Far beyond the Sun – I. The beating magnetic heart in Horologium

Julián D. Alvarado-Gómez, Gaitee A. J. Hussain, Jeremy J. Drake, Jean-François Donati, Jorge Sanz-Forcada, Beate Stelzer, Ofer Cohen, Eliana M. Amazo-Gómez, Jason H. Grunhut, Cecilia Garraffo, Sofia P. Moschou, James Silvester, Mary E. Oksala. 2018. Far beyond the Sun – I. The beating magnetic heart in Horologium. Monthly Notices of the Royal Astronomical Society 473, 4, 4326-4338, DOI: 10.1093/mnras/stx2642

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 similar to 1.6 yr). With an age of similar to 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 papers, 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 similar or equal to 1.97 +/- 0.02 yr and P-2 similar or equal to 1.41 +/- 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 +/- 4G and served to estimate the rotation period of the star (P-rot = 7.70(-0.67)(+0.18) d).

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