Observational Properties of
Solar Neighborhood Brown Dwarfs
PHY 688, Lecture 7
Stanimir Metchev
Outline
• Review of previous lecture
• The Milky Way Galaxy and the Solar
Neighborhood
• Observational Properties of Brown Dwarfs
Feb 9, 2009
PHY 688, Lecture 7
2
Previously in PHY 688…
Feb 9, 2009
PHY 688, Lecture 7
3
infrared spectra
visible spectra
Spectral Types: Temperature
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PHY 688, Lecture 7
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Spectral Classes: Luminosity (Gravity)
• luminosity, radius, surface gravity, and surface pressure
are mutually related
– L = 4πR2σTeff4, g = GM/R2, P = ρgl (l is photon m.f.p.)
• define “spectral class”
V: dwarfs, log g ~ 4.5 [cgs units]
IV: subgiants, log g ~ 3 (approximately as on Earth)
III: giants, log g ~ 1.5
II: (bright) giants, log g ~ 0.5
I: supergiants, log g ~ –0.5
• Sun: G2 V star (Teff = 5777K, log g = 4.43)
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PHY 688, Lecture 7
5
Dwarf Stars
Sun
stars
(G dwarf) M dwarf
Feb
9, 2009K
5700
~3500 K
brown dwarfs
L dwarf
T dwarf
PHY 688,
~2000
KLecture 7 ~1000 K
planets
Jupiter
160 K
6
Dwarf vs. Giant
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PHY 688, Lecture 7
7
(courtesy: D. Gray)
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PHY 688, Lecture 7
8
Hertzsprung
-Russell
(H-R)
Diagram
• log L vs. log Teff
• main sequence:
– locus of most stars
– bulk of stellar
lifetimes
– L ∝ M3.8
– τMS ≈
1010 yr (M/MSun)–2.8
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PHY 688, Lecture 7
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ColorMagnitude
Diagram
(CMD)
• proxy for the (TeffL) HertzsprungRussell diagram
• e.g., B–V vs. MV,
J–K vs. MK, etc.
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PHY 688, Lecture 7
10
Star Formation
• molecular clouds:
–
–
–
–
–
n ~ 1000 cm–3
ρ ~ 10–27 g cm–3
M ~ 103–105 MSun
T ~ 10–100 K
r ~ 100 pc
• Jeans mass
– minimum mass for gravitational
collapse
# " & c s3
MJ = % ( 3 2 1 2
$6'G )
Feb 9, 2009
sound speed
3
#
&
# n &+1 2
cs
* (2MSun )%
+1 ( %
3
+3 (
$ 0.2kms ' $ 10 cm '
PHY 688, Lecture 7
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Star Formation: the H-R Perspective
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PHY 688, Lecture 7
12
Evolution
Evolutionary Models
Models
•
Note: more
massive stars
reach the zero-age
main sequence
(ZAMS) more
quickly
Feb 9, 2009
PHY 688, Lecture 7
(Baraffe et al. 2002)
13
Post-Main Sequence Evolution
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PHY 688, Lecture 7
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Outline
• Review of previous lecture
• The Milky Way Galaxy and the Solar
Neighborhood
• Observational Properties of Brown Dwarfs
Feb 9, 2009
PHY 688, Lecture 7
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Feb 9, 2009
PHY 688, Lecture 7
(photo: Serge Brunier)
16
8 kpc
aSun ~ 8 kpc
PSun ~ 250 Myr
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PHY 688, Lecture 7
(artist: R. Hurt, NASA)17
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Outline
• Review of previous lecture
• The Milky Way Galaxy and the Solar
Neighborhood
• Observational Properties of Brown Dwarfs
– M dwarfs: nearing the end of the Main Sequence
– prototypes for the L and T spectral types
– L and T dwarf spectroscopic characteristics
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PHY 688, Lecture 7
20
M Dwarfs
• nearing the
end of the
main sequence
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PHY 688, Lecture 7
V–I (mag)
21
M Dwarfs: Nearing the End of the
Main
Sequence
TiO
TiO
TiO
M2 V
M4 V
M2 V
M4 V
M2.5 V
M4.5 V
M2.5 V
M4.5 V
M3 V
M5.5 V
M6 V
M3 V
M3.5 V
M6.5 V
M3.5 V
M3.5 V
M3.5 V
M9 V
6500 7000 7500 8000 8500 9000
6500 7000 7500 8000 8500 9000
Wavelength (Å)
Wavelength (Å)
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PHY 688, Lecture 7
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Dark Matter: Brown Dwarfs?
• galaxy rotation curves
are non-Keplerian
a typical galaxy rotation curve
– A: expected from
Keplerian rotation
– B: observed
• unseen mass has a
different spatial
distribution from
luminous mass
– faint stars (white
dwarfs, brown
dwarfs)?
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Kepler’s Third Law:
P3/a2 = const
PHY 688, Lecture 7
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GD 165B: The Original L Dwarf
1”
J
H
• GD 165A is a white dwarf
• direct imaging
• bizarre spectrum
– BD or polluted star?
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PHY 688, Lecture 7
K
(Zuckerman & Becklin et al. 1988)
near-IR tri-band color
composite (2MASS)
1”
24
GD 165B: The Original L Dwarf
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PHY 688, Lecture 7
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GD 165B: The Original L Dwarf
no TiO!
MJ = 11.7 mag
I – J = 3.4 mag
Teff = 2625 K
Sun (G2 dwarf):
MJ = 3.6 mag
I – J = 0.4 mag
Teff = 5777 K
Feb 9, 2009
MJ = 13.2 mag
I – J = 4.7 mag
Teff ≈ 2300 K
PHY 688, Lecture 7
(Kirkpatrick et al. 1993)
26
• the end of the
main
sequence
(MV)
L Dwarfs
• the beginning
of the
substellar
domain
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PHY 688, Lecture 7
V – I (mag)
27
Gl 229B: The Original T Dwarf
• Gl 229A is a nearby
low-mass M star
• Gl 229B:
unambiguously cool
and substellar
• direct imaging +
adaptive optics
– contrast enhancement
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PHY 688, Lecture 7
(Nakajima et al. 1995)
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Gl 229B: The Original T Dwarf
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Gl 229B: The
Original T Dwarf
•
•
•
no CrH or FeH as in L
dwarfs …
… but H2O and CH4 as on
solar system planets
GD 165B:
CrH, FeH
M9 (latest M dwarf)
L0 (earliest L dwarf)
L2 dwarf (like GD 165B)
– MJ = 13.2 mag
– I – J = 4.7 mag
– Teff ≈ 2300 K
•
Gl 229B:
– MJ ≈ 16.5 mag
• Lbol = 6.4 × 10–6 LSun
– I – J ~ 6.5 mag
– Teff ≈ 900 K
• Jupiter is 160 K
Feb 9, 2009
decreasing Teff
• Lbol ~ 10–4 LSun
L8 (latest L dwarf)
methane (T) dwarf (Gl 229B)
(Kirkpatrick
etLecture
al. 1999)
PHY 688,
7
CH4 H2O
30
Gl 229B: A Planet-Like Atmosphere
Geballe et al. (1996)
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• the coolest
known starlike objects
(MV)
T Dwarfs
• always
substellar
T
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PHY 688, Lecture 7
V – I (mag)
(550 nm – 800 nm)
32
T Dwarfs
Sun
mag
• the coolest
known starlike objects
• always
substellar
Feb 9, 2009
mag
V–I (mag)
PHY 688, Lecture (800
7
nm – 1200 nm)
33