The Spectral Energy Distributions of
Brown Dwarfs
PHY 688, Lecture 8
Stanimir Metchev
Outline
• Review of previous lecture
• Ultra-cool spectral energy distributions (SEDs)
– optical to IR spectra and broadband photometry
• Finding cool brown dwarfs
– all-sky surveys and color-color diagrams
Feb 11, 2009
PHY 688, Lecture 8
2
Previously in PHY 688…
Feb 11, 2009
PHY 688, Lecture 8
3
8 kpc
aSun ~ 8 kpc
PSun ~ 250 Myr
Feb 11, 2009
PHY 688, Lecture 8
(artist: R. Hurt, NASA) 4
“Local Bubble”
AV = 0.0 mag
r ~ 100 pc
Feb 11, 2009
PHY 688, Lecture 8
5
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)
Feb 11, 2009
PHY 688, Lecture 8
6
GD 165B: The Original L Dwarf
near-IR tri-band color
composite (2MASS)
no TiO!
1”
(Zuckerman & Becklin et al. 1988)
• GD 165B:
– MJ = 13.2 mag
• Lbol ~ 10–4 LSun
– I – J = 4.7 mag
• Teff ≈ 2300 K
• M ~ 0.07 MSun
Feb 11, 2009
(Kirkpatrick
al.Lecture
1993) 8
PHY et
688,
7
Gl 229B: The
Original T Dwarf
CrH, FeH
M9 (latest M dwarf)
L0 (earliest L dwarf)
L2 dwarf (like GD 165B)
(Nakajima et al. 1995)
– MJ ≈ 16.5 mag
• Lbol = 6.4 × 10–6 LSun
– I – J ~ 6.5 mag
• Teff ≈ 900 K
• Jupiter is 160 K
• M ~ 0.03 MSun
Feb 11, 2009
decreasing Teff
• Gl 229B:
L8 (latest L dwarf)
methane (T) dwarf (Gl 229B)
(Kirkpatrick
et al.
1999)8
PHY 688,
Lecture
CH4 H2O
8
Gl 229B: A Planet-Like Atmosphere
Geballe et al. (1996)
Feb 11, 2009
PHY 688, Lecture 8
9
L+T
Dwarfs
mag
• the lowest
mass stars are
early-L
dwarfs (≤ L4)
at >1 Gyr
ages
Sun
~ stellar / substellar
boundary at >1 Gyr
• late-L dwarfs
and T dwarfs
are always
substellar
Feb 11, 2009
mag
V–I (mag)
PHY 688, Lecture (800
8
nm – 1200 nm)
10
Outline
• Review of previous lecture
• Ultra-cool spectral energy distributions (SEDs)
– optical to IR spectra and broadband photometry
• Finding cool brown dwarfs
– all-sky surveys and color-color diagrams
Feb 11, 2009
PHY 688, Lecture 8
11
Ultra-Cool Dwarfs (UCDs):
Spectral Types >M6
• M7–M9, L, T
• Teff ≤ 2500–2700 K
• When evolved (> 1Gyr), R ~ 0.10 RSun ~ 1 RJup
Sun
Feb 11, 2009
stars
(G dwarf)
M dwarf
5700 K
~3000 K
brown dwarfs
L dwarf
~2000 K
planets
T dwarf
Jupiter
~800 K
160 K
visible
light 8
PHY
688, Lecture
R. Hurt (Caltech/IPAC)
12
SEDs: Visible
Wavelengths
• very red optical
continuum
• large numbers of
Ls and Ts found in
SDSS
M9 (latest M dwarf)
L0 (earliest L dwarf)
L2 dwarf (like GD 165B)
L8 (latest L dwarf)
methane (T) dwarf (Gl 229B)
Feb 11, 2009
(Kirkpatrick
et al.
1999)8
PHY 688,
Lecture
13
The Sloan Digital Sky Survey (SDSS)
SDSS 2.5 m telescope at Apache Point, NM
mapped 1/4 of sky at 300–1000 nm
2000–2006
Feb 11, 2009
PHY 688, Lecture 8
14
The Sloan Digital Sky Survey (SDSS)
u
g
r
i
(ansgtroms)
Feb 11, 2009
z
SDSS footprint on sky
(Aitoff projection)
PHY 688, Lecture 8
15
The Sloan Digital Sky Survey (SDSS)
u
g
r
i
z
• limiting magnitudes:
u : 22.0 mag
g : 22.2 mag
r : 22.2 mag
i : 21.3 mag
z : 20.5 mag
(angstroms)
Feb 11, 2009
PHY 688, Lecture 8
16
SEDs: Visible
Wavelengths
• very red optical
continuum
• large numbers os
Ls and Ts found in
SDSS
• SDSS i – z colors:
M5 V : ~1.1 mag
L5 (V): ~3.0 mag
T6 (V): ~4.0 mag
r
i
z
M9 (latest M dwarf)
L0 (earliest L dwarf)
L2 dwarf (like GD 165B)
L8 (latest L dwarf)
T6 dwarf (Gl 229B)
Feb 11, 2009
(Kirkpatrick
et al.
