cloudy with a chance of iron …

Cloudy with a Chance of

Iron…Clouds and Weather on

Brown Dwarfs

Adam BurgasserUCLA

J. Davy Kirkpatrick

Caltech/IPAC Katharina Lodders

Washington University

Andy Ackerman

& Mark MarleyNASA Ames

Didier SaumonLos Alamos NL

Adam BurgasserUCLA

Fermilab Colloquium, 6 August 2003

Summary (i.e., what I’ll try to convince you of!)

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).

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.


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 5060

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 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


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



Burrows et al. (2002)

T L

With rainout, Na and K persist well into the T dwarf regime.


Burgasser et al. (2002)


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)


Basics of the Cloudy Model

Simple treatment: assume transport of dust by diffusion and gravitational settling.

Horizontal homogeneity.No chemical mixing between clouds.

-κ (dqt/dz) – frain w* qcond = 0

qt = qcond + qvapor

eddy diffusioncoefficient

sedimentationefficiency convective

velocity scale


What is frain?

If L, qc/qt constant, scale height:

frain ~ 0 “dusty” atmosphere.

frain → ∞ “clear” atmosphere.

Earth: frain ~ 0.5 (stratocumulus) –

4 (cumulus).

Jupiter: frain ~ 1-3 (NH3 clouds).

qt(z) = q0 exp(- frain [qc/qt] [w*/κ] z)


Ackerman & Marley (2001)

frain determines extent of cloud, particle size distribution, and hence cloud opacity.

What is frain?




The cloud layer is generally confined to a narrow range of temperatures for cooler BDs, cloud will reside below the photosphere.




Condensate cloud may or may not influence spectrum of brown dwarf depending on its temperature – explains disappearance of dust in T dwarfs.

L5

L8

T5


cloudy, frain= 3


clear

•Accurately predicts M/L dwarf colors down to latest-type L dwarfs.

•Matches turnover in near-infrared colors in T dwarfs.

•Cannot explain J-band brightening across L/T transition.

Cloudy Model Results

dusty


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.

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!


Helling et al. (2001)

2D models of dust formation in BD atmospheres predict patchiness due to turbulence and rapid accumulation of condensate material.

Evidence for Cloud Disruption - Theory

Num

ber

densi

ty

Mean p

article

size


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.



Strengthening of K I higher-order lines around 1m reduced opacity at these wavelengths from late L to T.

Evidence for Cloud Disruption - Spectroscopy



Reappearance of condensate species progenitors (e.g., FeH) detected below cloud deck.



Presence of CO in Gliese 229B’s atmosphere 16,000x LTE abundance upwelling convective motion.

Oppenheimer et al. (1998)



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


Wavelength Matters!

FeH K I

I J Kz

1400 K

Relative brightening at z and J (~1 m)can be explained by holes in the clouds.



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?


Arguments Against the Model

Small numbers of objects with parallaxes, could be a statistical fluke. Recent parallaxes for 10-20 late-L/early-T show identical

trends – brightening is real.

Early T dwarfs could be young, late L dwarfs old. Fairly tight trend, some T dwarf companions are known to

be old, some late L dwarf companions known to be young.

May indicate different sedimentation efficiencies in different objects. Fit for L dwarfs is excellent for frain = 3, would require a

rapid shift in atmospheric dynamics – partial clouding is simpler.


Showman & Guillot (2002)

Extrasolar Planet Weather?

• 3D Hydrodynamic models of hot EGP atmospheres produce vertical winds/structure.

• Weak Na I in HD 209458b – high clouds?

• Presence of clouds affects detectability of EGPs.

Charbonneau et al. (2002)


More Work is Needed!! More data across L/T transition needed – new

discoveries (SDSS, 2MASS), distance measurements (USNO), better photometry.

Development of a fully self-consistent model – convective motions, cloud disruption – can be drawn from terrestrial/Jovian studies.

What are the cloud structures - Bands? Spots?

How do rotation, composition, age influence transition?

cloudy with a chance of iron …

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