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Surface activity of a Rossby sequence of cool Hyades stars

K. G. Strassmeier, I. Ilyin, M. Steffen, S. A. Barnes

TL;DR

The paper investigates how surface activity indicators—Li abundance $A({\rm Li})$, Ca II infrared triplet flux $R'_{\rm IRT}$, and large-scale magnetic fields $\langle|B|\rangle$—vary along a Rossby sequence in 21 Hyades dwarfs covering $T_{\rm eff}\approx6160$–3780 K and $Ro_n\approx0.14$–0.54. Using high-resolution PEPSI/LBT spectropolarimetry, the authors extract Li abundances, chromospheric fluxes, and Stokes $V$-based magnetic fields, deriving $P_{\rm rot}$, $v\sin i$, $\tau_c$, and $Ro_n$ for each star. They find a strong, two-slope dependence of $\langle|B|\rangle$ on $Ro_n$ (strong fields for $Ro_n<0.25$, weaker fields for $Ro_n>0.25$) and a complementary, inverse trend for $R'_{\rm IRT}$, while $A({\rm Li})$ increases with $Ro_n$; these trends are predominantly governed by convective turnover time rather than rotation alone. The results imply that convection-dominated processes set the global activity level in Hyades-age solar-like stars, with rotational mixing contributing secondary modulation; upcoming Zeeman-Doppler imaging will map magnetic topology to test dynamo scenarios.

Abstract

Aims. The Hyades cluster is key for the study of rotational, activity, and chemical evolution of solar-like low-mass stars. Here we present quantitative surface-activity information for a sequence of 21 Hyades dwarf stars. Conclusions. We conclude that the Rossby-number dependencies of the surface activity tracers A(Li), R(IRT), and B on our Hyades dwarf sequence primarily originate from convective motions, expressed by its turnover time, and only to a smaller and sometimes inverse extent from surface rotation and its related extra mixing.

Surface activity of a Rossby sequence of cool Hyades stars

TL;DR

The paper investigates how surface activity indicators—Li abundance , Ca II infrared triplet flux , and large-scale magnetic fields —vary along a Rossby sequence in 21 Hyades dwarfs covering –3780 K and –0.54. Using high-resolution PEPSI/LBT spectropolarimetry, the authors extract Li abundances, chromospheric fluxes, and Stokes -based magnetic fields, deriving , , , and for each star. They find a strong, two-slope dependence of on (strong fields for , weaker fields for ) and a complementary, inverse trend for , while increases with ; these trends are predominantly governed by convective turnover time rather than rotation alone. The results imply that convection-dominated processes set the global activity level in Hyades-age solar-like stars, with rotational mixing contributing secondary modulation; upcoming Zeeman-Doppler imaging will map magnetic topology to test dynamo scenarios.

Abstract

Aims. The Hyades cluster is key for the study of rotational, activity, and chemical evolution of solar-like low-mass stars. Here we present quantitative surface-activity information for a sequence of 21 Hyades dwarf stars. Conclusions. We conclude that the Rossby-number dependencies of the surface activity tracers A(Li), R(IRT), and B on our Hyades dwarf sequence primarily originate from convective motions, expressed by its turnover time, and only to a smaller and sometimes inverse extent from surface rotation and its related extra mixing.
Paper Structure (21 sections, 6 equations, 10 figures, 6 tables)

This paper contains 21 sections, 6 equations, 10 figures, 6 tables.

Table of Contents

  1. Introduction
  2. Observations
  3. Target sample
  4. Census of stellar parameters
  5. Temperature, gravity, luminosity, radius
  6. Metallicity
  7. Rossby number
  8. Projected rotational velocity, inclination
  9. Radial velocity
  10. Surface activity measurements
  11. Lithium abundance
  12. Chromospheric Caii IRT flux
  13. Magnetic field measurements
  14. Hyades activity-rotation relations
  15. Lithium dependency
  16. ...and 6 more sections

Figures (10)

  • Figure 1: Temperature distributions for the Hyads in our sample. $a.$ Versus rotation period. The target in the lower right corner is the cool M-dwarf binary RSP 348 (not included in the other panels). $b.$ Versus radius as obtained from the Gaia DR3 parallax and $c.$ versus luminosity $L$.
  • Figure 2: Comparison of the lithium 6707.8-Å region for our sample stars. Shown are the phase-combined spectra. Short vertical dashes indicate the wavelengths of blending lines: Bottom row of dashes is the line list from Strassmeier & Steffen (xiboo), middle row are the strongest CaH and TiO lines from the solar umbral atlas, and the upper four dashes indicate the two $^7$Li and $^6$Li doublets, respectively. The x-axis is wavelength in Å ngstroem.
  • Figure 3: Nineteen individual Li i 6707.8-Å line profiles of RSP 177 ($T_{\rm eff}$=5770 K). It indicates the expected range of line-core changes over time. The black profiles are from 2020 (season S20), the red profiles from 2022 (season S22), according to Table \ref{['T1-App']}. The maximum line-core variability is in this case 0.8%. The x-axis is wavelength in Å ngstroem.
  • Figure 3: (Continued.)
  • Figure 4: Representative Stokes-V data and results for RSP 177. $a.$ Top spectra: Overplot of all available 19 LSD Stokes-V line profiles in units of the continuum expanded by a factor 100 and shifted in intensity by 0.1 for better visibility. Bottom spectrum: Example LSD Stokes-I profile. $b.$ Phase-resolved disk-integrated Stokes-V LSD line profiles in units of Gauss. $c.$ Longitudinal magnetic field versus rotational phase. Indicated are the two observing seasons for this target (S22: filled dots, S20: rings) .The Appendix provides similar figures for the other targets (Fig. \ref{['F_App3']}).
  • ...and 5 more figures