Ecole Normale Supérieure, 22 March 2011, LA-UR-11-01402
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Transcript of Ecole Normale Supérieure, 22 March 2011, LA-UR-11-01402
Ecole Normale Supérieure, 22 March 2011, LA-UR-11-01402
Images: Cassini; Marois et al. (2008)
D. Saumon
Los Alamos National Laboratory
Clouds in brown dwarfs
and hot young planets
Modeling Mark Marley (NASA Ames)
Katharina Lodders (Washington U.) Richard Freedman (NASA Ames)
Observations/Analysis
Michael Cushing (IPAC)Sandy Leggett (Gemini Observatory)Tom Geballe (Gemini Observatory)
Adam Burgasser (UCSD)Denise Stephens (Brigham Young)
Spitzer/IRS Dim Suns Team: Tom Roellig, Amy Mainzer, John Wilson, Greg Sloan,
Davy Kirkpatrick, Jeff Van Cleve
Brown dwarfs and hot young planets
Clouds and the LT transition
Back to hot young planets
The basics of brown dwarfs
Main sequence stars
Brown dwarfs
Planets
Substellar in mass ~ 12 to 77 MJupiter
Compact! R ~ 0.09 to 0.16 Rsun (all ~ size of Jupiter) No stable source of nuclear energy Evolution = cooling: Lbol and Teff decrease with time
The basics of brown dwarfs
Two new spectral classes cooler than dM: L (Teff ~ 2400 1400K) T (Teff ~ 1400 600K) … and soon Y
Very strong molecular bands H2O, CO, CH4, NH3, FeH, TiO
Condensates form in the atmosphere for Teff 2000K
M6.5
L5
T4.5
From Cushing et al. (2006)
Spectral energy distributions: M, L and T dwarfs
NIR color-magnitude diagrams: Field brown dwarfs
Data from Knapp et al (2004), Burgasser et al. (2006), Liu & Leggett (2005)
L dwarfs naturally extend the dM sequence
T dwarfs are “blue” in near-IR colours!
Rapid shift in IR colours at the LT transition.
Early T’s are brighter in J than late L’s!?
Sample of local disk brown dwarfs
Hot young planets
HR 8799 b,c,d,e
Star: A5 V 30160Myr old
Inner & outer dust disks (R ~ 8AU and R ~ 66AU)
b c d e
a (AU) 68 38 24 12
Mass (MJ)* 5-7 7-10 7-10 7-10
Teff (K)* ~900 ~1100 ~1100 ?
Marois et al. (2008, 2010), Currie et al. (2011)* Masses and Teff are approximate
Near IR color-magnitude diagrams: Young planets
Planet data from Marois et al. (2008), Lafrenière et al. (2008) and Neuhäuser (2008)
In the near IR, hot young planets ~ consistent with the L dwarf sequence and its extension
Hot young planets compared to field brown dwarfs
Similarities
Young planets far from their star have effectively evolved in isolation
Their NIR colours extend the brown dwarf sequence
Teff range (L~ 2400-1400K, T~1400 – 600K)
Differences
Lower gravity (lower mass, younger)
Possibly metal-rich if formed in a protoplanetary disk
Hot young planets are more hip
Atmospheric physics & chemistry should be very similar
Brown dwarfs and hot young planets
Clouds and the LT transition
Back to hot young planets
Color-magnitude diagrams: Limiting cloud models
Cloudless models can explain late T dwarfs (>T4) A diffuse cloud model works best for optically thin clouds (<L4)
A more sophisticated cloud model is required (cloud deck)
condensation
level
diffuse cloud
clouddeck
nocloud
Condensation and qualitative cloud behaviour
Condensation:
Fe (Teff ~ 2000K)
Silicates (Teff ~ 1600K)
H2O (Teff ~ 300K)
Main chemical transitions:
CO CH4 (Teff ~ 1200K)
N2 NH3 (Teff ~ 800K)
photosphere
Teff
2000K
1600K
1200K
800K
400K
}}
L
T
A cloud layer will gradually disappear below the photosphere
as Teff decreases cloudless atmosphere!
A simple cloud model
A minimalist 1-D cloud model results from a balance between:
1) Vertical transport of particles (e.g. turbulent diffusion)2) Gravitational settling of the particles
0* =qwfz
qK csed
tzz
Kzz: coefficient of diffusive mixingfsed: dimensionless sedimentation parameter
(as fsed increases, cloud deck becomes thinner)
The location of the bottom of the cloud is determined by the condensation curve
Ackerman & Marley (2001)
Color-magnitude diagrams: Models with clouds
For L dwarfs, fsed ~ 1-2The LT transition occurs over Teff ~ 1400 1200K Transition can be accounted for by an increase of fsed
At constant fsed, the cloud layer gradually disappears below the photosphere
The L T transition as cloud clearing
The L T transition can also be explained as a gradual clearing of the cloud
Transition from fsed=2 to cloudless models.
