The orbits of subdwarf-B + main-sequence binaries

III. The period–eccentricity distribution

¹ Instituto de Física y Astronomía, Universidad de Valparaiso, Gran Bretaña 1111, Playa Ancha, 2360102 Valparaíso, Chile
e-mail: joris.vos@uv.cl
² Department of Physics, Astronomy, and Materials Science, Missouri State University, Springfield, MO 65804, USA
³ Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium

Received: 7 April 2017
Accepted: 6 June 2017

Abstract

Context. The predicted orbital-period distribution of the subdwarf-B (sdB) population is bi-modal with peaks at short (<10 days) and long (>500 days) periods. Observationally, many short-period sdB systems are known, but only few wide sdB binaries have been studied in detail. Based on a long-term monitoring programme the wide sdB sample has been increased, discovering an unexpected positive correlation between the eccentricity and orbital period.

Aims. In this article we present the orbital solution and spectral analysis of four new systems, BD−7°5977, EC 11031–1348, TYC 2084–448–1 and TYC 3871–835–1, and update the orbital solution of PG 1104+243. Using the whole sample of wide sdB binaries, we aim to find possible correlations between orbital and spectral properties. The ultimate goal is to improve theoretical models of Roche-lobe overflow.

Methods. High-resolution spectroscopic time series were obtained to determine the RVs of both the sdB and main sequence (MS) components. Literature photometry was used to construct the spectral-energy distributions, which were fitted with atmosphere models to determine the surface gravities and temperatures of both components in all systems. Spectral parameters of the cool companion were verified using the GSSP code. Furthermore the amount of accreted mass was estimated.

Results. Orbital parameters matching the earlier observed period−eccentricity relation were found for three systems, while TYC 2084–448–1 is found to have a lower eccentricity than expected from the period−eccentricity trend indicated by the other systems. Based on new observations, the orbit of PG 1104+243 has a small but significant eccentricity of 0.04 ± 0.02, matching that of the other systems with similar periods. Furthermore, a positive correlation between accreted mass and orbital period was found, as well as a possible relation between the initial mass-ratio and the final period−eccentricity.

Conclusions. The wide sdB-binary sample shows interesting possible correlations between orbital and spectral properties. However, a larger sample is necessary to statistically validate them.