Young Jupiters are Faint

Jonathan Fortney (NASA Ames)

Mark Marley (Ames), Olenka Hubickyj (Ames/UCSC),Peter Bodenheimer (UCSC), Didier Saumon (LANL)

Don Davis

•Review evolution at young ages

•Nucleated collapse models (Core accretion – Gas capture)

•Alternate early evolution

•Other detectability issues

Burrows et al. 2001

Teff

(K)

log Age (Gyr)

“Arbitrarily Hot Start”

•Initial conditions are uncertain

•initial radii too large for smallest masses

•collapse & accretion not spherical

•“...assigning an age to objects younger than a few Myr is totally meaningless when the age is based on models using oversimplified initial conditions.” Baraffe et al. (2003)

•When can the models be trusted?

•Can initial conditions be improved?

Early Model Evolution

Nucleated Collapse Model• Model for accretion of giant planets

• 10 to 20 M⊕ core

forms first, initiates collapse of nebula• Time to gas runaway sensitively depends on atmospheric opacity• Peak accretion luminosity, created by shock, is short lived• Gives initial boundary condition for subsequent evolution

Hubickyj, Bodenheimer & Lissauer (2005)

Deviations are greater at larger masses

Arbitrarily hot start overestimates radius and under-estimate gravity at all masses

• Opacity of proto-atmosphere affects formation time, as does surface density of the nebula

• Only Podolak (2003) has tried to calculate the opacity of the proto-atmospheres during formation

• When does t = 0?

• Agreement with standard cooling models is even worse if one assigns t=0 to the post-formation time

Hubickyj, et al (2005)

How long is the formation time?

A Potential Application: 2M1207

Companion•Companion to ~M8 brown

dwarf in TW Hydrae (age ~ 8 Myr)

•red J-K implies late L, Teff ~ 1250 K

•Models give M = 5 ± 2 MJup

Chauvin et al. (2004)

Burrows et al. 1997

Teff

(K)

log Age (Gyr)

Real mass closer to 10 MJ ?

Close et al. (2005) – young M star

Mohanty et al. (2004a,b)Comparisons with hi-res spectraMasses down to deuterium burning limit

Zapatero Osorio et al. (2004)Dynamical masses of GJ 569 Bab brown

dwarfs

AB Dor C

Similar Problem for Other Objects?

Reiners et al. (2005) – young M star

Moral•Discern mass from g, Teff indicators in spectra

& colors, not luminosity at young ages (This was just done for GQ Lup b)

•(Of course, this isn’t always easy…)

from Knapp et al. (2004)

SOri70

log g = 4

log g = 5.5

Which Bandpasses to Search?

M band Jupiter image courtesy Glenn OrtonJupiter’s M band flux has stories to tell!

Nonequilibrium CO dims M band

Saumon et al. 2003

Saumon et al. 2003

L’ May Be Comparable to M’

L’

Conclusions•Luminosity of young giant planets depends

sensitively on initial conditions

•Nucleated collapse models are cooler, dimmer, and smaller than generic ‘hot start’ evolution calculations. Differences...

•persist longer than “a few million years”

•are more significant at larger masses

•Use of ‘hot start’ evolution may result in substantially underestimating mass of observed objects, depending on actual formation mechanism