Substellar Atmospheres III.
Dusty Clouds and the L/T Transition
PHY 688, Lecture 20
Mar 13, 2009
Outline
• Course administration
– midterm exam date: April 1 (Wednesday)
– oral presentations: April 27– May 8
• individual; one per class period
• 30 min long
• example topics will appear on course website; suggestions welcome
• Review of previous lecture
– dust, clouds, meteorology
• Appearance and Disappearance of Dusty Clouds
– the dusty L dwarfs
– the L/T transition to the dust-free atmospheres
Mar 13, 2009
PHY 688, Lecture 20
2
Previously in PHY 688…
Mar 13, 2009
PHY 688, Lecture 20
3
UCD Spectral
Classification
• largely based on
strengths of atomic
or molecular
absorbers
• e.g.:
– CaH and TiO
indices for M
dwarfs
– CrH, Rb I, Cs I for
L dwarfs, among
others
CaH
TiO
TiO
M9 (latest M dwarf)
L0 (earliest L dwarf)
CrH
Rb
L2 dwarf (like GD 165B)
Cs
L8 (latest L dwarf)
T6.5 dwarf (Gl 229B)
Mar 13, 2009
(Kirkpatrick
et al.
1999)20
PHY 688,
Lecture
4
But Atoms and Simple Molecules
Do Not Make Up the Whole Picture
forsterite
(Mg2SiO4)
ruby corundum
(Al2O3)
Mar 13, 2009
PHY 688, Lecture 20
5
(Burrows et al. 2001)
Simplified Chemical Picture
•
As gas temperature of a (brown) dwarf drops, atoms:
– first favor an ionized state
• e.g., Ca II, Fe II in Sun
– then favor a neutral state
• e.g., Na I, K I in M/L/T dwarfs
– then form molecules
• e.g, H2O, TiO, FeH, CH4 in M/L/T dwarfs
– then condense into a solid or liquid
• e.g., Mg2SiO4, Al2O3 in L/T dwarfs
• dust clouds
•
•
More refractory elements tend to condense first
Exact sequence of molecule and condensate formation depends on
– gas pressure
– metallicity
– turbulent mixing from warmer or colder layers, etc
Mar 13, 2009
PHY 688, Lecture 20
6
Dust in Substellar Atmospheres
• Once dust condenses, it may:
– remain suspended at level of formation
– sediment to deeper, optically thick layers
• Either can occur, depending on temperature,
surface gravity
• Presence of suspended dust (clouds) is required to
explain very red colors of L dwarfs
Mar 13, 2009
PHY 688, Lecture 20
7
Dust Cloud
Chemistry
(Burrows et al. 2001)
Mar 13, 2009
PHY 688, Lecture 20
8
Cloud Formation: Meteorology 101
• A cloud appears where adiabatic cooling of an air parcel
in an upward draft results in saturation
• Further cooling condenses vapor in excess of saturation
onto cloud particles
• The particles grow through condensation and coalescence
until their sedimentation velocities exceed the updraft
speed and then fall out of the parcel
• Why is there convection in a supposedly radiative region
(the atmosphere)???
Mar 13, 2009
PHY 688, Lecture 20
9
Cloud Formation: Meteorology 102
Mar 13, 2009
PHY 688, Lecture 20
10
Cloud Level: Balance of Turbulent
Mixing and Sedimentation
•
•
Cloud condensates will settle under gravity to a level where there is enough
upward convective (turbulent) motion to keep them afloat.
Level and vertical extent of clouds depend on
– droplet size (i.e., mass)
– convective velocity, mixing efficiency
#qt
"K
" f rain w*qc = 0,
#z
13
$
'
H$ L'
RF
K = & ) &&
))
3 % H ( % µ* a c p (
4 3
• K – vertical eddy diffusion coefficient (~105–109 cm2 s–1)
!
– H = RT/µg – atmospheric scale height (~10 km); L – turbulent mixing length (~H); R –
universal gas constant; µ – atmospheric molecular weight (2.2 g mol–1 assumed); ρa –
atmospheric density; cp – specific heat of atmosphere at constant pressure (ideal gas); F = σTeff4
•
•
•
•
qc – condensate mixing ratio (mole of condensate per mole of atmosphere)
qt = qc+qv – total mixing ratio (condensate + vapor)
w* = K/L – convective velocity scale (~1 m s–1)
frain – sedimentation efficiency (~2–6 in bulk of cumulus clouds on Earth)
– ratio of mass-weighted droplet sedimentation velocity to w*
Mar 13, 2009
PHY 688, Lecture 20
(Ackerman & Marley 2001)
11
Condensate Clouds
(AM01 Baseline Models)
L dwarf
Mar 13, 2009
T dwarf
PHY 688, Lecture 20
giant
planet
(Ackerman & Marley 2001)
12
L Dwarfs Are
Dusty Objects
•
M
L
models that incorporate
suspended dust (DUSTY)
successfully reproduce L
dwarf colors
T
DUSTY models
(dust remains
suspended)
COND models
(dust is removed)
Mar 13, 2009
PHY 688, Lecture 20
13
(Baraffe et al. 2003)
Outline
• Course administration
– midterm exam date: April 1 (Wednesday)
– oral presentations: April 27– May 8
• individual; one per class period
• 30 min long
• example topics will appear on course website; suggestions welcome
• Review of previous lecture
