paper

The Degree of Alignment Between Circumbinary Disks and Their Binary Hosts

arXiv:1906.03269 · doi:10.3847/1538-4357/ab287b

Abstract

All four circumbinary (CB) protoplanetary disks orbiting short-period ( day) double-lined spectroscopic binaries (SB2s)---a group that includes UZ Tau E, for which we present new ALMA data---exhibit sky-plane inclinations which match, to within a few degrees, the sky-plane inclinations of their stellar hosts. Although for these systems the true mutual inclinations between disk and binary cannot be directly measured because relative nodal angles are unknown, the near-coincidence of and suggests that 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 . 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 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.

ApJ accepted. 30 pages, 16 figures. Transcription error in Table 4 fixed

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