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See also: Davy Kirkpatrick’s Annual Review article, 2005 New Light on Dark Stars by Neill Reid

See also: Davy Kirkpatrick’s Annual Review article, 2005 New Light on Dark Stars by Neill Reid and other sources cited. L and T Dwarfs.

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See also: Davy Kirkpatrick’s Annual Review article, 2005 New Light on Dark Stars by Neill Reid

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  1. See also: Davy Kirkpatrick’s Annual Review article, 2005 New Light on Dark Stars by Neill Reid and other sources cited L and T Dwarfs With contributions from:Andy Ackerman & Mark Marley (NASA Ames)Didier Saumon (Los Alamos NL) J. Davy Kirkpatrick(Caltech/IPAC) Katharina Lodders (Washington University) Based on a colloquium by Adam Burgasser (MIT) http://vmsstreamer1.fnal.gov/VMS_Site_02/Lectures/Colloquium/Presentations/Burgasser.ppt

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

  3. 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.

  4. A Little History • Substantial effort in ’80s and early ’90s to find very low mass M dwarfs • Parallax surveys of high proper motion red objects • Companions to M dwarfs, WDs (IR excesses) • Companion to vB8 – NOT • Companion to G29-38 – NOT • Companion to G165B – YES! the first L dwarf • Spectrum not understood until more found • Gl 229B the first T dwarf • IR Colors surprisingly blue Note change in slope – H2

  5. Brown Dwarfs Abound! • Many L and T dwarfs have now been found • Improved IR detectors • Better spatial resolution (seeing improvements, AO) • IR and multi-color surveys (2MASS, DENIS, and Sloan) • Breakthrough in understanding appearance of spectra • Significant progress in modeling low mass stellar and substellar objects • Understood in the late ’50s (Limber) that • low mass stars must be fully convective • Electron degeneracy must play a role • H2 formation also important (change in slope of main seq. at 0.5 MSun) • Kumar figured out (in the early ’60s) that a minimum mass is needed for H burning • Grossman et al. included deuterium burning (early ’70s) • Recent improvements include better equation of state and grain formation

  6. 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)

  7. 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)

  8. 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

  9. 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*

  10. 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).

  11. 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.

  12. M Dwarf Spectral Types • Molecular species switch from MgH to TiO • CaOH appears in later M dwarfs • Prominent Na D lines • Spectral types determined in the blue

  13. Later Spectral Classes • TiO disappears to be replaced by water, metal hydrides (FeH, CrH) • Alkali metal lines strengthen (note K I in the L8 dwarf) • Spectral types determined from red, far red spectra (blue too faint!)

  14. L-type Spectral Sequence • K I line strength increases with later spectral type • Li I appears in some low mass stars (m < 0.06 solar masses) • Appearance of FeH, CrH • Strength of Ca I • Strength of water • Disappearance of TiO • Absence of FeH, CrH in T dwarf, much increased strength of water

  15. Lithium in Brown Dwarfs • Li I appears in about a third of L dwarfs • EQW from 1.5 to 15 Angstroms • Li I can be used to distinguish between old, cooled brown dwarfs and younger, lower mass dwarfs

  16. Evolution of Lithium • At a given Teff,Stars with Li are lower mass than stars with Li depleted.

  17. M, L, and T Dwarfs in the IR 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.

  18. Alkali Lines • Alkali lines very prominent in L dwarf spectra (Li, Na, K, Cs, Rb) • Strong because of very low optical opacities • TiO, VO are gone • Dust formation also removes primary electron donors, so H- and H2- opacities are also reduced • High column density due to low optical opacity leads to very strong lines • K I lines at 7665 and 7699 A have EQWs of several hundred Angstroms • Na D lines also become very strong

  19. Stellar Models • General assumptions include • Plane parallel geometry • Homogeneous layers • LTE • Surface gravities: log g ~ 5.0 • Convection using mixing length • Convection is important even at low optical depth (t<0.01) • Strength of water absorption depends on detailed temperature structure and treatment of convection • For Teff < 3000 K, grains become important in atmospheric structure (scattering)

  20. Opacities • Bound-bound opacities – molecules • TiO, CaH + other oxides & hydrides in the optical • H2O, CO in the IR • ~109 lines! • Bound-bound molecular line opacities dominate the spectrum • Bound-free opacities • Atomic ionization, molecular dissociation • Free-free opacities – Thomson and Rayleigh scattering • In metal-poor low mass stars, pressure induced absorption of H2-H2 is important in the IR (longer than 1 micron) • H2 molecules have allowed transitions only at electric quadrupole and higher order moments, so H2 itself is not significant • Also significant van der Waals collisional (pressure) broadening of atomic and molecular lines, making these lines much stronger than they would otherwise be • At even cooler temperatures (T~1500-1200) CO is depleted by methane formation (CH3) – the transition from L to T dwarfs

  21. Dust and Clouds in Brown Dwarfs • 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).

  22. 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)

  23. 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)

  24. 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)

  25. 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)

  26. Evidence for Rainout - Abundances L T With rainout, Na and K persist well into the T dwarf regime. Burrows et al. (2002)

  27. 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)

  28. 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)

  29. CondensateClouds Clouds are not uniform!

  30. 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)

  31. 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)

  32. 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)

  33. Evidence for Cloud Disruption - Spectroscopy Reappearance of condensate species progenitors (e.g., FeH)  detected below cloud deck. Burgasser et al. (2002)

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

  35. 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

  36. 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.

  37. 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)

  38. Cooler Than T Dwarfs… • Proposed spectral class for ultra-cool dwarfs - Y stars • None yet discovered • Cooler than 770K (the coolest subclass of T dwarf) • Not clear (yet) whether the atmospheric chemistry will change enough to warrant a new spectral class • May be discovered with the next generation of deep IR surveys • not detected with DENIS (K<16.5) or 2MASS (K<15.8) • May be detected with UKIRT LAS (J<19.7) and UDS (J<24) • These surveys will also find many more L & T dwarfs

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