Marley, Saumon & Goldblatt (2010)
Clouds in brown dwarfs: Summary
Condensates (Fe & silicates) are present throughout the L spectral class (Teff > 1400K)
The condensates gravitationally settle into cloud decks
Mid- to late-T dwarfs appear to be free of clouds
Clearing of clouds at the LT transition indicated by CMDs
Physical mechanism: unknown
An increase of sedimentation efficiency?
A break up of the cloud layer?
There are hints that the LT transition is gravity sensitive
The LT transition must also occur in hot young planets
Brown dwarfs and hot young planets
Clouds and the LT transition
Back to hot young planets
HR 8799b, c, d photometry: Clouds
All 3 planets:
zJHKL’M photometry can only be fitted
with cloudy atmospheres models
Marois et al. (2008)
HR 8799b 2.2m spectrum: Detection of CH4
Bowler et al. (2010)
When fitted with field BD spectra, this spectrum matchesthat of T1-T3 dwarfs. CH4 is present in this spectrum
planet b
Trouble with HR 8799bcd
9 photometric channels spanning 1.03 to 5m d=39.4 1.0 pc
Luminosities of log L/L= - 5.1, - 4.7 and - 4.7 (0.1)
Age of the star: 30-160Myr
Evolution models then give M, R, Teff, gravity
Using a variety of brown dwarf atmosphere models, fits to the photometry give Teff for all 3 planets that are too high (or R too small).
Bowler et al. (2010)
planet b
Trouble with HR 8799bcd
Burrows, Sudarsky & Hubeny (2006)
Brown dwarf models used by Bowler et al (2010) & Currie et al (2011)
The model E sequences do not enter the mag-color space of the L8-L9dwarfs Can fit the color but not the magnitude (radius) of the planet
Model A cloud extends to the top of the atmosphere (much thicker than E) more promising (as found by Currie et al 2011)
Model E
MJ
J-K
MJ
J-K
A new class of objects?
Madhusudhan, Burrows & Currie (2011)
` Teff log g M/MJ Age (Myr)
b 850 4.3 12 150
c 1000 4.2 11 65
d 900 3.8 6 20
-1 0 1 2 3J-Ks
MJ
12
14
16
model E
mo
del A
E
A new, intermediate, “AE” cloud modelGood fits for all 3 planetsConsistent with evolution and age of the system (30-160Myr)
Claim: The hot young planets occupy a different NIR color space than BDs
Require models with very thick clouds to explain their colors
Our fits of HR 8799 bcd (work in progress)
These best fitting model spectra for c & d are too old and too massive!BUT: within 1.5, models agree with the upper age limit of 160Myr
Teff log g fsed M/MJ Age
b 900 4.25 1 11 150Myr
c 1200 5 1 38 500Myr
d 1100 5 2 38 1Gyr
Data from Marois et al. (2008), Hinz et al. (2010), Currie et al. (2011)
A different view of clouds in hot young planets
These planets look like very late L dwarfs only colder
Need only to delay the cloud clearing to lower Teff (< 1400K)
The Teff of the L T cloud clearing appears to be rather sensitive to gravity (HD 203030 B, 25 Mj, has Teff ~1150K, but is cloudy)
Hot young planets have much in common with field brown dwarfs and can be modelled with the same tools and physics
The study of clouds in field brown dwarf is highly relevant to hot young planets:
Cloud decks of iron and silicate particles
Clouds present for Teff > 1400K (all L), absent for Teff<1200K (late T) in field brown dwarfs
Clouds disappear over only 200K of cooling in Teff
Apparently gravity dependent (e.g. HD 203030 B, 2MASS 1207B?)
Mechanism presently unknown
Exoplanets: What have we learned from brown dwarfs?
HR 8799 planets:
NIR colors similar to those of the latest L dwarfs but b & d are fainter
Their NIR colors extend the cloudy L dwarf sequence
Good fits of the 1 – 4.7m photometry can be obtained with cloudy models
Claims that they have much thicker clouds than field brown dwarfs are not justified
The same cloud model (and same fsed range) as for BDs is adequate
Understanding these planets only requires that the cloud clearing of the L T transition occurs at lower Teff for lower gravities
Independent evidence of the latter has been accumulating
Exoplanets: What have we learned from brown dwarfs?