1999)8
PHY 688,
Lecture
17
SEDs: Near-IR Wavelengths
• reddish to neutral colors
because of molecular
opacity
• large numbers of Ls and
Ts found in 2MASS
CIA H2
Feb 11, 2009
PHY 688, Lecture 8
18
The Two-Micron All-Sky Survey
(2MASS)
2MASS North
Mt. Hopkins, AZ
2MASS South
Cerro Tololo, Chile
• pair of 1.3 m
telescopes: North
and South
• mapped entire sky
(99.998%) at
1.10–2.30 micron
• 1997–2001
(a.k.a., SMARTS 1.3 m)
Feb 11, 2009
PHY 688, Lecture 8
19
The Two-Micron All-Sky Survey
(2MASS)
• limiting magnitudes
J = 15.8 mag
H = 15.1 mag
Ks = 14.3 mag
Feb 11, 2009
PHY 688, Lecture 8
20
SEDs: Near-IR Wavelengths
• reddish to neutral colors
because of molecular
opacity
• large numbers of Ls and
Ts found in 2MASS
• 2MASS J – Ks colors:
M5 : ~0.9 mag
L5 : ~1.7 mag
T6 : ~0 mag
Feb 11, 2009
J
H
K
CIA H2
PHY 688, Lecture 8
21
T Dwarfs Are Very Red in
Visible Light …
Sun
stars
(G dwarf) M dwarf
5700 K
~3000 K
brown dwarfs
L dwarf
~2000 K
T dwarf
~800 K
visible light
Feb 11, 2009
PHY 688, Lecture 8
planets
Jupiter
160 K
R. Hurt (Caltech/IPAC)
22
… but T Dwarfs Can Be Blue in the
Near-Infrared!
Sun
stars
(G dwarf) M dwarf
5700 K
~3000 K
brown dwarfs
L dwarf
~2000 K
T dwarf
~800 K
near-infrared light
Feb 11, 2009
PHY 688, Lecture 8
planets
Jupiter
160 K
R. Hurt (Caltech/IPAC)
23
Near-IR CMD of Stars and Brown Dwarfs
Feb 11, 2009
PHY 688, Lecture 8
(Kirkpatrick 2005)
24
SEDs: Mid-IR Wavelengths
• neutral to reddish
continua
• individual UCDs
characterized by
Spitzer Space
Telescope
5.4 arc sec
J
H
KS
Binary T dwarf ε Indi Bab
resolved with VLT AO
(McCaughrean et al. 2004)
Feb 11, 2009
(Roellig et al. 2004; Kirkpatrick 2005)
PHY 688, Lecture 8
25
The Spitzer Space Telescope
• 0.85 m mirror
• 3–180 micron,
imaging +
spectroscopy
• cryo-cooled (5.5 K)
• last of NASA’s
Great Observatories
• 2003–2009
– “warm” mission
until 2011
Feb 11, 2009
PHY 688, Lecture 8
26
The Optical to IR SEDs of UCDs
Feb 11, 2009
(Cushing
et al. 2006; Marley & Leggett 2008)
PHY 688, Lecture 8
27
Outline
• Review of previous lecture
• Ultra-cool spectral energy distributions (SEDs)
– optical to IR spectra and broadband photometry
• Finding brown dwarfs in large imaging surveys
– color-color diagrams
Feb 11, 2009
PHY 688, Lecture 8
28
First L and T Dwarfs Discovered as
Companions to Stars …
Gl 229 B: first T dwarf
GD 165 B: first L dwarf
J
H
K
U Hawai’i 2.2 m telescope
Palomar 1.5 m
discovery
Hubble Telescope
confirmation
Becklin & Zuckerman (1988); Nakajima et al. (1995)
Feb 11, 2009
PHY 688, Lecture 8
29
… but Most L’s and T’s Now Found
from Large-Area Imaging Surveys
>500
~100
(DwarfArchives.org)
Feb 11, 2009
PHY 688, Lecture 8
30
1.2 µm
year 1999
0.6 µm
year 1951
1.6 µm
year 1999
2.1 µm
year 1999
0.6 µm
year 1989
the first L dwarf
from 2MASS
Feb 11, 2009
PHY 688, Lecture 8
(Kirkpatrick et al. 1997)
31
The First Field T Dwarf (SDSS)
Feb 11, 2009
PHY 688, Lecture 8
(Strauss et al. 1999)
32
Searching for Cool Brown Dwarfs
with Color-Color Diagrams
• near-IR:
– L dwarfs are red
– T dwarfs span a vast
color range
G–
visible
L
Feb 11, 2009
T
L
ma
in
se
qu
en
ce
• significant overlap with
dwarf and giant stars
• stars : T dwarfs ~106 in
2MASS
• challenging to find in
2MASS-only
M
ts
n
a
gi
T0–T4
T5–T8
near-IR
L
T
PHY 688, Lecture 8
33