Dynamics of passing-stars-perturbed binary star systems

In this work, we investigate the dynamical effects of a sequence of close encounters over 200 Myr varying in the interval of 10 000–100 000 au between a binary star system and passing stars with masses ranging from 0.1 to 10  $\, \mathrm{M}_{\odot }$ . We focus on binaries consisting of two Sun-like stars with various orbital separations a 0 from 50 to 200 au initially on circular planar orbits. We treat the problem statistically since each sequence is cloned 1000 times. Our study shows that orbits of binaries initially at a 0 = 50 au will slightly be perturbed by each close encounter and exhibit a small deviation in eccentricity (+0.03) and in periapsis distance (+1 and −2 au) around the mean value. However increasing a 0 will drastically increase these variances: up to +0.45 in eccentricity and between +63 and −106 au in periapsis, leading to a higher rate of disrupted binaries up to 50 per cent after the sequence of close encounters. Even though the secondary star can remain bound to the primary, ∼20 per cent of the final orbits will have inclinations greater than 10 ^○ . As planetary formation already takes place when stars are still members of their birth cluster, we show that the variances in eccentricity and periapsis distance of Jupiter- and Saturn-like planets will inversely decrease with a 0 after successive fly-bys. This leads to higher ejection rate at a 0  = 50 au but to a higher extent for Saturn-likes (60 per cent) as those planets’ apoapsis distances cross the critical stability distance for such binary separation.