Magnetic activity evolution on Sun-like stars

European Space Agency, ESTEC – Postbus 299, 2200 AG, Noordwijk, The Netherlands
e-mail: pgondoin@cosmos.esa.int

Received: 10 July 2017
Accepted: 27 May 2018

Abstract

Context. Characterising the time evolution of magnetic activity on Sun-like stars is important not only for stellar physics but also for determining the environment in which planets evolve.

Aims. In recent decades, many surveys of open clusters have produced extensive rotation periods measurements on Sun-like stars of different ages. The present study uses this information with the aim to improve the description of their magnetic activity evolution.

Methods. I present a method that infers the long-term evolution of Ca II chromospheric (R′_HK) and X-ray coronal (L_X) emission on solar mass stars by combining a best fit parametric model of their rotation evolution with empirical rotation-activity relationships.

Results. The inferred scenario reproduces the high chromospheric and coronal emission levels around R′_HK ≈ 10⁻⁴ and L_X ≈ 10³⁰ erg s⁻¹ that are observed on pre-main sequence (PMS) stars. At the end of the PMS contraction phase around the age of ~30 Myr, the slowest rotating stars experience a rapid transition of their magnetic activity to more moderate levels around R′_HK ≈ 4 × 10⁻⁵ and L₅ ≈ 10²⁹ erg s⁻¹. This transition occurs later on more rapidly rotating stars, up to an age of ~600 Myr for the fastest rotators. After this brief episode of large magnetic activity decay, the average chromospheric and coronal emission levels of solar-mass stars decrease steadily converging towards similar values (R′_HK ≈ 10⁻⁵ and L_X ≈ 10²⁷ erg s⁻¹) by the age of the Sun.

Conclusion. The study suggests that solar mass stars in open clusters with ages between ~30 and ~600 Myr exhibit bimodal distributions of their R′_HK chromospheric activity indices and coronal X-ray to bolometric luminosity ratios that can be traced back to their rotation evolution. This conclusion is consistent with available measurements of activity indices from Sun-like stars in nearby open clusters.