Geoscience Reference
In-Depth Information
13.3.2
Grain Condensation and Growth
The next stage of planet formation, the condensation of small grains and their
growth into larger pebbles and eventually kilometre-sized planetesimals, has not
been extensively studied in the context of binary systems. One main reason is
probably that this stage is the one that is currently the least understood even in the
“normal” context of single stars (e.g. Blum and Wurm
2008
), so that extrapolating
it to perturbed binaries might seem premature. A noteworthy exception is the study
by Nelson (
2000
) showing that for an equal-mass binary of separation 50 AU,
temperatures in the disc might stay too high to allow grains to condense. These
results were recently confirmed by the sophisticated 3-D modelling of radiative
discs by Picogna and Marzari (
2013
), who found strong disc heating, due to spiral
chocks and mass streaming between the circumprimary and circumsecondary discs,
in binaries of separation 30 and 50 AU. On a related note, Zsom et al. (
2011
)showed
that even if grains can condense, binary perturbations might impend their growth by
mutual sticking because of too high impact velocities.
13.4
Planetesimal Accumulation
13.4.1
Context
The next stage of planet formation starts once “planetesimals”, i.e. objects large
enough (typically sub-kilometre to kilometre sized) to decouple from the gas,
have formed. Within the standard planet-formation scenario, this stage leads, by
mutual accretion amongst these km-sized planetesimals, to the formation of large
planetary embryos. This stage is the one that has been by far the most extensively
studied within the context of binary systems. The main reason is that the accretion
of planetesimals and their growth to planetary embryos is potentially extremely
sensitive to dynamical perturbations, since it does not take much to hamper or
even halt the mutual accretion of kilometre-sized objects. Indeed, in the standard
simulations of planet formation around single stars, this stage proceeds through
fast runaway and oligarchic growth that require very low impact velocities between
colliding bodies, typically smaller than their escape velocity, i.e. just a few m.s
1
for
kilometre-sized objects (e.g. Lissauer
1993
; Kokubo and Ida
2000
). These values are
less than
10
4
that of typical orbital velocities, so that even moderate dynamical
perturbations can have a dramatic effect by increasing impact velocities beyond an
accretion-hostile threshold.
The crucial parameter sealing the fate of the planetesimal swarm is thus their
encounter velocities, and most studies of this stage have numerically investigated
how this parameter evolves under the coupled effect of stellar perturbations, gas
drag and physical collisions.
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