– dust, clouds, meteorology
• Appearance and Disappearance of Dusty Clouds
– the dusty L dwarfs
– the L/T transition to the dust-free atmospheres
Mar 13, 2009
PHY 688, Lecture 20
14
Direct Evidence for
Dust in L Dwarfs
•
•
Model photospheres are
excellent at reproducing the
spectra of L and T dwarfs in the
mid-IR L (unlike in the optical
and in the mid-IR)
However, 9–11µm “plateau” in
mid-L dwarfs was not
anticipated a priori
Mar 13, 2009
PHY 688, Lecture 20
(Cushing et al. 2006)
15
Direct Evidence for
Dust in L Dwarfs
•
•
Model photospheres are
excellent at reproducing the
spectra of L and T dwarfs in the
mid-IR L (unlike in the optical
and in the mid-IR)
However, 9–11µm “plateau” in
mid-L dwarfs was not
anticipated a priori
– flux deficiency
•
Likely the result of the direct
detection of electronic
transitions in silicates
– Si-O stretching mode
Mar 13, 2009
PHY 688, Lecture 20
16
(Cushing et al. 2006)
Visibility of Dusty Condensate Clouds
Depends on Wavelength of Observation
L dwarf
Mar 13, 2009
T dwarf
PHY 688, Lecture 20
giant
planet
(Ackerman & Marley 2001)
17
Conversely: Emergent Flux Depends on
Wavelength and Cloud Level
τcloud < 0.5; hcloud > hphotosphere
τcloud > 1; hcloud ~ hphotosphere
silicate cloud
(frain = 3)
τcloud > 1; hcloud < hphotosphere
Mar 13, 2009
PHY 688, Lecture 20
(Ackerman & Marley 2001) 18
From Lecture 8:
Near-IR CMD of Stars and Brown Dwarfs
F–K
M
L
•
dusty clouds are the reason that
L to early-T dwarfs are
unusually red
•
sedimentation (rain-out) of
clouds in mid-T dwarfs
removes clouds as a source of
opacity and reddening
•
… but why should T dwarfs be
bluer than M dwarfs in the
near-IR?
T
Mar 13, 2009
PHY 688, Lecture 20
19
From Lecture 8:
Near-IR SEDs of M/L/T Dwarfs
• 2MASS J – Ks colors:
J
M5 : ~0.9 mag
L5 : ~1.7 mag
T6 : ~0 mag
• neutral to blue colors of
T dwarfs in near-IR are
due to CH4 molecular
opacity
Mar 13, 2009
PHY 688, Lecture 20
H
K
CIA H2
20
Cloud Sedimentation Is Coincident
with CO to CH4 Transition
Mar 13, 2009
PHY 688, Lecture 20
(Burrows et al. 2001)
21
Modeling
L and T Dwarfs
•
•
•
Models that incorporate
suspended dust (DUSTY)
successfully reproduce L
dwarf colors
M
L
T
Late T dwarfs well fit by dustfree photospheres (e.g.,
COND models: dust removed
upon formation)
DUSTY models
(dust remains
suspended)
COND models
(dust is removed)
Transition can be explained by
sedimentation of silicate
clouds below visible
photosphere
Mar 13, 2009
PHY 688, Lecture 20
22
(Baraffe et al. 2003)
Revisit: Emergent Flux Depends on
Wavelength and Presence of Clouds
τcloud < 0.5
silicate cloud
(frain = 3)
Mar 13, 2009
τcloud > 1
PHY 688, Lecture 20
(Ackerman & Marley 2001) 23
The L/T Transition Problem
•
•
photospheres turn blue in the
near-IR unusually quickly
clouds sink comparatively
slowly
fr
ai
n
→
∞
– need to be “rained out”
(sedimented) faster
3
=
n
f rai
Mar 13, 2009
PHY 688, Lecture 20
(Burgasser et al. 2002)
24
For a Constant frain Parameter,
Silicate Clouds Sink Gradually
τcloud < 0.5
silicate cloud
(frain = 3)
Mar 13, 2009
τcloud > 1
PHY 688, Lecture 20
(Ackerman & Marley 2001) 25
From Lecture 8:
Effective Temperatures of Brown Dwarfs
•
rather than cooling between
late-L and mid-T spectral
types, brown dwarfs merely
change appearance
– change in temperature is
∆Teff < 150 K
•
models of constant frain
require ∆Teff ~ 600 K of
cooling to reproduce the
same effect on the emergent
SED
Mar 13, 2009
PHY 688, Lecture 20
26
(Kirkpatrick 2005)
The L/T Transition Problem
•
•
photospheres turn blue in the
near-IR unusually quickly
clouds sink comparatively
slowly
– need to be “rained out”
(sedimented) faster
Mar 13, 2009
→
ai
n
fr
reddest L dwarfs require
inefficient sedimentation
(frain < 3)
• early T dwarfs require frain > 3
• late T’s require no visible
clouds (frain → ∞)
∞
•
3
=
n
f rai
PHY 688, Lecture 20
(Burgasser et al. 2002)
27
Another L/T Problem: J-band Flux
Reversal in Early- to Mid-T Dwarfs
10 m Keck telescope laser-guide star adaptive optics images
T1
0.1336"
T5
• 2MASS 1404 AB: T1 + T5 dwarf binary
• the earlier-type T dwarf, which is bolometrically
brighter, is fainter at J band (only)
Mar 13, 2009
PHY 688, Lecture 20
(Looper et al. 2008)
28
The J-band Flux
Reversal in Earlyto Mid-T Dwarfs
Mar 13, 2009
PHY 688, Lecture 20
(Looper et al. 2008)
29
What Is the Weather on
an Early T Dwarf?
• partly cloudy?
• uniformly hazy?
• pouring “cats and dogs”?
– i.e., silicates and iron
Mar 13, 2009
PHY 688, Lecture 20
30