Tracking the spin axes orbital alignment in selected binary systems - Torun Rossiter-McLaughlin effect survey

TL;DR

This study measures the spin-orbit alignment in four eclipsing binary systems using the Rossiter-McLaughlin effect, finding mostly aligned systems and significantly increasing the number of known spin-orbit angles.

Contribution

It introduces a spectroscopic method to measure stellar spin-orbit angles in binary systems and provides new measurements, including a notable misalignment in one system.

Findings

Most systems show no significant misalignment.

Increased the number of measured spin-orbit angles from 9 to 14.

Detected a large misalignment of 87±17 degrees in one system.

Abstract

We have obtained high-resolution spectra of four eclipsing binary systems (FM Leo, NN Del, V963 Cen and AI Phe) with the view to gaining insight into the relative orientation of their stellar spin axes and orbital axes. The so called Rossiter-McLaughlin (RM) effect, i.e. the fact that the broadening and the amount of blue- or redshift in the spectra during an eclipse depend on the tilt of the spin axis of the background star, has the potential of reconciling observations and theoretical models if such a tilt is found. We analyse the RM effect by disentangling the spectra, removing the front component and measuring the remaining, distorted lines with a broadening function (BF) obtained from single value decomposition (SVD), weighting by the intensity centre of the BF in the eclipse. All but one of our objects show no significant misalignment, suggesting that aligned systems are dominant. We provide stellar as well as orbital parameters for our systems. With five measured spin-orbit angles we significantly increase (from 9 to 14) the number of stars for which it has been measured. The spin-orbit angle $\beta$ calculated for AI Phe's secondary component shows a misalignment of 87$\pm$17 degrees. NN Del, with a large separation of components and a long dynamical timescale for circularisation and synchronisation, is an example of a close to primordial spin-orbit angle measurement.