Backup slides
New spectral class: L dwarfs
K I
TiO TiO TiO TiO
VO VO
FeH
H2O
Cs I
Cs I
Rb I
CrH
FeH
K I
Adapted from Kirkpatrick et al. (1999)
TiO and VO bands peak at dM9 then weaken through L sequence
Strong CrH and FeH
FeH peaks at mid-L
Lines of alkali metals appear
CO at 2.3m
Strong H2O
J-K increases steadily
Teff ~ 2400 – 1400K
L dwarfs are nearly all brown dwarfs
New spectral class: T dwarfs
Spectra from Geballe et al. (2002)
CH4 appears in H and K bands
CO (2.3m) disappears by T2
Very strong H2O
No TiO or VO, weakening FeH and CrH
Lines of alkali metals
J-K decreases steadily
Teff ~ 1400 – 600K
T dwarfs are all brown dwarfs
1 2.5
Hot young planets
Fomalhaut b
Star:
A3 V 300500Myr old dust disk (R=133-160AU)
Planet:
a ~ 115 AU Mass* ~ 1 3 MJupiter
Teff * ~ 400K
Kalas et al., Science, 322, 1345 (2008)
* Mass and Teff are early estimates
Other hot young planets?
GQ Lup b2MASS 1207 b
AB Pic b Pic b1XRS
J1609 b
star K7 (TTau) M8 (BD) K2 V A5 V K7 V
age (Myr) 1-2 5-12 30 8-20 4-6
a (AU) >100 >54 >250 8-15 >330
Mass (MJ) 1-42 2-70 11-70 6-12 6-12
Teff (K) 2400-1600 2000-1100 2400-1600 ~1400 ~1800
notesToo young for models
Not bound to 2M1207A
Brown dwarf?
L’ data only
Lafrenière et al (2008), Lagrange et al. (2010) and data compiled in Neuhäuser (2008)
Teff=800 K
cloud= 2/3
Teff=1200 K
cloud= 2/3
Clouds and photospheres
All models: log g=5 fsed=3
Teff=1600 K
cloud= 2/3
Teff=2000 K
Fits of entire SED of brown dwarfs
Stephens et al.(2009)
Fits of entire SED of brown dwarfs
Stephens et al.(2009)
Brown dwarfs and hot young planets
Clouds and the L/T transition
Non-equilibrium chemistry
Back to hot young planets
Unexpected species in the atmosphere of Jupiter
AsH3
Noll, Larson & Geballe (1990)
Bjoraker, Larson & Kunde (1986)
CO
GeH4
Bézard et al. (2002)
Chemistry and vertical transport in brown dwarf atmospheres
Chemistry of carbon & nitrogen:
slow
CO + 3H2 CH4 + H2O fast
slow N2 + 3H2 2NH3
fast
Transport:
Convection (mix ~ minutes)
Radiative zone (mix ~ hours to years?)
e.g. meridional circulation
Two characteristic mixing time scales in the atmosphere
CO &N2: ms to > Hubble time!
sun
Effect of mixing on abundances
Net effect:
Excess of CO in the spectrum
Depletion of CH4, NH3 and H2O
For CO, CH4 and H2O, the faster the mixing in the radiative zone, the larger the effect (up to saturation)
A way to measure the mixing time scale in the atmosphere!
The most important opacity sources!}
A case study: Gl 570D (T8)
Teff=820K log g=5.23
[M/H]=0
Equilibrium model
Model with mixing
Model fluxes are computed at Earth
Saumon et al. (2006)
Gl 570D: 3-5m spectrum and photometry
Teff= 820K log g=5.23 [M/H]=0
Equilibrium (no CO) With mixing (excess CO)
Absolute fluxes
CH4
CO
Geballe et al. (2009)
Departure from chemical equilibrium: Summary
The consequence of very basic considerations (“universality”)
Important at low Teff: late L dwarfs and T dwarfs
Affects CO (), CH4 () , H2O () and NH3 (), all important sources of opacity
CO excess observed in all 6 T dwarfs subjected to detailed analysis (e.g. Gl 570D)
NH3 depleted in the IRS spectra of 3 T dwarfs analyzed so far
Additional evidence shows that departures from equilibrium chemistry (i.e. vertical mixing) are common in brown dwarfs
An opportunity to measure the mixing time scale in the atmosphere
Mixing mechanism: turbulent break up of gravity waves?
