Spectral Disentangling Reveals Deep CNO-cycle Exposure in ET Cru

TL;DR

This study provides a detailed spectroscopic analysis of the binary star ET Cru, revealing deep CNO-cycle exposure and chemical anomalies in the donor star, and establishing it as a benchmark for binary evolution.

Contribution

The paper introduces high-precision spectral disentangling of ET Cru, uncovering chemical anomalies and providing a comprehensive characterization of the system’s physical parameters.

Findings

Secondary star shows extreme carbon depletion and nitrogen enrichment.

Spectroscopic data confirms deep CNO-cycle exposure in the donor.

ET Cru's distance estimate differs from Gaia parallax, indicating potential systematic uncertainties.

Abstract

Binary stars undergoing mass transfer provide unique laboratories for testing stellar evolution. Here, we present a comprehensive photometric and spectroscopic analysis of the semi-detached system ET Cru. Using spectral disentangling, we independently determined the effective temperatures and chemical abundances of both components with high precision, including nine elements (eleven species). We find masses of $13.41\,M_\odot$ and $6.00\,M_\odot$ for the primary and secondary, respectively, with uncertainties of only $\sim$1.3%. The radii are $5.58\,R_\odot$ and $5.68\,R_\odot$, measured to within 0.4% and 0.5%. Surface gravities are constrained to better than 1%, while effective temperatures are determined to within 3-5%. The secondary exhibits extreme chemical anomalies, with severe carbon depletion and nitrogen enrichment far exceeding those reported in classical Algol systems. Multi-wavelength spectral energy distribution modelling yields a distance of $\sim$2.5 kpc, inconsistent with the $Gaia$ DR3 parallax, suggesting systematic astrometric uncertainties in the parallax distance. Together, these results establish ET Cru as a benchmark Algol-type binary, revealing direct spectroscopic evidence of deep CNO-cycle exposure in the donor and confirming the primary star as a rejuvenated gainer. ET Cru thus provides a chemically and dynamically illustrative case for understanding advanced binary interactions and the late evolutionary stages of massive-star evolution.