Analysis of apsidal motion in eclipsing binaries using TESS data

Abstract

Context. The change in the argument of periastron of eclipsing binaries, that is, the apsidal motion caused by classical and relativistic effects, can be measured from variations in the difference between the time of minimum light of the primary and secondary eclipses. Poor apsidal motion rate determinations and large uncertainties in the classical term have hampered previous attempts to determine the general relativistic term with sufficient precision to test general relativity predictions. Aims. As a product of the TESS mission, thousands of high-precision light curves from eclipsing binaries are now available. Using a selection of suitable well-studied eccentric eclipsing binary systems, we aim to determine their apsidal motion rates and place constraints on key gravitational parameters. Methods. We compute the time of minimum light from the TESS light curves of 15 eclipsing binaries with precise absolute parameters and with an expected general relativistic contribution to the total apsidal motion rate of greater than 60%. We use the changing primary and secondary eclipse timing differences over time to compute the apsidal motion rate, when possible, or the difference between the linear periods as computed from primary and secondary eclipses. For a greater time baseline we carefully combine the high-precision TESS timings with archival reliable timings. Results. We determine the apsidal motion rate of 9 eclipsing binaries, 5 of which are reported for the first time. From these, we are able to measure the general relativistic apsidal motion rate of 6 systems with sufficient precision to test general relativity for the first time using this method. This test explores a regime of gravitational forces and potentials that had not been probed before. We find perfect agreement with theoretical predictions, and we are able to set stringent constraints on two parameters of the parametrised post-Newtonian formalism.

THE BANANA PROJECT. VI. Close double stars are well aligned with noticeable exceptions; results from an ensemble study using apsidal motion and Rossiter-McLaughlin measurements

“…Combining 35 new, precise apsidal motion rates from Baroch et al (2021) and Claret et al (2021) with 16 additional data points of apsidal motion rates as well as measurements of the RM effect and gravity darkening from the literature, we are able to find consistent results of misalignment for the two binary systems DI Her and CV Vel where misalignment is already established. For AS Cam, we agree on the hypothesis by Pavlovski et al (2011) that the low apsidal motion rate of AS Cam can be explained by spin-orbit misalignment alone.…”

“…1 With these assumptions we then attribute any discrepancy between the observed and expected apsidal motion rate to a misalignment of stellar spin axes, or if agreement is found, determine upper limits on any potential obliquity. The recent high-precision apsidal motion determinations by Baroch et al (2021) and Claret et al (2021) using TESS light curves sparked this idea and their data compose the bulk the apsidal motion rates that we use.…”

“…An asphericity of a star may arise from the deformations caused by either tidal interactions between the components of a binary system or from rotational flattening that leads to a larger diameter of a star along its equatorial plane than its north-south axis. The total apsidal motion rate may thus be expressed as (Shakura 1985;Baroch et al 2021):…”

“…Finally, k 2, j are the internal structure constants (or apsidal motion constants) that relate to the radial mass-distribution of the stars (Kopal 1978) and are determined from measurements of the apsidal motion (Claret & Gimenez 1993). The three terms ωGR , ωtid and ωrot are all increasing functions of the eccentricity e according to their respective functions f GR , f tid and f rot (Sterne 1939;Baroch et al 2021), see also fig. 1:…”

Space-Based Photometry of Binary Stars: From Voyager to TESS

“…TESS can also contribute to period analysis for EBs with apsidal motion. Baroch et al [271] used TESS data to study apsidal motion in 15 EBs and determine the apsidal periods of nine of these. A comparison of these with theoretical models by Claret et al [272] found good agreement.…”