Barium and related stars, and their white-dwarf companions

Abstract

Context. Barium and S stars without technetium are red giants suspected of being all members of binary systems. Aims. This paper provides both long-term and revised, more accurate orbits for barium and S stars adding to previously published ones. The sample of barium stars with strong anomalies comprise all such stars present in the Lü et al. catalogue. Methods. Orbital elements are derived from radial velocities collected from a long-term radial-velocity monitoring performed with the HERMES spectrograph mounted on the Mercator 1.2 m telescope. These new measurements were combined with older, CORAVEL measurements. With the aim of investigating possible correlations between orbital properties and abundances, we collected as well an as homogeneous as possible set of abundances for barium stars with orbital elements. Results. We find orbital motion for all barium and extrinsic S stars monitored. We obtain the longest period known so far for a spectroscopic binary involving an S star, namely 57 Peg with a period of the order of 100 -500 yr. We present the mass distribution for the barium stars, which ranges from 1 to 3 M , with a tail extending up to 5 M in the case of mild barium stars. This high-mass tail comprises mostly high-metallicity objects ([Fe/H] ≥ −0.1). Mass functions are compatible with WD companions whose masses range from 0.5 to 1 M . Strong barium stars have a tendency to be found in systems with shorter periods than mild barium stars, although this correlation is rather lose, metallicity and WD mass playing a role as well. Using the initial -final mass relationship established for field WDs, we derived the distribution of the mass ratio q = M AGB,ini /M Ba (where M AGB,ini is the WD progenitor initial mass, i.e., the mass of the system former primary component) which is a proxy for the initial mass ratio (the more so, the less mass the barium star has accreted). It appears that the distribution of q is highly non uniform, and significantly different for mild and strong barium stars, the latter being characterized by values mostly in excess of 1.4, whereas mild barium stars occupy the range 1 -1.4. Conclusions. The orbital properties presented in this paper pave the way for a comparison with binary-evolution models.

Discovery of technetium- and niobium-rich S stars: The case for bitrinsic stars

“…We note that the atmospheric parameters of o 1 Ori quoted inJorissen et al (2019) are different than the ones used in this study. This is because log g iterations were not performed inJorissen et al (2019).…”

“…A tentative period of 575 d and a mass function of (2.7 ± 0.2) × 10 −5 M was obtained by Jorissen et al (2019), suggestive of an orbit seen almost pole-on (i = 4.7 • ± 1 • ). BD+79 • 156 is a confirmed binary with an orbital period of 10931 days, an eccentricity of 0.461, and a mass function of 0.025 M , compatible with a WD companion (Van der Swaelmen et al 2017;Jorissen et al 2019). An additional confirmation of the presence of a WD companion around BD+79 • 156 (and o 1 Ori) comes from the UV excess seen in their spectral energy distribution (SED), well visible when comparing photometric observations with model SED (Fig.…”

“…The derivation of the orbital elements of the o 1 Ori system has turned out to be very difficult (Jorissen et al 2019), because the radial-velocity data show a very limited excursion of 2 to 3 km s −1 with some superimposed radial-velocity jitter. A tentative period of 575 d and a mass function of (2.7 ± 0.2) × 10 −5 M was obtained by Jorissen et al (2019), suggestive of an orbit seen almost pole-on (i = 4.7 • ± 1 • ).…”

“…The derivation of the orbital elements of the o 1 Ori system has turned out to be very difficult (Jorissen et al 2019), because the radial-velocity data show a very limited excursion of 2 to 3 km s −1 with some superimposed radial-velocity jitter. A tentative period of 575 d and a mass function of (2.7 ± 0.2) × 10 −5 M was obtained by Jorissen et al (2019), suggestive of an orbit seen almost pole-on (i = 4.7 • ± 1 • ). BD+79 • 156 is a confirmed binary with an orbital period of 10931 days, an eccentricity of 0.461, and a mass function of 0.025 M , compatible with a WD companion (Van der Swaelmen et al 2017;Jorissen et al 2019).…”

“…Concerning o 1 Ori, this system has been considered as a prototype bitrinsic star for a very long time (Ake & Johnson 1988;Jorissen et al 1993Jorissen et al , 2019. As shown in the bottom panel of Fig.…”