With contributions from: Andy Ackerman & Mark Marley (NASA Ames) Didier Saumon (Los Alamos NL) J. Davy Kirkpatrick(Caltech/IPAC) Katharina Lodders (Washington University) See also: Davy Kirkpatrick’s Annual Review article, 2005 New Light on Dark Stars by Neill Reid and other sources cited Based on a colloquium by Adam Burgasser (MIT) /Presentations/Burgasser.ppt L and T Dwarfs

Cloudy with a Chance of Iron … Clouds and Weather on Brown Dwarfs Adam Burgasser UCLA

What are Brown Dwarfs? “Failed stars” : objects that form like stars but have insufficient mass to sustain H fusion. “Super-Jupiters” : objects with similar size and atmospheric constituents as giant planets, but form as stars.

A Little History Substantial effort in ’80s and early ’90s to find very low mass M dwarfs Parallax surveys of high proper motion red objects Companions to M dwarfs, WDs (IR excesses) Companion to vB8 – NOT Companion to G29-38 – NOT Companion to G165B – YES! the first L dwarf Spectrum not understood until more found Gl 229B the first T dwarf IR Colors surprisingly blue Note change in slope – H 2

Brown Dwarfs Abound! Many L and T dwarfs have now been found –Improved IR detectors –Better spatial resolution (seeing improvements, AO) –IR and multi-color surveys (2MASS, DENIS, and Sloan) –Breakthrough in understanding appearance of spectra Significant progress in modeling low mass stellar and substellar objects Understood in the late ’50s (Limber) that – low mass stars must be fully convective –Electron degeneracy must play a role –H 2 formation also important (change in slope of main seq. at 0.5 M Sun ) Kumar figured out (in the early ’60s) that a minimum mass is needed for H burning Grossman et al. included deuterium burning (early ’70s) Recent improvements include better equation of state and grain formation

Hayashi (1965) 1.Adiabatic contraction (Hayashi tracks) 2.Ignition, formation of radiative core, heating – dynamic equilibrium (Henyey tracks) 3.Settle onto Hydrogen main sequence – radiative equilibrium Stellar evolution Brown Dwarfs (1) (2) (3)

PPI chain: p + p → d + e + + e, T c = 3  10 6 K Kumar (1963) Below ~0.1 M , e - degeneracy becomes significant in interior (P core ~ 10 5 Mbar, T core ~ T Fermi ) and will inhibit collapse. Below ~ M , T core remains below critical PPI temperature  Cannot sustain core H fusion. Brown Dwarfs

With no fusion source, Brown dwarfs rapidly evolve to lower T eff and lower luminosities Stars BDs “… cool off inexorably like dying embers plucked from a fire.” A. Burrows Brown Dwarfs

Some Brown Dwarf Properties Interior conditions: ρ core ~ g/cm 3, T core ~ K, P core ~ 10 5 Mbar, fully convective, largely degenerate (~90% of volume), predominantly metallic H (exotic?). Atmosphere conditions: P phot ~ 1-10 bar, T phot ~ 3000 K and lower. All evolved brown dwarfs have R ~ 1 R Jupiter. Age/Mass degeneracy: old, massive BDs have same T eff, L as young, low-mass BDs. Below T eff ~ 1800 K, all objects are substellar. N BD ~ N *, M BD ~ 0.15 M *

Why Brown Dwarfs Matter Former dark matter candidates - no longer the case. Important and populous members of the Solar Neighborhood. End case of star formation, test of formation scenarios at/below M Jeans. Tracers of star formation history and chemical evolution in the Galaxy. Analogues to Extra-solar Giant Planets (EGPs), more easily studied. Last source of stars in distant future of non-collapsing Universe - Adams & Laughlin (RvMP, 69, 337, 1997).

Three spectral classes encompass Brown Dwarfs: M dwarfs ( K): Young BDs and low-mass stars. L dwarfs ( K): BDs and very low-mass, old stars. T dwarfs (< 1300 K): All BDs; coolest objects known. M, L, and T dwarfs

M Dwarf Spectral Types Molecular species switch from MgH to TiO CaOH appears in later M dwarfs Prominent Na D lines Spectral types determined in the blue

Later Spectral Classes TiO disappears to be replaced by water, metal hydrides (FeH, CrH) Alkali metal lines strengthen (note K I in the L8 dwarf) Spectral types determined from red, far red spectra (blue too faint!)

L-type Spectral Sequence K I line strength increases with later spectral type Li I appears in some low mass stars (m < 0.06 solar masses) Appearance of FeH, CrH Strength of Ca I Strength of water Disappearance of TiO Absence of FeH, CrH in T dwarf, much increased strength of water

Lithium in Brown Dwarfs Li I appears in about a third of L dwarfs EQW from 1.5 to 15 Angstroms Li I can be used to distinguish between old, cooled brown dwarfs and younger, lower mass dwarfs

Evolution of Lithium At a given Teff,Stars with Li are lower mass than stars with Li depleted.

