Nature or nurture of coplanar Tatooines: the aligned circumbinary Kuiper belt analogue around HD 131511

Abstract

A key discovery of the Kepler mission is of the circumbinary planets known as "Tatooines", which appear to be well aligned with their host stars' orbits. Whether this alignment is due to initially coplanar circumbinary planet-forming discs (i.e. nature), or subsequent alignment of initially misaligned discs by warping the inner disc or torquing the binary (i.e. nurture), is not known. Tests of which scenario dominates may be possible by observing circumbinary Kuiper belt analogues ("debris discs"), which trace the plane of the primordial disc. Here, the 140 au diameter circumbinary debris disc around HD 131511 is shown to be aligned to within 10$^\circ$ of the plane of the near edge-on 0.2 au binary orbit. The stellar equator is also consistent with being in this plane. If the primordial disc was massive enough to pull the binary into alignment, this outcome should be common and distinguishing nature versus nurture will be difficult. However, if only the inner disc becomes aligned with the binary, the HD 131511 system was never significantly misaligned. Given an initial misalignment, the $\sim$Gyr main-sequence lifetime of the star allows secular perturbations to align the debris disc out to 100 au at the cost of an increased scale height. The observed debris disc scale height limits any misalignment to less than 25$^\circ$. With only a handful known, many more such systems need to be characterised to help test whether the alignment of circumbinary planets is nature or nurture.