Orbital Characteristics of the Subdwarf-B and F V Star Binary EC~20117-4014(=V4640 Sgr)

TL;DR

This study precisely determined the orbital period and characteristics of the EC 20117-4014 binary system, supporting the stable Roche lobe overflow hypothesis for sdB star formation, and analyzed pulsation period changes indicating its evolutionary stage.

Contribution

First precise measurement of the orbital parameters of the EC 20117-4014 binary system using 20 years of data, revealing a nearly circular orbit and insights into sdB star evolution.

Findings

Orbital period of 792.3 days with minimal eccentricity.

Upper limit of eccentricity is 0.025, the lowest among similar binaries.

Pulsation period change indicates the star is near the end of core helium-burning.

Abstract

Among the competing evolution theories for subdwarf-B (sdB) stars is the binary evolution scenario. EC~20117-4014 (=V4640~Sgr) is a spectroscopic binary system consisting of a pulsating sdB star and a late F main-sequence companion (O'Donoghue et al. 1997), however the period and the orbit semi-major axes have not been precisely determined. This paper presents orbital characteristics of the EC 20117-4014 binary system using 20 years of photometric data. Periodic Observed minus Calculated (O-C) variations were detected in the two highest amplitude pulsations identified in the EC 20117-4014 power spectrum, indicating the binary system's precise orbital period (P = 792.3 days) and the light-travel time amplitude (A = 468.9 s). This binary shows no significant orbital eccentricity and the upper limit of the eccentricity is 0.025 (using 3 $\sigma$ as an upper limit). This upper limit of the eccentricity is the lowest among all wide sdB binaries with known orbital parameters. This analysis indicated that the sdB is likely to have lost its hydrogen envelope through stable Roche lobe overflow, thus supporting hypotheses for the origin of sdB stars. In addition to those results, the underlying pulsation period change obtained from the photometric data was $\dot{P}$ = 5.4 ($\pm$0.7) $\times$ $10^{-14}$ d d$^{-1}$, which shows that the sdB is just before the end of the core helium-burning phase.