral Features of M,L,T Dwarfs Convection Convection H 2 Dissociation H 2 Dissociation Recognizing Brown Dwarfs Recognizing Brown Dwarfs Degeneracy & the Minimum Mass for H- Burning Degeneracy & the Minimum Mass for H- Burning Lithium Depletion Lithium Depletion

9 Sept 2005 Stellar Astro II : Brown Dwarfs.ppt3 Basic Properties Spectral Classification System: 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.

9 Sept 2005 Stellar Astro II : Brown Dwarfs.ppt4 Basic Properties Brown Dwarf (BD) Characteristics: Brown Dwarf (BD) Characteristics: Not defined by spectral type, but includes late M,L,and T spectral types. Not defined by spectral type, but includes late M,L,and T spectral types. No central, stable H fusion. No central, stable H fusion. Convection is the dominant form of energy transport. (M < 0.3M  ) Convection is the dominant form of energy transport. (M < 0.3M  ) Defined by mass (but hard to directly determine): Defined by mass (but hard to directly determine): 15M Jup < M BD < 0.08 M  15M Jup < M BD < 0.08 M  No central stable H fusion: not a star No central stable H fusion: not a star Burns deuterium: not a planet Burns deuterium: not a planet T eff < 2800 K (M6 spectral type), the presence of lithium proves that they are substellar. T eff < 2800 K (M6 spectral type), the presence of lithium proves that they are substellar. Short luminous lifetimes. Short luminous lifetimes. Candidate for baryonic DM. Candidate for baryonic DM. L BD < L . L BD < L . R BD ~ R Jup (Degeneracy). R BD ~ R Jup (Degeneracy).

9 Sept 2005 Stellar Astro II : Brown Dwarfs.ppt5 Basic Properties

9 Sept 2005 Stellar Astro II : Brown Dwarfs.ppt6 Basic Properties Substellar-mass objects were first theorized by Kumar,S.S. 1963, ApJ, 137, Substellar-mass objects were first theorized by Kumar,S.S. 1963, ApJ, 137, Objects renamed BDs by Tarter J.C., 1974, PhD Thesis, Cal- Berkeley. Objects renamed BDs by Tarter J.C., 1974, PhD Thesis, Cal- Berkeley. Very few potential BDs few observed prior to 2MASS, SDSS, & other surveys in the mid 1990’s. Wien’s Displacement Law: Very few potential BDs few observed prior to 2MASS, SDSS, & other surveys in the mid 1990’s. Wien’s Displacement Law: max (  m) = 2898 / T BB (K) max (  m) = 2898 / T BB (K)

9 Sept 2005 Stellar Astro II : Brown Dwarfs.ppt7 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 & K I T: Exhibit H 2 0 & CH 4 absorption bands

9 Sept 2005 Stellar Astro II : Brown Dwarfs.ppt8 Very Low Mass Stars

9 Sept 2005 Stellar Astro II : Brown Dwarfs.ppt9 Very Low Mass Stars T Dwarfs: T Dwarfs: T eff < 1200 K T eff < 1200 K Methane absorption similar to Jupiter: causes a bluer color- see next slide… Methane absorption similar to Jupiter: causes a bluer color- see next slide…

9 Sept 2005 Stellar Astro II : Brown Dwarfs.ppt10 Very Low Mass Stars Expect dwarfs to get redder for late spectral types. Expect dwarfs to get redder for late spectral types. This trend is seen, except for the T class. This trend is seen, except for the T class. Methane absorption bands lower observed flux in the K s NIR band. Methane absorption bands lower observed flux in the K s NIR band.

9 Sept 2005 Stellar Astro II : Brown Dwarfs.ppt11 Very Low Mass Stars In very low mass (VLM) objects (stars & BDs), convection, not radiation, is the dominant form of energy transport. In very low mass (VLM) objects (stars & BDs), convection, not radiation, is the dominant form of energy transport. For VLMs below 0.3 M , the objects are fully convective. 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: Convective stability occurs when the adiabatic temp. gradient is less than that for radiation:  radiation >  adiabatic  adiabatic = (  - 1)/   = 1 + R g /C v  adiabatic = (  - 1)/   = 1 + R g /C v Temperatures are low enough such that H 2 can form. A larger amount of energy is needed to raise the temperature due to subsequent H 2 dissociation: The specific heat rises significantly. Temperatures are low enough such that H 2 can form. A larger amount of energy is needed to raise the temperature due to subsequent H 2 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 T eff. Change in slope of HR? 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 T eff. Change in slope of HR?

