Surface activity of a Rossby sequence of cool Hyades stars^⋆

¹ Leibniz-Institute for Astrophysics Potsdam (AIP), An der Sternwarte 16, D-14482 Potsdam, Germany
² Institute for Physics and Astronomy, University of Potsdam, D-14476 Potsdam, Germany

^⋆⋆ Corresponding author: kstrassmeier@aip.de

Received: 17 August 2025
Accepted: 22 October 2025

Abstract

Aims. The Hyades cluster is key for the study of the rotational, activity, and chemical evolution of solar-like low-mass stars. We present quantitative surface-activity information for a sequence of 21 Hyades dwarf stars with effective temperatures 6160–3780 K (all cooler than the red edge of the Li dip), rotation periods 5–16 d, and normalized Rossby numbers (Ro_n) between 0.14 to 0.54 with respect to the Sun (Ro(Sun) = 1).

Methods. High-resolution Stokes-V spectra and a least-squares deconvolution of thousands of spectral lines per spectrum were employed to measure the longitudinal surface magnetic field. We obtained the velocities, lithium abundances, metallicity, and chromospheric Ca II infrared-triplet (IRT) fluxes from Stokes-I spectra.

Results. The average metallicity, +0.186 ± 0.045 (rms), for our stars with T_eff ≥ 4200 K agrees well with the metallicity in the recent literature. The lithium abundances A(Li) range from 95-times solar (A(Li) ≈  + 3.0) on the warm end of the sample to 1/25 solar (A(Li) ≈  − 0.4) on the cool end. We confirm the tight relation of A(Li) with T_eff and extend it to K–M stars with even lower Li abundances than previously measurable. A formal relation with rotational period and velocity in the sense of a higher Li abundance for faster rotators is present. Targets that rotate faster than v sin i of 6 km s⁻¹ (P_rot ≈ 8 d) appear to be Li saturated at A(Li) ≈3.0 dex. The Ca II IRT fluxes for our sample indicate (logarithmic) chromospheric radiative losses R′_IRT in the range −4.0 to −4.9 in units of the bolometric flux. These radiative losses are also related to T_eff, P_rot, and v sin i, but opposite to A(Li), in an inverse sense with higher radiative losses for the slower, that is, cooler rotators. Longitudinal magnetic field strengths were measured in the range zero to −100 G and +150 G with phase-averaged disk-integrated unsigned values ⟨|B|⟩ of 15.4 ± 3.6(rms) G for targets warmer than ≈5000 K and 91 ± 61(rms) G for targets cooler than this. These unsigned field strengths are related to P_rot, v sin i, and Ro_n, but in a dual-slope fashion. The short-period bona fide single M-target RSP 348 was found to be a double-lined spectroscopic binary with a classification dM3e+dM5e.

Conclusions. We conclude that the dependence on Rossby number of the surface activity tracers A(Li), R′_IRT, and ⟨|B|⟩ on our Hyades dwarf sequence primarily originates from convective motions, expressed by its turnover time, and only to a smaller and sometimes inverse extent from surface rotation and its related additional mixing.