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

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

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)


Clouds and the L/T transition

Non-equilibrium chemistry


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



log Kzz (cm2/s) =2



log Kzz (cm2/s) =4



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



Evolution of brown dwarfs


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

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