Chemistry and vertical transport
Tem
per
atu
resurface
CO < mix
CH4 < mix
CO > CH4
(equilibrium)
CH4 > CO (excess CO)
CO > mix
CH4 < mix
slow CO CH4
fast CH4 CO
Cloudless model Teff=1000Klog g=5
equilibrium
Non-equilibrium abundances in brown dwarfs
Non-equilibrium abundances in brown dwarfs
Cloudless model Teff=1000Klog g=5
log Kzz (cm2/s) =2
Non-equilibrium abundances in brown dwarfs
Cloudless model Teff=1000Klog g=5
log Kzz (cm2/s) =4
Non-equilibrium abundances in brown dwarfs
Cloudless model Teff=1000Klog g=5
log Kzz (cm2/s) =6
Ground-based photometry: K, L’ & M’
The K L’ M’ color-color diagram provides strong evidencethat non-equilibrium chemistry (i.e. enhanced CO) isa common feature of T dwarfs
T dwarfs
L dwarfs
M dwarfs
Adapted from Leggett et al. (2007)
Effect on 4.6m CO band
Teff=1000K log g=5Kzz=0, 102, 104, 106 cm2/s
Kzz=0, 104 cm2/s
M’
4-5m M’ flux
Can affect searches for exoplanets
Effect on CO bands
Teff=1000K log g=5Kzz=0, 102, 104, 106 cm2/s
Kzz=0, 104 cm2/s
M’
K band 4-5m M’ flux
Can affect searches for exoplanets
Effect on NH3 bands
Teff=1000K log g=5
Kzz=0, 102, 106 cm2/s Kzz=0, 104 cm2/s & no NH3
log g=5
H band K band 10m
Gl 570D: M band spectrum
Teff=820K log g=5.23 [M/H]=0
Equilibrium model (no CO)
With mixing: mix=14d
With mixing: mix=6.7h
Model fluxes normalized to data
6.5h of integration on Gemini…
CO is present in the spectrum (4.4 detection)
Geballe et al. (2009)
Other examples of non-equilibrium abundances in brown dwarfs
ObjectSpectral
Type
Teff
(K)
log g
(cm/s2)[M/H]
log Kzz
(cm2/s)
Under abund of
NH3?
Excess of CO?
2MASS 0415
T8 725 - 775 5.00 - 5.37 0 > 4
Gl 570D T7.5 800 - 820 5.09 - 5.23 06.20.
7
2MASS 1217
T7.5 850 - 950 4.80 - 5.42 0.3 > 2Data too
noisy
Gl 229B T7p 870 - 1030 4.5 - 5.5-0.5 to
- 0.14 - 5
Marginal data
2MASS 0937
T6p 925 - 975 5.20 - 5.47 -0.3 4.30.3
Ind Ba T11300 -1340
~5.50 -0.2 7-8? No data
HR 8799c spectroscopy: Non-equilibrium?
Models in chemical equilibrium (with or without clouds) cannot reproduce the shape of the 4m spectrum
Janson et al. (2010)
Cloudy modelTeff=1100K, log g=4
HR 8799b, c, d photometry: Excess CO?
Models in chemical equilibrium are inconsistent with the M band upper limits
Hinz et al. (2010)
(800-900K)
(1000-1100K)
(1000-1100K)
Burrows, Sudarsky & Hubeny (2006) clouds
Model E
Model E
Brown dwarfs and hot young planets
Clouds and the LT transition
Evolution of brown dwarfs
Back to hot young planets
Evolution of brown dwarfs
end of the main sequence
time
BD evolution is primarily: Cooling, contraction and a phase of deuterium fusion
Controlled by surface boundary condition
Opacity of the atmosphere! (clear vs cloudy)
Brown dwarf evolution: a hybrid modelThe L-T transition can be reproduced by increasing fsed
Toy model of the evolution: Boundary Condition: cloudy (Teff > 1400K) to clear (Teff < 1200K) Interpolate atmospheric B.C. (not the evolution) in betweenSynthetic disk population IMF: dN/dM ~ M-1 SFR: Constant (0-10Gyr) Monte Carlo sampling Let evolve and…
cloudyclear
evolution
10 Gyr envelope
Reproduces CMD fairly well
Brown dwarf evolution: a hybrid model
clearcloudyhybrid
Observations: An apparent dearth of brown dwarfs in the transition!
A more complete sample should reveal a pile up at the transition
Pile up of brown dwarfs in the L-T transition A direct consequence of changes in the atmosphere
Synthetic galactic clusters
Synthetic clusters Hybrid evolution IMF: dN/dM ~ M-0.6 Constant SFR over age 1Myr (~ isochrone)
A distinctive feature due to deuterium fusion (50-100Myr)In principle, detectable with deep enough surveys
Synthetic disk population of brown dwarfs
Assume IMF: dN/dM ~ M-1
Constant SFR (0-10Gyr) Single BDs only
Monte Carlo sampling
Let evolve and…
evolution
10 Gyr envelope
Brown dwarf evolution: variations on hybrid model