How did the planet in the Gamma-Cephei binary system form? (Forwarded)

Observatoire de Paris
Paris, France

Contact:
Philippe Thébault, Observatoire de Paris, LESIA
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Email: Philippe.THÉBAULT @ obspm.fr

23 August 2004

How did the planet in the Gamma-Cephei binary system form?

The formation of a planet in a binary star system poses serious problems, in
particular when the two stars are very close, like in the system of
Gamma-Cephei. A giant planet was discovered there, close to the primary star,
but the perturbations from the secondary star should have prevented the
accretion of planetesimals. An international team of researchers, led by an
astronomer from Paris Observatory, has just shown how the presence of residual
gas from the initial nebula could allow the formation of the planet. The
problems are not all solved, however, since the predicted position of the planet
is too close to the primary star with respect to what is observed.

Gamma-Cephei, a challenge for the standard planetary formation scenario

Among the more than 100 extra-solar planets today discovered, 15 have been found
within binary systems. One of the more interesting of these systems is
Gamma-Cephei, since it is the one with the closest companion star (Figure1). The
detected planet is slightly more massive than Jupiter and orbits at 2.1 AU from
the central star.

Previous studies have shown that this planet is on a stable orbit, but the
question is how it could form within such a binary system: the companion star is
indeed so close to the primary that its perturbing effect could prevent
planetary accretion. Indeed, the "standard" scenario of planetary formation
requires a dynamically "quiet" environment. Only in this quiet environment can
impacts between planetesimals -- planetesimals being the primordial km-sized
"bricks" whose accretion will lead to ~1000km-sized planetary embryos -- be soft
enough in order to lead to mutual accretion rather than erosion.

Calling numerical simulations to the rescue

To study the dynamical conditions allowing accretion in such a system, the
crucial parameter is thus the impact velocity among planetesimals during the
accretion phase. This velocity distribution can be studied using numerical
simulations. Such simulations show that the secondary star induces strong
perturbations of planetesimal orbits in the 1-4 AU region (orbits beyond 4 AU
are unstable), with important eccentricity oscillations. These oscillations
yield relative velocities higher than 1 km/s, thus preventing accretion (Figure 2).

The situation becomes more favourable to planetary accretion when taking into
account gas drag effect on planetesimals. It is indeed likely that a substantial
amount of gas from the initial nebulae was still present as planetesimal
accretion started. Should this gas be dense enough, then it would tend to align
planetesimal orbits, hence lowering their impact velocities. Planetesimal sizes
are here a crucial parameter: the smaller there are, the more sensitive to
gaseous friction they get. Our simulations show that, for a gas nebulae slightly
denser than the "standard" minimum mass solar nebulae and for planetesimals
larger than 5 km, friction reduces encounter velocities to values allowing
accretion (Figure 3). The (yet unanswered) question is: did such a high gas
density stage really occur?

But even if planetary embryos of ~1000 km could form, problems are not over yet.
There is one important stage left before the completion of planetary formation,
i.e. the final mutual perturbations and accreting encounters between those large
embryos. Here again, perturbations by the companion star might hinder the
process. However, new specific simulations show that this stage might
successfully complete despite the perturbing star. There is nevertheless one
important problem: the final planet is never at the right location; it always
ends up within 1.5 AU from the central star regardless of the initial conditions
chosen (Figure 4). How can we reconcile this puzzling result with observational
data? Several hypothesis might be considered. It is for instance possible that
the separation between the 2 stars was larger in the beginning and was reduced
after the formation of the planet. This could be the case if the binary system
was initially in a clustered stellar environment where neighbouring stars might
have dynamically perturbed the system. It is also possible that one or several
additional yet-undetected giant planet(s) orbit around Gamma Cephei and that
mutual perturbations among planets might have affected their initial locations.

Conclusion

As it appears from this study the "standard" planetary formation scenario
encounters here several problems. It requires very specific initial conditions
in order to successfully complete, in particular a high initial gas density that
can sufficiently damp impacts between planetesimals but also specific dynamical
configurations (early perturbations by neighbour stars, presence of additional
giant planets, ...) in order to explain the final location of the planet at 2.1
AU. Upon looking at these difficulties, a possible solution might be to look for
alternative mechanisms for forming the planet. Such mechanism have been
proposed, in particular the possibility for giant planets to form in a "stellar"
way, by direct gravitational instabilities within the initial accretion nebulae.
The problems remains yet open ...

References

Thébault, P., Marzari, F., Scholl, H., Turrini, D., Barbieri, M., "Planetary
formation in the Gamma Cephei system", 2004, accepted in Astronomy & Astrophysics,
http://arxiv.org/abs/astro-ph/0408153

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IMAGE CAPTIONS:

[Figure 1:
http://www.obspm.fr/actual/nouvelle/aug04/Gamma-f1.gif (2KB)]
The Gamma Cephei system

[Figure 2:
http://www.obspm.fr/actual/nouvelle/aug04/Gamma-f2.gif (51KB)]
Effect of the companion star perturbations on a population of test
planetesimals. Particles orbits are initially circular (e = 0) between 1 and 5
AU and are gradually excited, with large eccentricity oscillations which get
narrower and narrower as time proceeds. At some point, these eccentricity
oscillations become so narrow that they lead to orbital crossing between
neighbouring planetesimals at very high encounter velocities, ~ 1km/s, thus
preventing any accretion.

[Figure 3:
http://www.obspm.fr/actual/nouvelle/aug04/Gamma-f3.gif (48KB)]
Same as Fig.2, but with gas drag. The effect of the gas is to damp the orbital
perturbations induced by the companion star but also to align neighbouring
planetesimals orbits. This prevents the large eccentricity oscillations of Fig.2
to set in, and thus strongly reduces encounter velocities between particles.
Another consequence of gaseous friction is the progressive orbital drift towards
the central star it induces on planetesimals, with in particular the progressive
emptying of the outer regions of the system.

[Figure 4:
http://www.obspm.fr/actual/nouvelle/aug04/Gamma-f4.gif (3KB)]
Mutual evolution of planetary embryos of initial size ~1000 km. As time goes on,
embryos progressively accrete each other to form fewer but increasingly larger
objects. At the end of the run, a 10 Earth-masses body, i.e. with a mass
sufficient to trigger the final gas infall leading to a Jupiter-sized planet,
have formed. However, it's final position is around 1,5 AU, well within the
actual location of the observed planet.