Revisiting the connection between magnetic activity, rotation period, and convective turnover time for main-sequence stars

Abstract

The connection between stellar rotation, stellar activity, and convective turnover time is revisited with a focus on the sole contribution of magnetic activity to the Ca II H&K emission, the so-called excess flux, and its dimensionless indicator R + HK in relation to other stellar parameters and activity indicators. Our study is based on a sample of 169 mainsequence stars with directly measured Mount Wilson S-indices and rotation periods. The R + HK values are derived from the respective S-indices and related to the rotation periods in various B − V -colour intervals. First, we show that stars with vanishing magnetic activity, i.e. stars whose excess flux index R + HK approaches zero, have a well-defined, colourdependent rotation period distribution; we also show that this rotation period distribution applies to large samples of cool stars for which rotation periods have recently become available. Second, we use empirical arguments to equate this rotation period distribution with the global convective turnover time, which is an approach that allows us to obtain clear relations between the magnetic activity related excess flux index R + HK , rotation periods, and Rossby numbers. Third, we show that the activity versus Rossby number relations are very similar in the different activity indicators. As a consequence of our study, we emphasize that our Rossby number based on the global convective turnover time approaches but does not exceed unity even for entirely inactive stars. Furthermore, the rotation-activity relations might be universal for different activity indicators once the proper scalings are used.

Chromospheric activity in 55 Cancri: II. Theoretical wave studies versus observations

“…This behavior stems from its old age (Mamajek & Hillenbrand 2008;von Braun et al 2011;Yee et al 2017) and slow rotation (Henry et al 2000;Bourrier et al 2018). Stellar evolution models indicate that late-type main-sequence stars gradually lose angular momentum resulting in a steady decrease of stellar activity (e.g., Charbonneau et al 1997;Mittag et al 2018).…”

“…A large body of previous work, see, e.g., Schrijver & Zwaan (2000) and references therein, indicates that the atmospheres of highly active stars are dominantly heated by magnetic processes, including magnetic waves (e.g., Shoda et al 2020), whereas for the atmospheric heating of low-activity stars non-magnetic processes are expected to play a more prominent role (e.g., Schrijver 1995;Buchholz et al 1998;Cuntz et al 1999), even though magnetic phenomena. When stars age, the relative importance of atmospheric magnetic processes tends to subside, a process closely related to the evolution of angular momentum (e.g., Keppens et al 1995;Charbonneau et al 1997;Wolff & Simon 1997;Matt et al 2015;Mittag et al 2018).…”

Zodiacal exoplanets in time – X. The orbit and atmosphere of the young ‘neptune desert’-dwelling planet K2-100b

“…From the sample of France et al (2018) we identified several stars (Table 1) as potential analogs to K2-100 based on spectral type and Rossby number Ro = P/τ, where τ is the global convective overturn timescale. We use the τ vs. B − V relationship developed by Mittag et al (2018) combined with periods to estimate Ro. The two most similar stars are probably HD 25825 and V993 Tau, members of the Hyades cluster which itself may have the same age and metallicity as Praesepe (Brandt & Huang 2015).…”

The corona -- chromosphere connection studied with simultaneous eROSITA and TIGRE observations

“…Comparisons of the X-ray flux and the Ca ii H&K excess emission were also performed for more solar-type stars (Schrijver 1983;Mittag et al 2018). The power-law relation between the chromospheric Ca ii H&K emission and the coronal X-ray flux holds over four orders of magnitude in X-ray flux; the Sun also follows this relation during its activity cycle.…”