Planet Formation in Binaries - Planetary Exploration and Science: Recent Results and Advances - page 307

Fig. 13.2 Observational constraints on discs in binaries. Left panel : Frequency of disc-bearing
systems in close binaries as a function of age in several young stellar clusters (From Kraus et al.
2012 , courtesy of the Astrophysical Journal). Right panel : Measured excess flux in the millimetre
as a function of binary projected separation (From Harris et al. 2012 , courtesy of the Astrophysical
Journal)
There is, however, a potentially bigger problem that is inherent to truncated
discs, i.e. that they should be short-lived. The viscous evolution of a compact
disc is indeed much faster than that of an extended system, and its mass gets
drained, by accretion onto the central star, on shorter timescales. This reduces the
timespan within which gaseous planet can form. As an example, Müller and Kley
( 2012 )haveshownthatfor Cephei-type systems, only for unrealistically low disc
viscosities do they obtain disc lifetimes that are long enough to allow for in situ
giant planet formation by core accretion. Observational confirmation of this short-
lived-disc trend has been obtained by Kraus et al. ( 2012 ), who compared the disc
frequency as a function of age for close ( 40 AU) binaries in several nearby young
associations. They found that the majority of such systems lose their disc in less than
1 Myr, even if a small fraction is able to retain discs to ages close to 10 Myr. Their
preliminary conclusion is that “ 2/3 of all close binary systems clear their disks
extremely quickly, within 1 Myr of the end of envelope accretion. The other 1/3
of close binary systems evolve on a timescale similar to that of single stars”. There
seems thus to exist a disc-in-binary category for which the theoretically expected
faster mass drain does not occur and which could thus be more friendly to planet
formation. A good example for this category could be the young L1551 system,
harbouring two resolved 10 AU wide circumstellar discs in a binary of 45 AU
separation (Rodriguez et al. 1998 ). Note, however, that even for this population of
long-lived binary discs, most estimated disc masses are much lower than for single-
star cases (with, however, some important exceptions, such as L1551).