The orbital period–mass ratio relation of wide sdB+MS binaries and its application to the stability of RLOF

Abstract

Wide binaries with hot subdwarf-B (sdB) primaries and main sequence companions are thought to form only through stable Roche lobe overflow (RLOF) of the sdB progenitor near the tip of the red giant branch (RGB). We present the orbital parameters of eleven new long period composite sdB binaries based on spectroscopic observations obtained with the UVES, FEROS and CHIRON spectrographs. Using all wide sdB binaries with known orbital parameters, 23 systems, the observed period distribution is found to match very well with theoretical predictions. A second result is the strong correlation between the orbital period (P) and the mass ratio (q) in the observed wide sdB binaries. In the P-q plane two distinct groups emerge, with the main group (18 systems) showing a strong correlation of lower mass ratios at longer orbital periods. The second group are systems that are thought to be formed from higher mass progenitors. Based on theoretical models, a correlation between the initial mass ratio at the start of RLOF and core mass of the sdB progenitor is found, which defines a mass-ratio range at which RLOF is stable on the RGB.

New hot subdwarf variables from the EC survey

“…Wide sdB + MS binaries have periods of ∼1000 days and orbit determination is necessarily more challenging than in the case of close sdB + MS binaries. As noted above, EC 01441-6605 after further study could be an addition to the Vos et al (2019) sample.…”

“…Objects of particular interest were then identified. Vos et al (2019) study the orbital period-mass ratio relation of wide sdB + MS binaries and their probable formation through the Roche Lobe Overflow channel. Wide sdB + MS binaries have periods of ∼1000 days and orbit determination is necessarily more challenging than in the case of close sdB + MS binaries.…”

Asteroseismic Observations of Hot Subdwarfs

“…Some hot subdwarfs appear to be single stars and many exist in binary systems. Two broad categories of binary system are found to host hot subdwarfs: those with short ( ∼ 0.1 days) orbital periods (Kupfer et al, 2015) where the companion is typically a M-dwarf or a white dwarf and those with long ( ∼ 1000 days) orbital periods (Vos et al, 2019) where the companion is a F, G or K Main Sequence star.…”

Composite spectrum hot subdwarf binaries

“…XTgrid has been employed to solve subdwarf binary orbits for 28 systems with UVES spectroscopy (Vos et al, 2018) and showed consistent results with the independent GSSP code (Tkachenko, 2015) for the cool companions. These analyses found a positive correlation between orbital period and eccentricity and an anti-correlation between orbital period and mass ratio (Vos et al, 2019). In some systems the companion shows chromospheric activity based on the near infrared calcium triplet.…”