Binary-induced magnetic activity?

Abstract

Context. Multi-wavelength time-series observations with high cadence and long duration are needed to resolve and understand the many variations of magnetically active late-type stars, which is an approach often used to observe the Sun. Aims. We present a first and detailed study of the bright and active K0IV-III star HD 123351. Methods. We acquired a total of 955 high-resolution STELLA echelle spectra during the years 2006-2010 and a total of 2260 photometric VI C data points during 1998-2010. These data are complemented by some spectra from CFHT and KPNO. Results. The star is found to be a single-lined spectroscopic binary with a period of 147.8919 ± 0.0003 days and a large eccentricity of e = 0.8086 ± 0.0001. The rms of the orbital solution is just 47 m s −1 , making it the most precise orbit ever obtained for an active binary system. The rotation period is constrained from long-term photometry to be 58.32 ± 0.01 days. It shows that HD 123351 is a very asynchronous rotator, rotating five times slower than the expected pseudo-synchronous value. Two spotted regions persisted throughout the 12 years of our observations. We interpret them as active longitudes on a differentially rotating surface with a ΔP/P of 0.076. Four years of Hα, Ca ii H&K and He i D3 monitoring identifies the same main periodicity as the photometry but dynamic spectra also indicate that there is an intermittent dependence on the orbital period, in particular for Ca ii H&K in 2008. Line-profile inversions of a pair of Zeeman sensitive/insensitive iron lines yield an average surface magnetic-flux density of 542 ± 72 G. The time series for 2008 is modulated by the stellar rotation as well as the orbital motion, such that the magnetic flux is generally weaker during times of periastron and that the chromospheric emissions vary in anti-phase with the magnetic flux. We also identify a broad and asymmetric lithium line profile and measure an abundance of log n(Li) = 1.70 ± 0.05. The star's position in the H-R diagram indicates a mass of 1.2 ± 0.1 M and an age of 6-7 Gyr. Conclusions. We interpret the anti-phase relation of the magnetic flux with the chromospheric emissions as evidence that there are two magnetic fields present at the same time, a localized surface magnetic field associated with spots and a global field that is oriented towards the (low-mass) secondary component. We suggest that the inter-binary field is responsible for the magnetic-flux dilution at periastron. It is also likely to be responsible for the unexpected slow and asynchronous rotation of the primary star.

Evidence for enhanced mixing on the super-meteoritic Li-rich red giant HD 233517

“…These lines are typical magnetic-activity indicators (see, e.g., Strassmeier et al 2011) and are most prominent in young stars. The nondetection is surprising because the large rotational line broadening and the detection of a rotation period from broadband photometry, as well as from radial velocities, would normally hint at the existence of (magnetic) cool spots and thus also of chromospheric radiative losses.…”

“…Enhanced surface Li with rapid rotation is what is generally observed . A recently-studied example of a moderately Li-rich K0 subgiant is HD 123351 (Strassmeier et al 2011). This star not only turned out to be in a highly eccentric (e = 0.81) orbit with its unseen companion but also to be a very slow and strongly asynchronous rotator.…”

Antisolar differential rotation of the K1-giantσ Geminorum revisited

“…On the other hand, long-term photometric observations exist for a much wider sample of spotted stars in binary systems (e.g., Strassmeier et al 1997Strassmeier et al , 2011Oláh & Strassmeier 2002;Oláh et al 2003Oláh et al , 2013Lindborg et al 2013, etc. ), which can also be used for investigating the role of binarity in the formation of magnetic activity.…”

Lithium abundance and6Li/7Li ratio in the active giant HD 123351

“…The target is a recently studied example of a moderately Li-rich K0 III-IV giant (Strassmeier et al 2011). It is heavily covered by spots and exhibits a disk-averaged surface magnetic field of ≈540 G. This star was previously found to exhibit strong Ca ii H&K core emission (Strassmeier et al 2000) which is known to be a fingerprint of chromospheric magnetic activity.…”

“…The star is magnetically active and is a component of a binary system in a highly eccentric orbit (e = 0.81) with an orbital period of 148 d (Strassmeier et al 2011). Its rotation period is 58 d and thus not synchronized to the orbital motion (rotating five times faster than the expected pseudo-synchronous rotation rate).…”