The spin axes orbital alignment of both stars within the eclipsing binary system V1143Cyg using the Rossiter-McLaughlin effect

TL;DR

This study develops a robust method to analyze the Rossiter-McLaughlin effect in eclipsing binary systems, revealing that both stars in V1143Cyg have aligned spin axes with the orbital axis, informing binary formation theories.

Contribution

The paper introduces a new approach to measure stellar spin axis orientation in binary systems with nearly equal brightness components using high-resolution spectra.

Findings

Both stars' spin axes are aligned with the orbital axis within uncertainties.

The measured angles are beta_p=0.3+-1.5 deg and beta_s=-1.2+-1.6 deg.

The alignment explains the discrepancy in theoretical and observed apsidal motion.

Abstract

Context: The Rossiter-McLaughlin (RM) effect, a rotational effect in eclipsing systems, provides unique insight into the relative orientation of stellar spin axes and orbital axes of eclipsing binary systems. Aims: Our aim is to develop a robust method to analyze the RM effect in an eclipsing system with two nearly equally bright components. This gives access to the orientation of the stellar rotation axes and may shed light on questions of binary formation and evolution. Methods: High-resolution spectra have been obtained both out of eclipse and during the primary and secondary eclipses in the V1143Cyg system, using the high-resolution Hamilton Echelle Spectrograph at the Lick Observatory. The Rossiter-McLaughlin effect is analyzed in two ways: (1) by measuring the shift of the line center of gravity during different phases of the eclipses and (2) by analysis of the line shape change of the rotational broadening function during eclipses. Results: The projected axes of both stars are aligned with the orbital spin within the observational uncertainties, with the angle of the primary rotation axis beta_p=0.3+-1.5 deg, and the angle of the secondary rotation axis beta_s=-1.2+-1.6 deg, thereby showing that the remaining difference between the theoretical and observed apsidal motion for this system is not due to a misalignment of the stellar rotation axes. Both methods utilized in this paper work very well, even at times when the broadening profiles of the two stars overlap.[abridged]