The orbits of subdwarf-B + main-sequence binaries

Abstract

Context. The predicted orbital-period distribution of the subdwarf-B (sdB) population is bi-modal with a peak 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 program 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 at finding possible correlations between orbital and spectral properties. The ulitmate goal is to improve theoretical models of Roche-lobe overflow (RLOF). Methods. High-resolution spectroscopic time series were obtained to determine the radial velocities of both the sdB and 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.

Summary of Eighth Meeting on Hot Subdwarfs and Related Objects

“…Wide binaries involving a hot subdwarf and a MS companion currently attract considerable interest (Vos et al 2017(Vos et al , 2018 because radial velocities can be easily observed for both components, allowing orbital parameters and stellar masses to be determined; Chen et al (2013) show how these systems may be formed through the RLOF channel, reproducing the observed period distribution and period dependence on mass-ratio. Vos described his ongoing observational studies of wide binaries, for which preliminary reports are available (Vos et al 2017(Vos et al , 2018.…”

“…Vos described his ongoing observational studies of wide binaries, for which preliminary reports are available (Vos et al 2017(Vos et al , 2018. Element abundances are determined for both stars in wide binaries studied to date, and mass accreted by the companion during subdwarf formation found to be negligible.…”

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“…The long orbital period slows down observing programs and therefore only a handful of systems are solved. Vos et al (2017) have investigated such binaries and found a positive correlation between orbital period and eccentricity, which makes long-period binaries important in finetuning population synthesis models. A very compelling region is in between the two period regimes, which is almost empty.…”

Constraining Roche-Lobe Overflow Models Using the Hot-Subdwarf Wide Binary Population

“…Furthermore, the amount of accreted matter can be estimated by studying the rotational velocity of the companions. By assuming that the systems were circularised and synchronised before the onset of mass-loss, and attributing the increase of rotational velocity of the secondary to mass accretion during RLOF, the amount of accreted mass can be calculated (Vos et al 2017b). Furthermore, the evolved state of some companions argues in favour of low mass accretion rates (e.g.…”

“…While short period sdB binaries are the product of CE ejection, wide sdB binaries are formed through stable RLOF near the tip of the RGB (Han et al 2002(Han et al , 2003. Currently 11 wide sdB binaries have solved orbits (Vos et al 2017a), most of which are the result of a long term observing campaign using HERMES at the 1.2m Mercator and UVES at the 8m VLT telescope (Vos et al 2017b). Although this is still a small sample, several interesting result have appeared.…”