PG1018-047: the longest period subdwarf B binary

TL;DR

This paper reports the discovery of PG1018-047, the longest period subdwarf B binary with a 760-day orbit, providing new insights into the formation mechanisms of such systems.

Contribution

It presents the first measurement of an orbital period exceeding 30 days for a subdwarf B binary, supporting the stable mass transfer formation channel.

Findings

Orbital period of 760 days for PG1018-047

Mass ratio M_{MS}/M_{sdB} = 1.6±0.2

Companion identified as mid K spectral type

Abstract

About 50% of all known hot subdwarf B stars (sdBs) reside in close (short period) binaries, for which common envelope ejection is the most likely formation mechanism. However, Han et al. 2003 predict that the majority of sdBs should form through stable mass transfer leading to long period binaries. Determining orbital periods for these systems is challenging and while the orbital periods of ~100 short period systems have been measured, there are no periods measured above 30 days. As part of a large program to characterise the orbital periods of subdwarf B binaries and their formation history, we have found that PG1018-047 has an orbital period of 760\pm6 days, easily making it the longest period ever detected for a subdwarf B binary. Exploiting the Balmer lines of the subdwarf primary and the narrow absorption lines of the companion present in the spectra, we derive the radial velocity amplitudes of both stars, and estimate the mass ratio M_{MS}/M_{sdB} = 1.6\pm0.2. From the combination of visual and infrared photometry, the spectral type of the companion star is determined to be mid K.