ιHorologi, the first coronal activity cycle in a young solar-like star^⋆

¹ Departamento de Astrofísica, Centro de Astrobiología (CSIC–INTA), ESAC Campus, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
e-mail: jsanz@cab.inta-csic.es
² INAF – Osservatorio Astronomico di Palermo G. S. Vaiana, Piazza del Parlamento 1, 90134 Palermo, Italy
e-mail: stelzer@oapa.inaf.it
³ Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301, USA
e-mail: travis@spacescience.org
⁴ Stellar Astrophysics Centre (SAC), Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark

Received: 1 March 2013
Accepted: 8 April 2013

Abstract

Context. The shortest chromospheric (Ca ii H&K) activity cycle (1.6 yr) has been recently discovered in the young (~600 Myr) solar-like star ι Hor. Coronal X-ray activity cycles have only been discovered in a few stars other than the Sun, all of them with an older age and a lower activity level than ι Hor.

Aims. We intended to find the X-ray coronal counterpart of the chromospheric cycle for ι Hor. This represents the first X-ray cycle observed in an active star, as well as the paradigm of the first coronal cycles in the life of a solar-like star.

Methods. We monitored ι Hor with XMM-Newton observations spanning almost two years. The spectra of each observation are fit with two-temperature coronal models to study the long-term variability of the star.

Results. We find a cyclic behavior in X-rays very similar to the contemporaneous chromospheric cycle. The continuous chromospheric monitoring for more than three cycle lengths shows a trend toward decreasing amplitude, apparently modulated by a longer term trend. The second cycle is disrupted prior to reaching its maximum, followed by a brief episode of chaotic variability before the cyclic behavior resumes, only to be disrupted again after slightly more than one cycle.

Conclusions. We confirm the presence of an activity cycle of ~1.6 yr in ι Hor both in X-rays and Ca ii H&K. It is likely subject to the modulation of a longer, not yet constrained second cycle. The 1.6 yr cycle is the shortest coronal one observed to date, and ι Hor represents the most active star for which a coronal activity cycle has been found. This cycle is probably representative of the first coronal cycles in the life of a solar-like star, at the age when life started on Earth.