Planet Formation in Binaries - Planetary Exploration and Science: Recent Results and Advances - page 306

large, often exceeding 100 times the radial distance of the exoplanet to the primary,
that binarity should have had a very limited effect in the planet-forming regions.
The situation should, however, be radically different for the handful of planets
in 20 AU binaries, notably for Cephei Ab, HD 196885 Ab and HD 41004
Ab, where the planet is located close to the orbital stability limit (Fig. 13.1 ). It is
unlikely that planet formation in these highly perturbed environments could proceed
unaffected by the presence of the companion star.

13.3 Early Stages of Planet Formation

13.3.1 Protoplanetary Disc Truncation

The planet-formation process can be affected by binarity right from the start, during
the formation of the initial massive gaseous protoplanetary disc. Artymowicz and
Lubow ( 1994 ) and Savonije et al. ( 1994 ) have shown that such a disc can be
tidally truncated by a stellar companion. This truncation distance depends on several
parameters, such as the disc's viscosity, but is in most cases roughly comparable to
the outer limit for dynamical stability (see previous section), i.e. typically at 1/3-1/4
of the binary's separation for non-extreme value of the orbital eccentricity.
Surveys of discs around young stellar objects (YSOs) have given observational
confirmation that discs in close binaries indeed tend to be both less frequent and
less massive than around single stars. For the 2 Myr old Taurus-Auriga association,
Kraus et al. ( 2012 ) have shown that whilst the disc frequency in wide binaries
remains comparable to that of single stars ( 80 %), it significantly drops for
separations 40AUandisaslowas 35 % for binaries tighter than 10 AU

(Fig. 13.2 left). Moreover, the millimetre-wave dust continuum imaging survey of
Harris et al. ( 2012 ) for discs in the same Taurus-Auriga cluster has shown that the
estimated dust mass contained in these discs strongly decreases with decreasing
binary separation (Fig. 13.2 right). This agrees very well with the theoretical
prediction of more compact and thus less massive discs in tight binaries.
This less frequent and more tenuous disc trend leads to two major problems
when considering planet formation. This first one is that truncated discs might
not contain enough mass to form planets, especially Jovian objects such as those
that have been observed for the three most “extreme” systems: HD 196885, HD
41004 and Cephei. This issue has been numerically investigated for the specific
case of Ceph-type systems, and Jang-Condell et al. ( 2008 ) have found that
for the most reasonable assumptions regarding the disc's viscosity and accretion
rate, there is just enough mass left in the truncated disc to form the observed
giant planet. This encouraging result has been later confirmed by Müller and
Kley ( 2012 ) using a different numerical approach. Note, however, that there might
not be enough mass left in the truncated Ceph disc to form another Jovian
planet.