Abstract
With hydrodynamical simulations we determine the conditions under which an initially coplanar planet-disc system that orbits a member of a misaligned binary star evolves to form a planet that undergoes Kozai-Lidov (KL) oscillations once the disc disperses. These oscillations may explain the large orbital eccentricities, as well as the large misalignments with respect to the spin of the central star, observed for some exoplanets. The planet is assumed to be massive enough to open a gap in the disc. The planet's tilt relative to the binary orbital plane is subject to two types of oscillations. The first type, present at even small inclination angles relative to the binary orbital plane, is due to the interaction of the planet with the disc and binary companion and is amplified by a secular resonance. The second type of oscillation is the KL oscillation that operates on both the planet and disc at larger binary inclination angles. We find that for a sufficiently massive disc, even a relatively low inclination planet-disc system can force a planet to an inclination above the critical KL angle, as a consequence of the first type of tilt oscillation, allowing it to undergo the second type of oscillation. We conclude that the hydrodynamical evolution of a sufficiently massive and inclined disc in a binary system broadens the range of systems that form eccentric and misaligned giant planets to include a wide range of initial misalignment angles (20 to 160 degrees).
Highlights
To date, a total of 1642 exoplanets have been confirmed, of which an estimated 40 − 50% are in binary systems (Horch et al 2014)
In Fu, Lubow & Martin (2015b), we found that self–gravity may suppress the KL oscillations of a disc, but only in relatively high mass discs, Md 0.01 M
The dispersal time will be short compared to the KL timescale for sufficiently wide binaries, with separations of the order of 103 AU. In this limit the dispersal is effectively instantaneous, and the planets shown in Fig. 3 would typically enter KL oscillations
Summary
A total of 1642 exoplanets have been confirmed (see exoplanets.org, Wright et al 2011; Han et al 2014), of which an estimated 40 − 50% are in binary systems (Horch et al 2014). It is important to understand planet formation and evolution in binary systems in order to explain the observed exoplanet orbital and physical properties. We would like to quantify the role of binaries in the formation of eccentric and inclined planetary systems. For (typically) smaller separation systems, where the misalignment of the planetary orbit to the spin of the host star can be measured with the Rossiter–McLaughlin effect (Rossiter 1924; McLaughlin 1924; Queloz et al 2000), misalignments are common (e.g. Triaud et al 2010; Schlaufman 2010; Winn et al 2010). Some planets even have retrograde orbits (e.g Winn et al 2009; Lund et al 2014).
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