Magnetic activity of F stars observed byKepler

Abstract

Context. The study of stellar activity is important because it can provide new constraints for dynamo models when combined with surface rotation rates and the depth of the convection zone. We know that the dynamo mechanism, which is believed to be the main process that rules the magnetic cycle of solar-like stars, results from the interaction between (differential) rotation, convection, and magnetic field. The Kepler mission has already been collecting data for a large number of stars during four years allowing us to investigate magnetic stellar cycles. Aims. We investigated the Kepler light curves to look for magnetic activity or even hints of magnetic activity cycles. Based on the photometric data we also looked for new magnetic indexes to characterise the magnetic activity of the stars. Methods. We selected a sample of 22 solar-like F stars that have a rotation period shorter than 12 days. We performed a timefrequency analysis using the Morlet wavelet yielding a magnetic proxy for our sample of stars. We computed the magnetic index S ph as the standard deviation of the whole time series and the index S ph , which is the mean of standard deviations measured in subseries of length five times the rotation period of the star. We defined new indicators, such as the contrast between high and low activity, to take into account the fact that complete magnetic cycles are not observed for all the stars. We also inferred the Rossby number of the stars and studied their stellar background. Results. This analysis shows different types of behaviour in the 22 F stars. Two stars show behaviour very similar to magnetic activity cycles. Five stars show long-lived spots or active regions suggesting the existence of active longitudes. Two stars in our sample seem to have a decreasing or increasing trend in the temporal variation of the magnetic proxies. Finally, the last group of stars shows magnetic activity (with the presence of spots) but no sign of cycle.

Rotation and magnetism ofKeplerpulsating solar-like stars

“…Unfortunately, the highest peak in the periodogram can sometimes be a higher overtone of the fundamental rotation period and the extracted rotation rate would be a multiple of the true value (see for further details McQuillan et al 2013a). To minimise this problem, we computed the wavelet power spectrum (WPS) of the light curves and projected it onto the period axis, which reinforces the power of the fundamental peak and reduces the heights of the overtones (for further details about the methodology see Mathur et al 2014a). McQuillan et al (2013aMcQuillan et al ( ,b, 2014) also studied the autocorrelation function of the Kepler time series to determine rotation rates of M dwarfs, Kepler objects of interests (KOI), and about 34 000 main-sequence stars using PDC-MAP data from Q1 to Q4 for the first study and from Q3 to Q14 for the last two.…”

Photometric and Astrometric Vagaries of the Enigma Star Kic 8462852

“…On the other hand, F-type stars have shorter periods of rotation and more persistent photometric features. Mathur et al (2014) investigated the light curves of 22 F-type stars with periods between 2 and 12 days and found a group of five stars with modulations which remained coherent in phase over the entire duration of observation. They interpreted these stars as rotators with very stable active longitudes where groups of starspots predominantly emerge.…”

“…They interpreted these stars as rotators with very stable active longitudes where groups of starspots predominantly emerge. Mathur et al (2014) discarded the possibility of stable classical pulsations because the stars do not reside in the known instability zones. The morphology of the 0.88 day variations in the light curve, which are flat at the top and sharp at the bottom, also favor the rotational modulation interpretation.…”

Keplerobservations of the asteroseismic binary HD 176465

“…They computed the standard deviation of the time series, which has been used as a photometric magnetic activity index, denoted S ph Mathur et al 2014). García et al (2014) found S ph = 248 ± 4 ppm, which is close to the value for the Sun during its maximum activity of Cycle 23, as measured from VIRGO/SPM (see Mathur et al 2014). Furthermore, the S ph obtained is a lower limit since the inclination angle is ∼ 50 • .…”

“…García et al (2014) also investigated the level of photospheric magnetic activity in HD 176465. They computed the standard deviation of the time series, which has been used as a photometric magnetic activity index, denoted S ph Mathur et al 2014). García et al (2014) found S ph = 248 ± 4 ppm, which is close to the value for the Sun during its maximum activity of Cycle 23, as measured from VIRGO/SPM (see Mathur et al 2014).…”