Cloudy with a Chance of Iron …
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Cloudy with a Chance of
Iron…Clouds and Weather on
Brown Dwarfs
Adam BurgasserUCLA
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J. Davy Kirkpatrick
Caltech/IPAC Katharina Lodders
Washington University
Andy Ackerman
& Mark MarleyNASA Ames
Didier SaumonLos Alamos NL
Adam BurgasserUCLA
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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).
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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.
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Fermilab Colloquium, 6 August 2003
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)
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Fermilab Colloquium, 6 August 2003
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
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Fermilab Colloquium, 6 August 2003
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
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Fermilab Colloquium, 6 August 2003
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*
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Fermilab Colloquium, 6 August 2003
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).
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Fermilab Colloquium, 6 August 2003
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
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Fermilab Colloquium, 6 August 2003
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
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Fermilab Colloquium, 6 August 2003
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)
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Fermilab Colloquium, 6 August 2003
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.
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Fermilab Colloquium, 6 August 2003
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
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Fermilab Colloquium, 6 August 2003
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
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Fermilab Colloquium, 6 August 2003
Evidence for Rainout - Abundances
Burrows et al. (2002)
T L
With rainout, Na and K persist well into the T dwarf regime.
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Fermilab Colloquium, 6 August 2003
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.
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Fermilab Colloquium, 6 August 2003
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)
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Fermilab Colloquium, 6 August 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
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Fermilab Colloquium, 6 August 2003
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)
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Fermilab Colloquium, 6 August 2003
Ackerman & Marley (2001)
frain determines extent of cloud, particle size distribution, and hence cloud opacity.
What is frain?
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Fermilab Colloquium, 6 August 2003
Ackerman & Marley (2001)
Basics of the Cloudy Model
The cloud layer is generally confined to a narrow range of temperatures for cooler BDs, cloud will reside below the photosphere.
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Fermilab Colloquium, 6 August 2003
Basics of the Cloudy Model
Ackerman & Marley (2001)
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
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Fermilab Colloquium, 6 August 2003
cloudy, frain= 3
Burgasser et al. (2002)
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
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Fermilab Colloquium, 6 August 2003
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.
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CondensateClouds
Clouds are not uniform!
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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!
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Fermilab Colloquium, 6 August 2003
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
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Fermilab Colloquium, 6 August 2003
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.
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Fermilab Colloquium, 6 August 2003
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
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Fermilab Colloquium, 6 August 2003
Burgasser et al. (2002)
Reappearance of condensate species progenitors (e.g., FeH) detected below cloud deck.
Evidence for Cloud Disruption - Spectroscopy
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Fermilab Colloquium, 6 August 2003
Presence of CO in Gliese 229B’s atmosphere 16,000x LTE abundance upwelling convective motion.
Oppenheimer et al. (1998)
Evidence for Cloud Disruption - Spectroscopy
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Fermilab Colloquium, 6 August 2003
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
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Fermilab Colloquium, 6 August 2003
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.
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Fermilab Colloquium, 6 August 2003
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?
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Fermilab Colloquium, 6 August 2003
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.
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Fermilab Colloquium, 6 August 2003
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)
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Fermilab Colloquium, 6 August 2003
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?
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