A former member of the Hyades cluster , \iota Horologii ( \iota Hor ) is a planet-hosting Sun-like star which displays the shortest coronal activity cycle known to date ( P _ { cyc } \sim 1.6 yr ) .
With an age of \sim 625 Myr , \iota Hor is also the youngest star with a detected activity cycle .
The study of its magnetic properties holds the potential to provide fundamental information to understand the origin of cyclic activity and stellar magnetism in late-type stars .
In this series of articles , we present the results of a comprehensive project aimed at studying the evolving magnetic field in this star and how this evolution influences its circumstellar environment .
This paper summarizes the first stage of this investigation , with results from a long-term observing campaign of \iota Hor using ground-based high-resolution spectropolarimetry .
The analysis includes precise measurements of the magnetic activity and radial velocity of the star , and their multiple time-scales of variability .
In combination with values reported in the literature , we show that the long-term chromospheric activity evolution of \iota Hor follows a beating pattern , caused by the superposition of two periodic signals of similar amplitude at P _ { 1 } \simeq 1.97 \pm 0.02 yr and P _ { 2 } \simeq 1.41 \pm 0.01 yr. Additionally , using the most recent parameters for \iota Hor b in combination with our activity and radial velocity measurements , we find that stellar activity dominates the radial velocity residuals , making the detection of additional planets in this system challenging .
Finally , we report here the first measurements of the surface longitudinal magnetic field strength of \iota Hor , which displays varying amplitudes within \pm 4 G and served to estimate the rotation period of the star ( P _ { rot } = ~ { } 7.70 ^ { +0.18 } _ { -0.67 } d ) .