Mass Transfer as an Explanation for the Lifetime-Travel Time Discrepancy in IT Librae

TL;DR

This study uses spectroscopy and Kepler data to analyze the binary system IT Librae, revealing it as a post-mass transfer system that resolves the discrepancy between its age and travel time from the galactic disk.

Contribution

The paper provides a detailed characterization of IT Librae, demonstrating it as a post-mass transfer binary and explaining the lifetime-travel time discrepancy through evolutionary modeling.

Findings

IT Librae is a post-mass transfer binary with a rejuvenated primary.

The secondary star is overluminous and fills its Roche lobe.

The true age of the system exceeds its travel time from the galactic disk.

Abstract

The eclipsing binary IT Librae is an unusual system of two B-type stars that is situated about 1 kpc above the galactic plane. The binary was probably ejected from its birthplace in the disk, but the implied time-of-flight to its current location exceeds the evolutionary lifetime of the primary star. Here we present a study of new high dispersion spectroscopy and an exquisite light curve from the Kepler K2 mission in order to determine the system properties and resolve the timescale discrepancy. We derive a revised spectroscopic orbit from radial velocity measurements and determine the component effective temperatures through comparison of reconstructed and model spectra ($T_1 = 23.8 \pm 1.8$ kK, $T_2 = 13.7 \pm 2.5$ kK). We use the Eclipsing Light Curve (ELC) code to model the K2 light curve, and from the inclination of the fit, we derive the component masses ($M_1 = 9.6 \pm 0.6 M_\odot$, $M_2 = 4.2 \pm 0.2 M_\odot$) and mean radii ($R_1 = 6.06 \pm 0.16 R_\odot$, $R_2 = 5.38 \pm 0.14 R_\odot$). The secondary star is overluminous for its mass and appears to fill its Roche lobe. This indicates that IT~Librae is a post-mass transfer system in which the current secondary was the mass donor star. The current primary star was rejuvenated by mass accretion, and its evolutionary age corresponds to the time since the mass transfer stage. Consequently, the true age of the binary is larger than the ejection time-of-flight, thus resolving the timescale discrepancy.