Planet Formation in Binaries - Planetary Exploration and Science: Recent Results and Advances - page 312

a (AU) a (AU)

0.6 0.8 1.0 1.2 1.4 1.6 1.8 0.6 0.8 1.0 1.2 1.4 1.6 1.8

0.15

6 1 km

5 km

gas

5

t= 5680 yr
q b =0.500
e b =0.3
a b =10.0 AU

0.10

4

3

0.05

2

1

0 0.00

1-5 km
1-1 km
5-5 km

1-5 km
1-1 km
5-5 km

1500 1500

1000 1000

500 500

0 0

0.6 0.8 1.0 1.2 1.4 1.6 1.8 0 2000 4000 6000

r (AU) t (yr)

Fig. 13.5 Planetesimal evolution in an evolving gas disc for a tight binary of separation 10 AU.
To p l e f t : longitude of periastron, after 5,680 years, for 1 km planetesimals, for 5 km ones and for
the gas disc. Top right : orbital eccentricity. Bottom left : encounter velocities between planetesimals
as a function of their sizes. Bottom right : average encounter velocities at 1 AU as a function of time
(From Paardekooper et al. 2008 , courtery of MNRAS)

nevertheless shown that if the gas disc is very massive and almost axisymmetric, its
gravity could in fact reduce encounter velocities amongst planetesimals. However,
this massive axisymmetric disc prerequisite is probably not likely to be generic,
as all hydrodynamical studies of gas discs in binaries have shown that they reach
eccentricities that are at least one order of magnitude higher than the few 10 3
needed in the Rafikov ( 2013 ) model (Kley and Nelson 2007 ; Marzari et al. 2009 ,
2012 ;Zsometal. 2011 ; Müller and Kley 2012 ).
On a more positive note, Xie and Zhou ( 2008 ) showed that during the late stages
when gas dissipates from the disc, the dispersal of planetesimal orbits narrows and
the systems can get accretion friendly again. However, this behaviour probably
occurs too late, once most planetesimals have already been either grounded to dust
or spiralled onto the star because of gas friction (Thebault et al. 2008 ).