Transcript L and T Dwarfs - Indiana University

L and T Dwarfs*
Often Brilliant Astronomers Find Great
Knowledge Meeting Late Together
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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
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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
Brown Dwarfs Abound!
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Many L and T dwarfs have now been found
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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
Minimum Mass for H Burning
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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
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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
H2O
H2O
methane
H2O
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 Hcontinuum spectra
shown as dashed lines
• Real spectrum doesn’t
match either
• Spectrum dominated
by sources of opacity
Opacities
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Bound-bound opacities – molecules
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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
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Free-free opacities – Thomson and Rayleigh scattering
In metal-poor low mass stars, pressure induced absorption of H2H2 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
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
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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
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” (NaAlSi3O8)
• CrH condenses at T=1400 K
• Alkali elements expected to form
chlorides at T < 1200
Temperature Calibration
Spectral
Type
Teff
(K)
Radius
(R/Rsun)
Mass
L/LSun
Log g
M0
3800
0.62
0.60
0.072
4.65
M2
3400
0.44
0.44
0.023
4.8
M4
3100
0.36
0.20
0.006
4.9
M6
2600
0.15
0.10
0.0009
5.1
M8
2200
0.12
~0.08
0.0003
5.2
L0
2000
~0.1
L2
1900
~0.1
L4
1750
~0.1
L6
1600
~0.1
L8
1400
~0.1
T
<1200
Loooooooooong Term Evolution
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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