Magnetic activity evolution on Sun-like stars

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 (LX) 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 LX ≈ 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 LX ≈ 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.

Rossby number and chromospheric activity of solar mass stars

“…Figure 3 shows that this episode occurs at the end of the contraction phase for stars that are initially slow rotators ( P rot (5 Myr) ≥ 6 d) and, later, up to an age of 600 Myr for initially fast rotators ( P rot (5 Myr) < 6 d). The Rossby Number evolution displayed in Figure 3 is derived from the rotation model evolution of Gondoin (2017, 2018) and a convective turnover time calculated with an evolutionary model that takes into account some structural effects of rotation (Landin et al 2010 and references therein), including mixing caused by rotational instabilities, angular momentum redistribution in radiative regions, and angular momentum losses due to magnetized stellar wind.…”

“…Left: measured indices of 0.9–1.1 M ⊙ in the Pleiades, the Hyades, M67, and the field compared with simulated evolutions of the average Ca II indices of 1.0 M ⊙ stars with initial rotation periods of 1 (top), 2, 3, 6, 10, and 20 days (bottom) at 5 Myr. Right: same graph with simulated evolutions of the Ca II indices of 1.0 M ⊙ stars, including their estimated dispersion around the average values due to the short‐term variability of the magnetic activity (Gondoin 2018)…”

“…This method was initially applied to the Pleiades (∼112 Myr), M50 (∼130 Myr), M35 (∼133 Myr), and M37 (∼550 Myr) open clusters and to the Sun. Gondoin (2018) later used rotation periods measurements in NGC 6811 (∼1 Gyr; Meibom et al 2011), NGC 6819 (∼2.4 Gyr; Meibom et al 2015), and M67 (∼4 Gyr; Barnes et al 2016). As these old clusters show narrow distributions of rotation periods, a few stellar members with known masses and rotation periods are sufficient to constrain the rotation of solar mass stars at the corresponding ages.…”

“…This conclusion is actually consistent with histograms of available measurements of activity indices. Gondoin (2018) provided a detailed comparison between simulated distributions of the index in an open cluster of various ages with measured histograms of chromospheric activity indices among 0.9–1.1 M ⊙ stars in the Pleiades (Duncan et al 1991; Soderblom et al 1993; White et al 2007), the Hyades (Duncan et al 1991; Paulson et al 2002; White et al 2007), and M67 (Giampapa et al 2006), compiled by Mamajek & Hillenbrand (2008). Despite the limited number of measurements, the comparison (Figure 5) corroborates that solar mass stars in the Pleiades that are 112 Myr old group into two distinct activity levels around to −4.2 and to −3.8.…”

Characterization of chromospheric activity based on Sun‐as‐a‐star spectral and disk‐resolved activity indices

“…The existence and the drivers of magnetic cycles raise fundamental questions in stellar physics (Egeland et al 2017). Major efforts are directed to studying the relation between the observed activity and the star's intrinsic properties such as age, rotation, and spectral type (Gondoin 2018; Judge & Thompson 2012; Radick et al 2018). This information serves as input for models, which facilitates the inference of stellar internal structure and dynamos (Baliunas et al 1996; Pesnell 2012; Pipin & Kosovichev 2016).…”