Spin-Orbit Alignment for the Circumbinary Planet Host Kepler-16A

TL;DR

This study finds that the primary star in the Kepler-16 circumbinary system has a rotation axis closely aligned with the orbital plane, supporting a disk-based formation scenario or tidal realignment.

Contribution

It provides the first measurement of the primary star's obliquity in a circumbinary planet system, indicating near alignment of angular momentum vectors.

Findings

Primary star's rotation period is 35.1 days.

Obliquity with respect to binary orbit is 1.6 degrees.

Angular momentum vectors are closely aligned.

Abstract

Kepler-16 is an eccentric low-mass eclipsing binary with a circumbinary transiting planet. Here we investigate the angular momentum of the primary star, based on Kepler photometry and Keck spectroscopy. The primary star's rotation period is 35.1 +/- 1.0 days, and its projected obliquity with respect to the stellar binary orbit is 1.6 +/- 2.4 degrees. Therefore the three largest sources of angular momentum---the stellar orbit, the planetary orbit, and the primary's rotation---are all closely aligned. This finding supports a formation scenario involving accretion from a single disk. Alternatively, tides may have realigned the stars despite their relatively wide separation (0.2 AU), a hypothesis that is supported by the agreement between the measured rotation period and the "pseudosynchronous" period of tidal evolution theory. The rotation period, chromospheric activity level, and fractional light variations suggest a main-sequence age of 2-4 Gyr. Evolutionary models of low-mass stars can match the observed masses and radii of the primary and secondary stars to within about 3%.