Projected spin-orbit alignments from Kepler asteroseismology and Gaia astrometry

TL;DR

This study combines Gaia astrometry and asteroseismology to measure projected spin-orbit alignments in binary systems, providing initial evidence against random obliquity distributions and highlighting future prospects with TESS and PLATO data.

Contribution

It presents the first combined analysis of Gaia and asteroseismic data to estimate projected spin-orbit alignments in binary systems, offering new insights into their formation.

Findings

Systems are generally aligned, but data are inconclusive due to measurement ambiguities.

Likelihood of random obliquity distribution is low, suggesting some alignment.

Future missions could significantly expand the sample size for better understanding.

Abstract

The angle between the rotation and orbital axes of stars in binary systems -- the obliquity -- is an important indicator of how these systems form and evolve but few such measurements exist. We combine the sample of astrometric orbital inclinations from Gaia DR3 with a sample of solar-like oscillators in which rotational inclinations have been measured using asteroseismology. We supplement our sample with one binary whose visual orbit has been determined using speckle interferometry and present the projected spin-orbit alignments in five systems. We find that each system, and the overall sample, is consistent with alignment but there are important caveats. First, the asteroseismic rotational inclinations are fundamentally ambiguous and, second, we can only measure the projected (rather than true) obliquity. If rotational and orbital inclinations are independent and isotropically-distributed, however, the likelihood of drawing our data by chance is less than a few per cent. Though small, our data set argues against uniformly random obliquities in binary systems. We speculate that dozens more measurements could be made using data from NASA's TESS mission, mostly in red giants. ESA's PLATO mission will likely produce hundreds more spin-orbit measurements in systems with main-sequence and subgiant stars.