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Planet formation
in binaries
Philippe Thébault
Planet formation in binaries
why bother?
• a majority of solar-type stars in multiple systems
• >90 detected exoplanets in binaries
•Test bench for planet-formation scenarios
Outline
•I Introduction
- exoplanets and circumstellar discs in binaries
- orbital stability
•II Planet formation: the different stages that can go wrong
- disc truncation / grain condensation
- embryo formation
•III Planetesimal accretion: the stage that goes really wrong
•IV Light at the end of the tunnel?
•V Circumbinary planets
I
Exoplanets in Binaries
>12% of detected extrasolar planets in
multiple systems
But...
~2-3% (4-5 systems) ”interesting” cases in
close binaries with ab≈20AU
(Raghavan et al., 2006, Roel et al., 2012)
I
(circumstellar) Exoplanets in Binaries
Gliese 86
HD 41004A
γ Cephei
(Raghavan et al., 2006)
I
Statistical analysis Are planets-in-binaries different?
Roel et al., 2012
Roel et al., 2012
Desidera&Barbieri, 2007
more massive planets on short-period
orbits around close (<100AU) binaries
Different formation process??
(Duchene, 2010)
short period planets
I
Long-term stability analysis
acrit
 0.46  0.38  0.63eb  0.59eb  0.15eb2  0.20eb2
ab
(Holman&Wiegert, 1999)
(David et al., 2003)
(Fatuzzo et al., 2006)
I
M1/M2=0.56
ab= 18AU
eb=0.40
Stability regions:
a few examples
aP= 0.11AU
eP=0.05
Gl 86
M1/M2=0.35
ab= 21AU
eb=0.42
M1/M2=0.25
ab= 19AU
eb=0.41
aP= 2.6AU
eP=0.48
aP= 2AU
eP=0.12
HD196885
 Cephei
I
Statistical distribution of
binary systems
a0 ~30 AU
~50% binaries wide
enough for stable
Earths on S-type orbits
~10% close enough for
stable Earths on P-type
orbits
(Duquennoy&Mayor, 1991)
II
The « standard » model of planetary formation
How could it be affected by binarity?
•Step by Step scenario:
√ 1-protoplanetary disc formation
 2-Grain condensation
x 3-formation of planetesimals
√ 4-Planetesimal accretion
√ 5-Embryo accretion
√ 6-Later evolution, resonances, migration
II
Grain condensation
(Nelson, 2000)
II
Protoplanetary discs in binaries: theory
Artymowicz & Lubow (1994)
tidal truncation of
circumprimary &
circumbinary discs
Müller & Kley (2012)
•Is there enough mass left to
form planet(s)?
•Shorter viscous lifetime for
discs in binaries
Protoplanetary discs
in binaries:
observations
Discs in close binaries do have
shorter lifetimes and are fainter
(Kraus et al., 2012)
Most single stars have 3-5Myr
to form giant planets, but
most (but not all!) tight
binaries have <1 Myr
Different formation process??
II
Last stages of planet
formation:
embryos to planets
(Barbieri et al. 2002, Quintana et al., 2002, 2007,
Thebault et al. 2004, Haghighipour& Raymond
2007, Guedes et al., 2008,...)
