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

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Presentation on theme: "Cloudy with a Chance of Iron … Clouds and Weather on Brown Dwarfs Adam Burgasser UCLA."— Presentation transcript:

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

2 J. Davy Kirkpatrick Caltech/IPAC Katharina Lodders Washington University Andy Ackerman & Mark Marley NASA Ames Didier Saumon Los Alamos NL Adam Burgasser UCLA

3 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).

4 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.

5 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)

6 Fermilab Colloquium, 6 August 2003 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

7 Fermilab Colloquium, 6 August 2003 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

8 Fermilab Colloquium, 6 August 2003 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 *

9 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 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).

10 Fermilab Colloquium, 6 August 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

11 Fermilab Colloquium, 6 August 2003 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

12 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 → TiO 2 (s), CaTiO 3 (s) VO → VO(s) Fe → Fe(l) SiO → SiO 2 (s), MgSiO 3 (s)

13 Fermilab Colloquium, 6 August 2003 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.

14 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

15 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

16 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.

17 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.

18 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)

19 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 (dq t /dz) – f rain w * q cond = 0 q t = q cond + q vapor eddy diffusion coefficient sedimentation efficiency convective velocity scale

20 Fermilab Colloquium, 6 August 2003 What is f rain ? If L, q c /q t 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 (NH 3 clouds). q t (z) = q 0 exp(- f rain [q c /q t ] [w * /κ] z)

21 Fermilab Colloquium, 6 August 2003 Ackerman & Marley (2001) f rain determines extent of cloud, particle size distribution, and hence cloud opacity. What is f rain ?

22 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.

23 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

24 Fermilab Colloquium, 6 August 2003 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. Cloudy Model Results dusty

25 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.

26 Condensate Clouds Clouds are not uniform!

27 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!

28 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 Number density Mean particle size

29 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.

30 Fermilab Colloquium, 6 August 2003 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

31 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

32 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

33 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

34 Fermilab Colloquium, 6 August 2003 Wavelength Matters! FeHK I I JK z 1400 K Relative brightening at z and J (~1  m) can be explained by holes in the clouds.

35 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?

36 Fermilab Colloquium, 6 August 2003 Arguments Against the Model Small numbers of objects with parallaxes, could be a statistical fluke.  Recent parallaxes for 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.

37 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 b – high clouds? Presence of clouds affects detectability of EGPs. Charbonneau et al. (2002)

38 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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