Surface activity of a Rossby sequence of cool Hyades stars

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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.

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