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)

http://vmsstreamer1.fnal.gov/VMS_Site_02/Lectures/Colloquium/Presentations/Burgasser.ppt

L and T Dwarfs

Cloudy with a Chance of

Iron…Clouds and Weather on

Brown Dwarfs

Adam BurgasserUCLA

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 – H2

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

– H2 formation also important (change in slope of main seq. at 0.5 MSun)

• 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:PPI chain:p + p → d + ep + p → d + e+ + + + e, e, TTcc = 3 = 3 10 1066 KK

Kumar (1963)

Below ~0.1 M, e- degeneracy becomes significant in interior (Pcore ~ 105 Mbar, Tcore ~ TFermi) and will inhibit collapse.

Below ~ 0.075 M, Tcore remains below critical PPI temperature Cannot sustain core H fusion.

Brown Dwarfs

With no fusion source, Brown dwarfs rapidly evolve to lower Teff and lower

luminosities.

10 20 30 40 5060

70

75

80

90

Stars

BDs

“… cool off inexorably like dying embers plucked from a fire.”

A. Burrows

Brown Dwarfs

Some Brown Dwarf Properties

• Interior conditions: ρcore ~ 10-1000 g/cm3, Tcore ~ 104-106

K, Pcore ~ 105 Mbar, fully convective, largely degenerate

(~90% of volume), predominantly metallic H (exotic?).

• Atmosphere conditions: Pphot ~ 1-10 bar, Tphot ~ 3000 K

and lower.

• All evolved brown dwarfs have R ~ 1 RJupiter.

• Age/Mass degeneracy: old, massive BDs have same Teff,

L as young, low-mass BDs.

• Below Teff ~ 1800 K, all objects are substellar.

• NBD ~ N*, MBD ~ 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 MJeans.

• 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).

10 20 30 40 50

60

70

75

80

90 Three spectral classes encompass Brown Dwarfs:

M dwarfs (3800-2100 K): Young BDs and low-mass stars.

L dwarfs (2100-1300 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, H2O, CO absorption plus metal/alkali lines.

L dwarfs replace oxides with hydrides (FeH, CrH, MgH, CaH) and alkalis are prominent.

T dwarfs exhibit strong CH4 and H2O 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

– H2O, CO in the IR

– ~109 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 H2-H2 is important in the IR (longer than 1 micron)

• H2 molecules have allowed transitions only at electric quadrupole and higher order moments, so H2 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~1500-1200) CO is depleted by methane formation (CH3) – 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 → TiO2(s), CaTiO3(s) • VO → VO(s)• Fe → Fe(l)• SiO → SiO2(s), MgSiO3(s)

Evidence for Condensation - Spectroscopy

Kirkpatrick et al. (1999)

• Relatively weak H2O 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

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)

CondensateClouds

Clouds are not uniform!

IRTF NSFCam 1995 July 26

c.f., Westphal, Matthews, & Terrile (1974)

At 5 m, holes in Jupiter’s NH3 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 1m 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