Far beyond the Sun: III. The magnetic cycle of $\boldsymbolι$ Horologii

TL;DR

This study provides the first dense Zeeman-Doppler Imaging time series for the young solar analogue ι Hor, revealing a full magnetic cycle of about 100 rotations with multiple polarity reversals in both poloidal and toroidal components. By constructing stellar butterfly diagrams, the authors trace latitudinal migration of large-scale magnetic features and extract rough large-scale flow speeds, finding faster poleward and equator-ward motions than solar values. The magnetic activity cycle shows connections to chromospheric and coronal cycles, offering empirical constraints for dynamos in young Sun-like stars and enabling direct comparisons with the solar cycle. Together, these results advance our understanding of stellar dynamos, magnetic cycle phenomenology, and the interplay between surface fields and activity proxies in solar-type stars.

Abstract

We present a comprehensive investigation of the magnetic cycle of the young, active solar analogue $ι$ Horologii ($ι$ Hor) based on intensive spectropolarimetric monitoring using HARPSpol. Over a nearly three-year campaign, the technique of Zeeman-Doppler Imaging (ZDI) was used to reconstruct 18 maps of the large-scale surface magnetic field of the star. These maps trace the evolution of the magnetic field morphology over approximately 139 stellar rotations. Our analysis uncovers pronounced temporal evolution, including multiple polarity reversals and changes in field strength and geometry. We examine the evolution of the poloidal and toroidal field components, with the toroidal component showing strong modulation in concert with the chromospheric activity. Furthermore, for the first time, we reconstruct stellar magnetic butterfly diagrams which are used to trace the migration of large-scale magnetic features across the stellar surface, determining a magnetic polarity reversal timescale of roughly 100 rotations ($\sim773$ d). In addition, by tracking the field-weighted latitudinal positions, we obtain the first estimates of the large-scale flow properties on a star other than the Sun, identifying possible pole-ward and equator-ward drift speeds for different field polarities. These results provide critical insights into the dynamo processes operating in young solar-type stars and offer a direct comparison with the solar magnetic cycle.

Figures (17)

• Figure 1: Unconstrained ZDI reconstruction of the large-scale magnetic field maps of $\iota$ Hor in latitude-longitude equirectangular projection. Columns contain the radial ($B_{\rm R}$, left), azimuthal ($B_{\rm A}$, middle), and meridional ($B_{\rm M}$, right) field components, with the color scale indicating the magnitude (in Gauss) and field polarity in each case. Each row corresponds to a different observing epoch as indicated (Epochs 1 to 6), with the observed phases denoted by black tick marks in longitude (upper x-axes). The campaign day and number of stellar rotations (measured from ${\rm BJD} = 2457300.78580$ to the first night of observations of a given epoch) are listed in each map. The segmented line shows the visibility limit imposed by the inclination of the star ($i = 60$ deg).
• Figure 2: Reconstructed large-scale magnetic field maps of $\iota$ Hor using ZDI (Epochs 7 to 12). See caption of Fig. \ref{['fig_1']}.
• Figure 3: Reconstructed large-scale magnetic field maps of $\iota$ Hor using ZDI (Epochs 13 to 18). See caption of Fig. \ref{['fig_1']}.
• Figure 4: Recovered LSD profiles from the spectropolarimetric observations of $\iota$ Hor. Each row contains a different observing epoch (Epochs 1 to 5), with individual observing phases ($\Phi$) shown in each sub-panel. The left column contains the ZDI fits (red) to the circularly polarized data (Stokes V, black), with the optimal reduced $\chi^2_{\rm R}$ achieved in each case. The corresponding diagnostic null (N) spectra are presented in the right column. Both profiles have been enhanced (by a factor of $5.0 \times 10^3$) and normalized to the continuum intensity ($I_{\rm C}$) for visualization purposes. The ZDI fits to the remaining epochs (6 to 18) are presented in Figs. \ref{['fig_5']}-\ref{['fig_7']}.
• Figure 5: The evolving global magnetic field properties for $\iota$ Hor plotted as a function of the chromospheric S-index and time. The red solid line shows the best 2-period model fit to Ca II H&K observations from 2023MNRAS.524.5725A. Day "0" corresponds to ${\rm BJD} = 2457300.78580$. The symbol sizes scale with $\log B^2$, and symbols for poloidal- and toroidal-dominated fields range from red to blue. The degree of non-axisymmetry is represented by symbol shapes, ranging from circular for fully axisymmetric field and star-shaped symbols for non-axisymmetry. Top: the axisymmetry and $\log B^2_{\rm tot}$ for the total field and the middle and bottom panels show the axisymmetry and $\log B^2$ for the poloidal field and toroidal field components, respectively.