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Cloudy with a Chance of Iron …

Cloudy with a Chance of Iron …. Clouds and Weather on Brown Dwarfs. Adam Burgasser UCLA. Andy Ackerman & Mark Marley NASA Ames. Didier Saumon Los Alamos NL. J. Davy Kirkpatrick Caltech/IPAC. Katharina Lodders Washington University. Adam Burgasser UCLA.

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Cloudy with a Chance of Iron …

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  1. Cloudywith a Chance ofIron… Clouds and Weather onBrown Dwarfs Adam Burgasser UCLA

  2. Andy Ackerman& Mark MarleyNASA Ames Didier SaumonLos Alamos NL J. Davy KirkpatrickCaltech/IPAC Katharina LoddersWashington University Adam Burgasser UCLA

  3. 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). Fermilab Colloquium, 6 August 2003

  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. Brown Dwarfs Stellar evolution (1) (2) • Adiabatic contraction (Hayashi tracks) • Ignition, formation of radiative core, heating – dynamic equilibrium(Henyey tracks) • Settle onto Hydrogen main sequence – radiative equilibrium (3) Hayashi (1965) Fermilab Colloquium, 6 August 2003

  6. Brown Dwarfs PPI chain: p + p → d + e+ + e, Tc = 3  106 K 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. Kumar (1963) Fermilab Colloquium, 6 August 2003

  7. 90 80 Stars BDs 75 70 60 30 40 50 10 20 Brown Dwarfs With no fusion source, Brown dwarfs rapidly evolve to lower Teff and lower luminosities. “… cool off inexorably like dying embers plucked from a fire.” A. Burrows Fermilab Colloquium, 6 August 2003

  8. 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* Fermilab Colloquium, 6 August 2003

  9. 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). Fermilab Colloquium, 6 August 2003

  10. 90 80 75 70 60 30 40 50 10 20 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. Fermilab Colloquium, 6 August 2003

  11. 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. Fermilab Colloquium, 6 August 2003

  12. Condensation in BD Atmospheres • 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) Marley et al. (2002) Fermilab Colloquium, 6 August 2003

  13. Evidence for Condensation - Spectroscopy • 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. Kirkpatrick et al. (1999) Fermilab Colloquium, 6 August 2003

  14. Evidence for Condensation - Photometry 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 Gliese 229B Cond Chabrier et al. (2000) Fermilab Colloquium, 6 August 2003

  15. Evidence for Rainout - Abundances L T Without the rainout of dust species, Na and K would form Feldspars and atomic species would be depleted in the late L dwarfs. Burrows et al. (2002) Fermilab Colloquium, 6 August 2003

  16. Evidence for Rainout - Abundances L T With rainout, Na and K persist well into the T dwarf regime. Burrows et al. (2002) Fermilab Colloquium, 6 August 2003

  17. Evidence for Rainout - Abundances K I (and Na I) absorption is clearly present in the T dwarfs  dust species must be removed from photosphere. Burgasser et al. (2002) Fermilab Colloquium, 6 August 2003

  18. 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) Fermilab Colloquium, 6 August 2003

  19. sedimentation efficiency eddy diffusion coefficient convective velocity scale -κ (dqt/dz) – frain w* qcond = 0 qt = qcond + qvapor Basics of the Cloudy Model • Simple treatment: assume transport of dust by diffusion and gravitational settling. • Horizontal homogeneity. • No chemical mixing between clouds. Fermilab Colloquium, 6 August 2003

  20. 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) Fermilab Colloquium, 6 August 2003

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

  22. 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. Ackerman & Marley (2001) Fermilab Colloquium, 6 August 2003

  23. Basics of the Cloudy Model L5 Condensate cloud may or may not influence spectrum of brown dwarf depending on its temperature – explains disappearance of dust in T dwarfs. L8 T5 Ackerman & Marley (2001) Fermilab Colloquium, 6 August 2003

  24. Cloudy Model Results • 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 clear cloudy, frain= 3 Burgasser et al. (2002) Fermilab Colloquium, 6 August 2003

  25. 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 mm. • Apparently narrow temperature range: Gl 584C (L8) ~ 1300 K 2MASS 0559 (T5) ~ 1200 K. Fermilab Colloquium, 6 August 2003

  26. CondensateClouds Clouds are not uniform!

  27. 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)

  28. 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 Helling et al. (2001) Fermilab Colloquium, 6 August 2003

  29. 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. Enoch, Brown, & Burgasser (2003) Fermilab Colloquium, 6 August 2003

  30. 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. Burgasser et al. (2002) Fermilab Colloquium, 6 August 2003

  31. Evidence for Cloud Disruption - Spectroscopy Reappearance of condensate species progenitors (e.g., FeH)  detected below cloud deck. Burgasser et al. (2002) Fermilab Colloquium, 6 August 2003

  32. Evidence for Cloud Disruption - Spectroscopy Presence of CO in Gliese 229B’s atmosphere 16,000x LTE abundance  upwelling convective motion. Oppenheimer et al. (1998) Fermilab Colloquium, 6 August 2003

  33. 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 Fermilab Colloquium, 6 August 2003

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

  35. 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? Burgasser et al. (2002) Fermilab Colloquium, 6 August 2003

  36. 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. Fermilab Colloquium, 6 August 2003

  37. 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. Showman & Guillot (2002) Charbonneau et al. (2002) Fermilab Colloquium, 6 August 2003

  38. 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? Fermilab Colloquium, 6 August 2003

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