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L and T Dwarfs*

L and T Dwarfs*. History of discovery Spectral types/properties Interiors of low mass stars Evolution of low mass stars Photospheres of low mass stars. O ften B rilliant A stronomers F ind G reat K nowledge M eeting L ate T ogether.

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L and T Dwarfs*

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  1. L and T Dwarfs* • History of discovery • Spectral types/properties • Interiors of low mass stars • Evolution of low mass stars • Photospheres of low mass stars Often Brilliant Astronomers Find Great Knowledge Meeting Late Together *Discussion and figures taken from Reid and Hawley’s New Light on Dark Stars, 2000

  2. 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

  3. 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

  4. Minimum Mass for H Burning • As protostar collapses, core temperature rises • Low mass stars must collapse to higher densities before temperature high enough for fusion • As density increases, core becomes partially degenerate • An increasing fraction of energy from collapse goes into compressing degenerate gas • Degeneracy stops star from collapsing below 0.1 RSun (and the core temperature can’t get any higher than this) • What happens to the star? • If M>0.09MSun, core fusion is possible and sustainable for many Hubble times • For 0.08-0.085 MSun, degeneracy lowers central temperature, but it’s still hot enough for hydrogen fusion (main sequence) • At 0.075 Msun, core temperature is initially hot enough, but degeneracy cools the core and fusion stops – “transition object” • For lower masses (M<0.07MSun), the core is never hot enough for fusion, brown dwarf cools to oblivion • Stellar mass limit somewhere between transition object and brown dwarf

  5. Evolutionary Models • Deuterium burning • Hydrogen burning • Transition objects may burn for ~10 Gyr • At a given luminosity, it is hard to distinguish between young brown dwarfs and older stars

  6. 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

  7. 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!)

  8. 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

  9. Li 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

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

  11. IR Spectra L dwarf IR spectra are dominated by water and CO T dwarf IR spectra dominated by water and methane H2O H2O H2O methane methane

  12. M Dwarf Spectra Are a Mess • Observed spectrum of M8 V dwarf VB10 • Black body and H- continuum spectra shown as dashed lines • Real spectrum doesn’t match either • Spectrum dominated by sources of opacity

  13. 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

  14. Opacities at 2800K Solar metallicity [Fe/H]=-2.5

  15. 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)

  16. Dust • Dust formation is important in M, L, and T dwarfs • Depletes metals, including Ti • Dust includes • Corundum (Al2O3) • Perovskite (CaTiO3), condensing at T < 2300-2000K • Iron (Fe) • VO, condensing at T < 1900-1700 K • Enstatite (MgSiO3) • Forsterite (Mg2SiO4) • Double-metal absorbers weaken (VO, TiO) • Hydride bands dominate • Dust opacity causes greenhouse heating – outgoing IR radiation trapped by extra dust-grain opacity • Heating dissociates H2O, giving weaker water bands • Dust settles gravitationally, depleting metals and leaving reduced opacities (time scales unclear) • Dusty models fit observed flux distributions better

  17. 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

  18. And More Dust • As temperature falls: • CO depleted to form methane at temperatues < 1500-1200 K • But Na may condense onto “high albite” (NaAlSi3O8) • CrH condenses at T=1400 K • Alkali elements expected to form chlorides at T < 1200

  19. Temperature Calibration

  20. Loooooooooong Term Evolution • After 1400 Gyr, increased He fraction in core causes temperature increase, more complete H burning • Surface temperature increases • After 5740 Gyr, only 16% of H is left, opacity is lower, radiative core develops • H burning shell forms • Teff, L continue to rise until 6000 Gyr • When H depleted, degenerate He star with thin (1% by mass) H envelope finally cools

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