L and T Dwarfs*

Often Brilliant Astronomers Find Great Knowledge Meeting Late Together

• History of discovery • Spectral types/properties • Interiors of low mass stars • Evolution of low mass stars • Photospheres of low mass stars *Discussion and figures taken from Reid and Hawley’s New Light on Dark Stars, 2000

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 – H 2

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 – H 2 formation also important (change in slope of main seq. at 0.5 M Sun ) • 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

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 R Sun (and the core temperature can’t get any higher than this) • What happens to the star?

– If M>0.09M

Sun , core fusion is possible and sustainable for many Hubble times – For 0.08-0.085 M Sun , degeneracy lowers central temperature, but it’s – At 0.075 Msun, core temperature is initially hot enough, but degeneracy cools the core and fusion stops – “transition object” – For lower masses (M<0.07M

Sun ), the core is never hot enough for fusion, • Stellar mass limit somewhere between transition object and brown dwarf

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

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

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!)

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

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

Evolution of Lithium

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

IR Spectra

L dwarf IR spectra are dominated by water and CO H 2 O H 2 O methane H 2 O methane T dwarf IR spectra dominated by water and methane

M Dwarf Spectra Are a Mess

• Observed spectrum of M8 V dwarf VB10 • Black body and H match either continuum spectra shown as dashed lines • Real spectrum doesn’t • Spectrum dominated by sources of opacity

Opacities

• Bound-bound opacities – molecules – TiO, CaH + other oxides & hydrides in the optical – H 2 O, CO in the IR – ~10 9 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 H 2 H 2 is important in the IR (longer than 1 micron) • H 2 molecules have allowed transitions only at electric quadrupole and higher order moments, so H • 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 (CH 3 2 itself is not significant ) – the transition from L to T dwarfs

Opacities at 2800K

Solar metallicity [Fe/H]=-2.5

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)

Dust

• Dust formation is important in M, L, and T dwarfs • Depletes metals, including Ti • Dust includes • Corundum (Al 2 O 3 ) • Perovskite (CaTiO 3 ), condensing at T < 2300-2000K • Iron (Fe) • VO, condensing at T < 1900-1700 K • Enstatite (MgSiO 3 ) • Forsterite (Mg 2 SiO 4 ) • 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

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

And More Dust

• As temperature falls: • CO depleted to form methane at temperatues < 1500-1200 K • But Na may condense onto “high albite” (NaAlSi 3 O 8 ) • CrH condenses at T=1400 K • Alkali elements expected to form chlorides at T < 1200

Temperature Calibration

Spectral Type

M0 M2 M4 M6 M8 L0 L2 L4 L6 L8 T

Teff (K)

3800 3400 3100 2600 2200 2000 1900 1750 1600 1400 <1200

Radius (R/R sun )

0.62

0.44

0.36

0.15

0.12

~0.1

~0.1

~0.1

~0.1

~0.1

Mass

0.60

0.44

0.20

0.10

~0.08

L/L Sun

0.072

0.023

0.006

0.0009

0.0003

Log g

4.65

4.8

4.9

5.1

5.2

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