SHORT-PERIOD STELLAR ACTIVITY CYCLES WITHKEPLERPHOTOMETRY

Abstract

We study the short-periodic component of stellar activity with a cycle periods P cyc < 10 3 days using the Kepler mission photometry of fast-rotating (1 < Ρ < 4 days) stars with spectra of M4V to F3V. Applying the originally developed two non-spectral methods, we measured the effective period of stellar cycles in 462 objects. The obtained results are in accordance with previous measurements by , do not seem to result from a beating effect. The performed measurements of P cyc cluster in a specific branch which covers the previously unstudied region in the Saar-Brandenburg (1999) diagram, and connects the branch of inactive stars with the area populated by super-active objects. It is shown that the formation of the discovered branch is due to the cu-quenching effect, which saturates the magnetic dynamo and decreases the cycle periods with the increase of inverted Rossby number. This finding is important in the context of the discussion on catastrophic quenching and other heuristic approximations of the non-linear α-effect.Subject headings: stars: activity -stars: magnetic field -(stars:) starspots -stars: statistics IntroductionStellar activity is a key factor of space weather and a valuable information channel on processes in stellar interiors (e.g., differential rotation, circulation and convection). At the same time, the connection of these processes with surface activity is poorly known. Various theories of magnetic dynamos need further observational tests and verification. That is why the empirical approach plays an important role in studies of stellar activity phenomenology. The matter of this approach consists in the search for the relation between physical parameters of stars and their activity parameters (e.g., the cycle periods P cyc and the average activity levels A) and comparison of that with theoretical predictions.An interesting view on stellar activity is given in Fig. 1 (reproduced from Saar & Brandenburg, 1999, Fig. 5; Herein after SB diagram). It shows that the stars in relation to their ω cyc /Ω and the inversed Rossby number Ro -1 =2τ MLT Ω are clustered in 3 branches: inactive (I), active (A) and super-active (S) objects. Here ω cyc = 2π/P cyc is the angular velocity of the magnetic cycle, Ω = 2π/Ρ is the angular velocity of the stellar rotation, Ρ is the rotation period of star and TMLT is the turnover time according to the mixing length theory (MLT; Bohm-Vitense, 1958).Unfortunately, the limited sample of the considered stars in the SB diagram does not cover the region of log(ω cyc /Ω) > -2 at Ro -1 >1, which still has to be explored. The uncovered region corresponds to the fast rotating stars and the shortest stellar activity cycles. In the present paper, we investigate this unstudied area using new data from the Kepler mission.Our study is focused on the stars with 1 < Ρ < 4 days and shortest scale of activity cycles P cyc < 10 3 days. A few similar objects with short activity cycles were initially investigated using Earth-based data (Vida & Olah, 2006; Olah et al., 2009;Vida et al., 2013)...

Stellar activity and rotation of the planet host Kepler-17 from long-term space-borne photometry

“…Similar cycles have been observed in, e.g., CoRoT-2 (Lanza et al 2009) and in some young late-type stars with an age between 4 and 95 Myr investigated by Distefano et al (2017). Short-term cycles, possibly of Rieger type, have also been investigated using CoRoT (Ferreira Lopes et al 2015) and Kepler (Arkhypov et al 2015) time series.…”

Deep Mixing in Stellar Variability: Improved Method, Statistics, and Applications

“…The differential rotation and the variability of spot latitudes may typically cause a few percent variation in the visible P (Reinhold et al 2013). It has been shown in our previous studies (Arkhypov et al 2015a(Arkhypov et al , 2015b that such uncertainty in the estimation of P leads to negligible variations in A 1 2 and A 2 2 . The squared amplitudes of the rotational harmonics A m 2 are analyzed because of their suspected proportionality to the number of starspots.…”

“…Detailed descriptions of the approach used can be found in our previous papers (Arkhypov et al 2015a(Arkhypov et al , 2015b. Therefore, below we simply briefly describe the key ideas of the improved algorithms of light-curve processing to explain the modifications.…”

“…This approach ensures the correct operation of the flare removing algorithm in all light curves. Preliminary tests of this improvement have been performed in Arkhypov et al (2015b).…”

“…In our previous studies (Arkhypov et al 2015a(Arkhypov et al , 2015b, we used high-precision light curves of 513 main-sequence stars from the Kepler mission database (http://exoplanetarchive. ipac.caltech.edu/applications/ETSS/Kepler_index.html) with known rotation periods of 1 < P < 4 days (Nielsen et al 2013), effective temperatures of 3504 < T KIC < 6740 K on the scale of the Kepler Input Catalog (KIC), and clear spot variability.…”