Multi-wavelength variability of the young solar analog ι Horologii

X-ray cycle, star spots, flares, and UV emission

¹ Departamento de Astrofísica, Centro de Astrobiología (CSIC-INTA), ESAC Campus, Camino bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
e-mail: jsanz@cab.inta-csic.es
² Institut für Astronomie and Astrophysik Tübingen (IAAT), Eberhard-Karls Universität Tübingen, Sand 1, 72076, Germany
³ INAF – Osservatorio Astronomico di Palermo G. S. Vaiana, Piazza del Parlamento 1, Palermo 90134, Italy
⁴ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA

Received: 16 April 2019
Accepted: 1 August 2019

Abstract

Context. Chromospheric activity cycles are common in late-type stars; however, only a handful of coronal activity cycles have been discovered. ι Hor is the most active and youngest star with known coronal cycles. It is also a young solar analog, and we are likely facing the earliest cycles in the evolution of solar-like stars, at an age (~600 Myr) when life appeared on Earth.

Aims. Our aim is to confirm the ~1.6 yr coronal cycle and characterize its stability over time. We use X-ray observations of ι Hor to study the corona of a star representing the solar past through variability, thermal structure, and coronal abundances.

Methods. We analyzed multi-wavelength observations of ι Hor using XMM-Newton, TESS, and HST data. We monitored ι Hor throughout almost seven years in X-rays and in two UV bands. The summed RGS and STIS spectra were used for a detailed thermal structure model, and the determination of coronal abundances. We studied rotation and flares in the TESS light curve.

Results. We find a stable coronal cycle along four complete periods, more than covered in the Sun. There is no evidence for a second longer X-ray cycle. Coronal abundances are consistent with photospheric values, discarding any effects related to the first ionization potential. From the TESS light curve we derived the first photometric measurement of the rotation period (8.2 d). No flares were detected in the TESS light curve of ι Hor. We estimate the probability of having detected zero flares with TESS to be ~2%.

Conclusions. We corroborate the presence of an activity cycle of ~1.6 yr in ι Hor in X-rays, more regular than its Ca II H&K counterpart. A decoupling of the activity between the northern and southern hemispheres of the star might explain the disagreement. The inclination of the system would result in an irregular behavior in the chromospheric indicators. The more extended coronal material would be less sensitive to this effect.