Creating retrogradely orbiting planets by prograde stellar fly-bys. (arXiv:1812.04104v1 [astro-ph.EP])
<a href="http://arxiv.org/find/astro-ph/1/au:+Breslau_A/0/1/0/all/0/1">Andreas Breslau</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Pfalzner_S/0/1/0/all/0/1">Susanne Pfalzner</a>
Several planets have been found that orbit their host star on retrograde
orbits (spin-orbit angle {phi} > 90{deg}). Currently, the largest measured
projected angle between the orbital angular momentum axis of a planet and the
rotation axis of its host star has been found for HAT-P-14b to be $approx$
171{deg}. One possible mechanism for the formation of such misalignments is
through long-term interactions between the planet and other planetary or
stellar companions. However, with this process, it has been found to be
difficult to achieve retrogradely orbiting planets, especially planets that
almost exactly counter-orbit their host star ({phi} $approx$ 180{deg}) such
as HAT-P-14b. By contrast, orbital misalignment can be produced efficiently by
perturbations of planetary systems that are passed by stars. Here we
demonstrate that not only retrograde fly-bys, but surprisingly, even prograde
fly-bys can induce retrograde orbits. Our simulations show that depending on
the mass ratio of the involved stars, there are significant ranges of planetary
pre-encounter parameters for which counter-orbiting planets are the natural
consequence. We find that the highest probability to produce counter-orbiting
planets ($approx$ 20%) is achieved with close prograde, coplanar fly-bys of an
equal-mass perturber with a pericentre distance of one-third of the initial
orbital radius of the planet. For fly-bys where the pericentre distance equals
the initial orbital radius of the planet, we still find a probability to
produce retrograde planets of $approx$ 10% for high-mass perturbers on
inclined (60{deg} < i < 120{deg}) orbits. As usually more distant fly-bys are
more common in star clusters, this means that inclined fly-bys probably lead to
more retrograde planets than those with inclinations < 60{deg}. (...)
Several planets have been found that orbit their host star on retrograde
orbits (spin-orbit angle {phi} > 90{deg}). Currently, the largest measured
projected angle between the orbital angular momentum axis of a planet and the
rotation axis of its host star has been found for HAT-P-14b to be $approx$
171{deg}. One possible mechanism for the formation of such misalignments is
through long-term interactions between the planet and other planetary or
stellar companions. However, with this process, it has been found to be
difficult to achieve retrogradely orbiting planets, especially planets that
almost exactly counter-orbit their host star ({phi} $approx$ 180{deg}) such
as HAT-P-14b. By contrast, orbital misalignment can be produced efficiently by
perturbations of planetary systems that are passed by stars. Here we
demonstrate that not only retrograde fly-bys, but surprisingly, even prograde
fly-bys can induce retrograde orbits. Our simulations show that depending on
the mass ratio of the involved stars, there are significant ranges of planetary
pre-encounter parameters for which counter-orbiting planets are the natural
consequence. We find that the highest probability to produce counter-orbiting
planets ($approx$ 20%) is achieved with close prograde, coplanar fly-bys of an
equal-mass perturber with a pericentre distance of one-third of the initial
orbital radius of the planet. For fly-bys where the pericentre distance equals
the initial orbital radius of the planet, we still find a probability to
produce retrograde planets of $approx$ 10% for high-mass perturbers on
inclined (60{deg} < i < 120{deg}) orbits. As usually more distant fly-bys are
more common in star clusters, this means that inclined fly-bys probably lead to
more retrograde planets than those with inclinations < 60{deg}. (…)
http://arxiv.org/icons/sfx.gif
