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advocate such a km-size-straddling formation mode. However, it remains to be seen
if these formation mechanisms can operate in the highly perturbed environment of
abinary.
Even if the kilometre-sized planetesimal phase cannot be bypassed, recent studies
have shown that there is a possibility that these bodies might after all grow even in
the presence of high-v collisions. Xie et al. ( 2010b ) have analytically investigated
the “snowball” growth mode, in which planetesimals accrete mass preferentially by
sweeping up of dust particles instead of mutual collisions with other planetesimals.
If the mass density contained in dust exceeds that contained in large bodies, then
this mechanism could provide a viable growth mode in binaries, because dust-on-
planetesimal impacts can result in accretion for v much higher than planetesimal-
on-planetesimal ones. These results have been strengthened by Paardekooper and
Leinhardt ( 2010 ), who incorporated for the first time a collisional evolution model
in their simulations, taking into account the fragments produced by high-velocity
impacts. Their runs have shown that many planetesimals are able to re-accrete, by
sweeping, a large fraction of the mass that has been lost to small fragments by earlier
high-v collisions. Moreover, frequent collisions with these small fragments can
prevent km-sized planetesimals from reaching their equilibrium secular+gas-drag
imposed orbits, so that their mutual impact velocities never fully reach the high
values resulting from the differential phasing effect. However, the collision outcome
prescription of Paardekooper and Leinhardt ( 2010 ) is still relatively simplistic, with
all bodies smaller than 1 km being treated at dust coupled to the gas, and more
sophisticated models have to be tested.
Another promising explanation for the presence of S-type planets in very close
binaries is that these binaries were initially wider than today. As it happens, there
is in fact theoretical support for this hypothesis. If indeed most stars are born in
clusters, then they should experience many close encounters in their early history.
Malmberg et al. ( 2007 ) have shown that, for a typical stellar cluster, some very wide
binaries get broken by close encounters, but for binaries that survive, the main effect
of these encounters is to shrink their orbits. Interestingly, for the cluster setup they
considered, Malmberg et al. ( 2007 ) found that around half of the binaries having
a present separation of 20 AU have had a significant orbit-shrinking encounter
(see Fig. 13.9 ). 2 However, for the specific case of HD 196885, we run a series
of simulations, each time increasing the binary separation, and found that the
present-day planet location became accretion friendly only for a binary separation
of 45 AU. This means that this separation should have shrunk by at least a factor 2
during the system's evolution, and it remains to be seen if such a change is realistic.
Perspectives are however more optimistic for the HZ of the Ǜ Centauri system, for
which only a moderately larger and/or less eccentric initial binary orbit would be
enough to allow planetary accretion there (see Fig. 13.10 ).
2 The simulations of Malmberg et al. ( 2007 ) were, however, only assuming a single, hypothetical,
initial setup. More generic numerical investigations, exploring a wider range of possible initial
conditions, should be carried out.
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