# Presentation: 561629

## Cloudy with a Chance of Iron...

*Clouds and Weather on**Brown Dwarfs*

**Adam Burgasser**
**UCLA**

##  Andy Ackerman& Mark MarleyNASA Ames

** ****Katharina Lodders****Washington University**

** ****Andy Ackerman****& Mark Marley****NASA Ames**

**Didier Saumon****Los Alamos NL**

**Adam Burgasser**
**UCLA**

## 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?

**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.

## Brown Dwarfs

- Hayashi (1965)

- Adiabatic contraction *(Hayashi tracks)*
- Ignition, formation of radiative core, heating – **dynamic equilibrium**   *(Henyey tracks)*
- Settle onto Hydrogen main sequence – **radiative equilibrium**

- Stellar evolution

- (1)

- (2)

- (3)

## Brown Dwarfs

**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 (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

- “... cool off inexorably like dying embers plucked from a fire.”
- A. Burrows

## 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*

**Notes:**
Extrapolate Salpeter to 0.001(0.005) Msun => 4(2)x more mass in BDs
Current estimates: NBD ~ N*, MBD ~ 0.15 M*
MJeans ~ 1 Msun
BD/EGP models – Baraffe et al. 2003, Burrows et al. 1997, etc
Degeneracy era – 1015-23 yr, ~1/2 of all objects BDs, ~50 BD collision-made stars in Galaxy at any time

## 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).

**Notes:**
Extrapolate Salpeter to 0.001(0.005) Msun => 4(2)x more mass in BDs
Current estimates: NBD ~ N*, MBD ~ 0.15 M*
MJeans ~ 1 Msun
BD/EGP models – Baraffe et al. 2003, Burrows et al. 1997, etc
Degeneracy era – 1015-23 yr, ~1/2 of all objects BDs, ~50 BD collision-made stars in Galaxy at any time

## M, L, and T dwarfs

- 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.

## 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.

## Evidence for Condensation - Photometry

**Gliese 229B**

- 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***

## Evidence for Rainout -         Abundances

- 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.

## Evidence for Rainout -         Abundances

- 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.

**Notes:**
K = 105 (minimum turbulence) – 108 (Jupiter) cm2/s ~ 109 BD
W* ~ 1 m/s Jupiter
Frain ~ 0.2-0.5 stratocumulus Earth
     ~ 2-6 trade cumulus Earth
     ~ 1-3 Jupiter
Frain determines particle size distribution

## What is frain?

- If L, qc/qt constant, scale height:
*f*rain ~ 0  “dusty” atmosphere.
*f*rain → ∞  “clear” atmosphere.
- Earth: *f*rain ~ 0.5 (stratocumulus) – 4 (cumulus).
- Jupiter: *f*rain ~ 1-3 (NH3 clouds).

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

**Notes:**
K = 105 (minimum turbulence) – 108 (Jupiter) cm2/s ~ 109 BD
W* ~ 1 m/s Jupiter
Frain ~ 0.2-0.5 stratocumulus Earth
     ~ 2-6 trade cumulus Earth
     ~ 1-3 Jupiter
Frain determines particle size distribution

## What is frain?

- Ackerman & Marley (2001)

*f*rain determines extent of cloud, particle size distribution, and hence cloud opacity.

- What is *f*rain?

## Basics of the Cloudy Model

- Ackerman & Marley (2001)

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

## 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***

## Cloudy Model Results

***cloudy, ***
***f******rain******= 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**.

***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.

## CondensateClouds

**Condensate****Clouds**

***Clouds are ******not****** uniform!***

## At 5 m, holes in Jupiter’s NH3 clouds produce “Hot Spots” that dominate emergent flux  horizontal structure important!

- 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!**

## Evidence for Cloud Disruption -         Theory

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

- Number density

- Mean particle size

## 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**.

## Evidence for Cloud Disruption -         Spectroscopy

- 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

- 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

- K

- z

- 1400 K

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

## Success...?

- Burgasser et al. (2002)

- 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 *f*rain = 3, would require a rapid shift in atmospheric dynamics – partial clouding is simpler.

## 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?

## Slide 39