Possible in almost the
whole dynamically
stable region
it takes a lot to prevent large
embryos from accreting
(Guedes et al., 2008)
II
very last stages of planet formation:
planetary core migration
“under the condition that protoplanetary cores
can form …, it is possible to evolve and grow a
core to form a planet with a final configuration
similar to what is observed”
(Kley & Nelson, 2008)
III
planetesimal accretion:
Crucial parameter: impact velocity distribution
3 possible regimes :
•dV < Vesc
=> runaway accretion
•Vesc< dV < Verosion => accretion (slowed down)
•Verosion < dV
=> erosion (no-accretion)
It doesn’t take much to stop planetesimal accretion
•Vesc(1km) ~ 1-2m/s
•Vero(1km on 1km) ~ 10-20m/s
III
(e,a) evolution: purely gravitational case
secular
oscillations with
phased orbits
V 
(e2 + i2)1/2 VKep
no <dV>
increase untill
orbit crossing
occurs
III
M2=0.5M1
e2=0.3 a2=20AU
(Thebault et al., 2006))
III
(e,a) evolution: with gas
1km<R<10km
tfinal=5x104yrs
differential
orbital phasing
according to
size
III
typical gas drag run
(Thebault et al., 2006)
(Thebault et al., 2006))
5km planetesimals
1km planetesimals
Differential orbital alignement between objects
of different sizes
dV increase
III
<dV(R1,R2)> distribution
(Thebault et al., 2008)
high <dV> as
soon as R1≠R2
at 1AU from α Cen A and at t=104yrs
III
Benz&Asphaug
, 1999
Critical fragmentation Energy (Q*)
conflicting estimates
III
Accretion/Erosion behaviour
(Thebault et al., 2008)
Vero2<dV
erosion
Vero1<dV<Vero2
unsure
Vesc<dV<Vero1
perturbed
accretion
Vesc<dV<Vero1
”normal”
accretion
at 1AU from α Cen A and at t=104yrs
a Centauri B
III
New
Planet !
erosion
HZ
perturbed
accretion
unsure
”normal”
accretion
0.04AU
(Thebault
et al.,
2009)
”nominal
case”
III
HD196885
PARADOX?
Planet
At least 2 exoplanets
are located in
accretion-hostile
regions
(Thebault, 2011)
IV
“big” (10-50km) planetesimals ?
at 1AU from the primary and at t=104yrs
IV
•
large initial planetesimals?
how realistic is a large « initial » planetesimals population?
depends on planetesimal-formation scenario
-> maybe possible if quick formation by instabilities (for ex.
of Johanssen 2007)
but how do instabilities. proceed in the dynamically perturbed
environment of a binary?
->more difficult if progressive sticking
always have to pass through a km-sized phase
•
in any case, it cannot be « normal » (runaway) accretion
-> « type II » runaway? (Kortenkamp, 2001)
model
IV
coupled hydro/N-body simulations
Paardekooper, Thebault & Mellema, 2008
<dv> always higher
than in the
axisymmetric gas
disc case!
evolving gas disc
IV
The role of the gas disc’s gravity
Fragner, Nelson & Kley (2011)
Inclined disc & circular binary
• dv are increased with respect to the gas-drag-only cases
• High dv even for equal-sized planetesimals
IV
outward migration after the formation of embryos
Payne, Wyatt &Thébault (2009)
IV
different initial binary configuration?
 most stars are born in clusters
early encounters and binary compaction/exchanges are possible:
Initial and final (e,a) for
binaries in a typical cluster
(Malmberg et al., 2007)
IV
different initial orbit for the binary?
Thebault et al., 2009
IV
a slightly inclined binary might help
(Xie & Zhou, 2009)
Favours accretion-friendly impacts between
equal-sized bodies
IV
a slightly inclined binary might help….but
Xie & Zhou, 2009
…collision *rates* decrease dramatically
IV
“realistic” treatment of collisions
• Collisions prevent the onset of
size-phased orbits
• The production of collisional
fragments favours growth by
« dust » sweeping
HZ
HZ
Paardekooper & Leinhardt (2010)
Conclusions
•Gas drag works against planetesimal accretion
•In coplanar systems, in-situ planet formation is difficult in the HZ of
binaries with ~20AU separation
•Outward migration of embryos by a/a ~ 0.25 is possible
•Moderate 1<iB<10o helps, but slows down the accretion
•~50% (?) chance that a 20AU binary was initially wider
•Fragment production and dust sweeping might help
• Different, binary-specific planet-formation scenario? Instabilities?
V
Circumbinary planets: observations
•
Most planets are close
to the inner orbital
stability limit
V
Circumbinary planets: modelling
Paardekooper et al.(2012)
no dust accretion
even in the most favourable
case, no in-situ accretion for
the circumbinary planets
with dust accretion
…but inward type I or type II
migration might solve the
problem…and also explain
the current location of the
planets close to the inner
stability limit
FIN