9 Sept 2005 Stellar Astro II : Brown Dwarfs.ppt12 Very Low Mass Stars More H 2 dissoc. Means larger T eff means bluer color? More H 2 dissoc. Means larger T eff means bluer color? Note change in slope – H 2

9 Sept 2005 Stellar Astro II : Brown Dwarfs.ppt13 Recognizing a Brown Dwarf As a protostar collapses, core temperature rises. 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. 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) 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). 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.

9 Sept 2005 Stellar Astro II : Brown Dwarfs.ppt14 Recognizing a Brown Dwarf Solid: boundaries Solid: boundaries Dotted:.5 M  Dotted:.5 M  Dash:.085 M  Dash:.085 M  Dot-dash: 0.05 M  Dot-dash: 0.05 M  Degeneracy becomes increasingly important for decreasing mass!! Degeneracy becomes increasingly important for decreasing mass!!

9 Sept 2005 Stellar Astro II : Brown Dwarfs.ppt15 Recognizing a Brown Dwarf VLM objects evolve along 4 possible paths: H fuses at 3x10 6 K. VLM objects evolve along 4 possible paths: H fuses at 3x10 6 K. 1. H fusion begins and is sustained: M > 0.09 M . No degeneracy. Stable low mass star is produced. 2. Degeneracy reduces the temperature, but still sustains fusion: M > 0.08 M . Stable low mass star is produced. 3. Fusion begins, but degeneracy lowers temperature: transition object M ~ M  4. 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 L nuc /L tot never exceeds 50%. Stellar mass limit somewhere between transition object and brown dwarf. This is arbitrarily placed at a mass where L nuc /L tot never exceeds 50%.

9 Sept 2005 Stellar Astro II : Brown Dwarfs.ppt16 Recognizing a Brown Dwarf M HBL = M 

9 Sept 2005 Stellar Astro II : Brown Dwarfs.ppt17 Recognizing a Brown Dwarf Deuterium burning Deuterium burning Hydrogen burning Hydrogen burning At a given luminosity, it is hard to distinguish between young brown dwarfs and older stars At a given luminosity, it is hard to distinguish between young brown dwarfs and older stars

9 Sept 2005 Stellar Astro II : Brown Dwarfs.ppt18 Recognizing a Brown Dwarf Lithium (Li) is a Big Bang nucleosynthetic product, so every VLM object contains Li at the beginning of its life. 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. 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 T crit = 3x10 6 K, corresponding to a minimum mass of 0.06 M . Thus, Li is quickly destroyed in dwarfs whose mass exceeds 0.06 M . The minimum core temperature for Li to burn is T crit = 3x10 6 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. Therefore, VLM objects with detectable Li absorption must NOT be undergoing fusion, and are therefore classified as Brown Dwarfs.

9 Sept 2005 Stellar Astro II : Brown Dwarfs.ppt19 Recognizing a Brown Dwarf

9 Sept 2005 Stellar Astro II : Brown Dwarfs.ppt20 Recognizing a Brown Dwarf Presence of Li absorption (6708 Å lithium doublet) means the mass is below ~ 0.06 M . Thus, no fusion. Presence of Li absorption (6708 Å 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. For spectral types later than M6, this is conclusive evidence of a BD.

9 Sept 2005 Stellar Astro II : Brown Dwarfs.ppt21References New Light of Dark Stars, I. Neill Reid and Suzanne L. Hawley, Springer (and references therein) New Light of Dark Stars, I. Neill Reid and Suzanne L. Hawley, Springer (and references therein) Kirkpatrick, Reid, Liebert, ApJ 519, 802, 1999 Kirkpatrick, Reid, Liebert, ApJ 519, 802, 1999