The Degree of Alignment Between Circumbinary Disks and Their Binary Hosts. (arXiv:1906.03269v1 [astro-ph.EP])

The Degree of Alignment Between Circumbinary Disks and Their Binary Hosts. (arXiv:1906.03269v1 [astro-ph.EP])
<a href="http://arxiv.org/find/astro-ph/1/au:+Czekala_I/0/1/0/all/0/1">Ian Czekala</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Chiang_E/0/1/0/all/0/1">Eugene Chiang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Andrews_S/0/1/0/all/0/1">Sean M. Andrews</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Jensen_E/0/1/0/all/0/1">Eric L. N. Jensen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Torres_G/0/1/0/all/0/1">Guillermo Torres</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wilner_D/0/1/0/all/0/1">David J. Wilner</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Stassun_K/0/1/0/all/0/1">Keivan G. Stassun</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Macintosh_B/0/1/0/all/0/1">Bruce Macintosh</a>

All four circumbinary (CB) protoplanetary disks orbiting short-period ($P < 20$ day) double-lined spectroscopic binaries (SB2s)---a group that includes UZ Tau E, for which we present new ALMA data---exhibit sky-plane inclinations $i_{rm disk}$ which match, to within a few degrees, the sky-plane inclinations $i_star$ of their stellar hosts. Although for these systems the true mutual inclinations $theta$ between disk and binary cannot be directly measured because relative nodal angles are unknown, the near-coincidence of $i_{rm disk}$ and $i_star$ suggests that $theta$ is small for these most compact of systems. We confirm this hypothesis using a hierarchical Bayesian analysis, showing that 68% of CB disks around short-period SB2s have $theta < 3.0^circ$. Near co-planarity of CB disks implies near co-planarity of CB planets discovered by Kepler, which in turn implies that the occurrence rate of close-in CB planets is similar to that around single stars. By contrast, at longer periods ranging from $30-10^5$ days (where the nodal degeneracy can be broken via, e.g., binary astrometry), CB disks exhibit a wide range of mutual inclinations, from co-planar to polar. Many of these long-period binaries are eccentric, as their component stars are too far separated to be tidally circularized. We discuss how theories of binary formation and disk-binary gravitational interactions can accommodate all these observations.

All four circumbinary (CB) protoplanetary disks orbiting short-period ($P <
20$ day) double-lined spectroscopic binaries (SB2s)—a group that includes UZ
Tau E, for which we present new ALMA data—exhibit sky-plane inclinations
$i_{rm disk}$ which match, to within a few degrees, the sky-plane inclinations
$i_star$ of their stellar hosts. Although for these systems the true mutual
inclinations $theta$ between disk and binary cannot be directly measured
because relative nodal angles are unknown, the near-coincidence of $i_{rm
disk}$ and $i_star$ suggests that $theta$ is small for these most compact of
systems. We confirm this hypothesis using a hierarchical Bayesian analysis,
showing that 68% of CB disks around short-period SB2s have $theta <
3.0^circ$. Near co-planarity of CB disks implies near co-planarity of CB
planets discovered by Kepler, which in turn implies that the occurrence rate of
close-in CB planets is similar to that around single stars. By contrast, at
longer periods ranging from $30-10^5$ days (where the nodal degeneracy can be
broken via, e.g., binary astrometry), CB disks exhibit a wide range of mutual
inclinations, from co-planar to polar. Many of these long-period binaries are
eccentric, as their component stars are too far separated to be tidally
circularized. We discuss how theories of binary formation and disk-binary
gravitational interactions can accommodate all these observations.

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