M dwarfs are dominated by TiO, VO, H 2 O, CO absorption plus metal/alkali lines. L dwarfs replace oxides with hydrides (FeH, CrH, MgH, CaH) and alkalis are prominent. T dwarfs exhibit strong CH 4 and H 2 O and extremely broadened Na I and K I. M, L, and T Dwarfs in the IR

Alkali Lines Alkali lines very prominent in L dwarf spectra (Li, Na, K, Cs, Rb) Strong because of very low optical opacities –TiO, VO are gone –Dust formation also removes primary electron donors, so H - and H2 - opacities are also reduced –High column density due to low optical opacity leads to very strong lines K I lines at 7665 and 7699 A have EQWs of several hundred Angstroms Na D lines also become very strong

Stellar Models General assumptions include –Plane parallel geometry –Homogeneous layers –LTE Surface gravities: log g ~ 5.0 Convection using mixing length Convection is important even at low optical depth (  <0.01) Strength of water absorption depends on detailed temperature structure and treatment of convection For Teff < 3000 K, grains become important in atmospheric structure (scattering)

Opacities Bound-bound opacities – molecules –TiO, CaH + other oxides & hydrides in the optical –H 2 O, CO in the IR –~10 9 lines! –Bound-bound molecular line opacities dominate the spectrum Bound-free opacities –Atomic ionization, molecular dissociation Free-free opacities – Thomson and Rayleigh scattering In metal-poor low mass stars, pressure induced absorption of H 2 -H 2 is important in the IR (longer than 1 micron) H 2 molecules have allowed transitions only at electric quadrupole and higher order moments, so H 2 itself is not significant Also significant van der Waals collisional (pressure) broadening of atomic and molecular lines, making these lines much stronger than they would otherwise be At even cooler temperatures (T~ ) CO is depleted by methane formation (CH 3 ) – the transition from L to T dwarfs

Dust and Clouds in Brown Dwarfs Cool brown dwarf atmospheres have the right conditions to form condensates or dust. Observations support the idea that these condensates form cloud structures. Cloud structures are probably not uniform, likely disrupted by atmospheric turbulence. Clouds have significant effects on the spectral energy distributions of these objects and analogues (e.g., Extra-solar giant planets).

Condensation in BD Atmospheres Marley et al. (2002) At the atmospheric temperatures and pressures of late-M and L dwarfs, many gaseous species are capable of forming condensates. e.g.: TiO → TiO 2 (s), CaTiO 3 (s) VO → VO(s) Fe → Fe(l) SiO → SiO 2 (s), MgSiO 3 (s)

Evidence for Condensation - Spectroscopy Kirkpatrick et al. (1999) Relatively weak H 2 O bands in NIR compared to models require additional smooth opacity source. The disappearance of TiO and VO from late-M to L can be directly attributed to their accumulation onto condensate species.

Gliese 229B Evidence for Condensation - Photometry Chabrier et al. (2000) The NIR colors of late-type M and L dwarfs are progressively redder – can only be matched by models that allow dust formation in their atmospheres. However, bluer colors of T dwarfs require a transparent atmosphere – dust must be removed. Dusty Cond

Burrows et al. (2002) T L Without the rainout of dust species, Na and K would form Feldspars and atomic species would be depleted in the late L dwarfs. Evidence for Rainout - Abundances

Burrows et al. (2002) T L With rainout, Na and K persist well into the T dwarf regime.

Burgasser et al. (2002) Evidence for Rainout - Abundances K I (and Na I) absorption is clearly present in the T dwarfs  dust species must be removed from photosphere.

Cloudy Models for BD Atmospheres Condensate clouds dominate visual appearance and spectrum of every Solar giant planet – likely important for brown dwarfs. Condensates in planetary atmospheres are generally found in cloud structures. Requires self-consistent treatment of condensable particle formation, growth, and sedimentation. Ackerman & Marley (2001); Marley et al. (2002); Tsuji (2002); Cooper et al. (2003); Helling et al. (2001); Woitke & Helling (2003)

Condensate Clouds Clouds are not uniform!

IRTF NSFCam 1995 July 26 c.f., Westphal, Matthews, & Terrile (1974) At 5  m, holes in Jupiter’s NH 3 clouds produce “Hot Spots” that dominate emergent flux  horizontal structure important!

Enoch, Brown, & Burgasser (2003) Evidence for Cloud Disruption - Variability Many late-type L and T dwarfs are variable, P ~ hours, similar to dust formation rate. Atmospheres too cold to maintain magnetic spots  clouds likely. Periods are not generally stable  rapid surface evolution.

Burgasser et al. (2002) Strengthening of K I higher-order lines around 1  m  reduced opacity at these wavelengths from late L to T. Evidence for Cloud Disruption - Spectroscopy

Burgasser et al. (2002) Reappearance of condensate species progenitors (e.g., FeH)  detected below cloud deck. Evidence for Cloud Disruption - Spectroscopy

Presence of CO in Gliese 229B’s atmosphere 16,000x LTE abundance  upwelling convective motion. Oppenheimer et al. (1998) Evidence for Cloud Disruption - Spectroscopy

A Partly Cloudy Model for BD Atmospheres An exploratory model. Linear interpolation of fluxes and P/T profiles of cloudy and clear atmospheric models. New parameter is cloud coverage percentage (0-100%). Burgasser et al. (2002), ApJ, 571, L151

The Transition L → T Dramatic shift in NIR color (ΔJ-K ~ 2). Dramatic change in spectral morphology. Loss of condensates from the photosphere. Objects brighten at 1  m. Apparently narrow temperature range: Gl 584C (L8) ~ 1300 K 2MASS 0559 (T5) ~ 1200 K.

Burgasser et al. (2002) Success…? Cloud disruption allows transition to brighter T dwarfs. Requires very rapid rainout at L/T transition, around 1200 K. Data fits, model is physically motivated, but is it a unique solution?

Cooler Than T Dwarfs… Proposed spectral class for ultra-cool dwarfs - Y stars None yet discovered Cooler than 770K (the coolest subclass of T dwarf) Not clear (yet) whether the atmospheric chemistry will change enough to warrant a new spectral class May be discovered with the next generation of deep IR surveys –not detected with DENIS (K<16.5) or 2MASS (K<15.8) –May be detected with UKIRT LAS (J<19.7) and UDS (J<24) –These surveys will also find many more L & T dwarfs