DI Herculis Revisited: Starspots, Gravity Darkening, and 3-D Obliquities

TL;DR

This study refines the understanding of the DI Herculis binary system by analyzing TESS data to measure starspot activity, gravity darkening, and 3-D obliquities, leading to a better match between observed and theoretical apsidal precession rates.

Contribution

It provides the first detailed measurement of stellar obliquities and starspot effects in DI Herculis using TESS data, resolving previous discrepancies in precession rate predictions.

Findings

Observed and theoretical precession rates are in close agreement.

Stellar obliquities are approximately 75-80 degrees for both stars.

Starspots and gravity darkening significantly influence light curve features.

Abstract

DI Herculis is an eclipsing binary famous for a longstanding disagreement between theory and observation of the apsidal precession rate, which was resolved when both stars were found to be severely misaligned with the orbit. We used data from the Transiting Exoplanet Survey Satellite (TESS) to refine our knowledge of the stellar obliquities and sharpen the comparison between the observed and theoretical precession rates. The TESS data show variations with a 1.07-day period, which we interpret as rotational modulation from starspots on the primary star. This interpretation is supported by the detection of photometric anomalies during primary eclipses consistent with starspot crossings. The secondary eclipse light curve shows a repeatable asymmetry which we interpret as an effect of gravity darkening. By combining the TESS data with previously obtained data, we determined the three-dimensional spin directions of both stars. Using this information, the updated value of the theoretical apsidal precession rate (including the effects of tides, rotation, and general relativity) is $1.35^{+0.58}_{-0.50}$ arcsec/cycle. The updated value of the observed rate (after including new TESS eclipse times) is $1.41^{+0.39}_{-0.28}$ arcsec/cycle. Given the agreement between the observed and theoretical values, we fitted all the relevant data simultaneously assuming the theory is correct. This allowed us to place tighter constraints on the stellar obliquities, which are $75^{+3}_{-3}$ and $80^{+3}_{-3}$ degrees for the primary and secondary stars, respectively.