Magnetic activity of six young solar analogues II. Surface Differential Rotation from long-term photometry

Abstract

Abstract. The present paper is the second of a series dedicated to the study of the magnetic activity in a selected sample of young solar analogues. The sample includes five single G0-G5V stars with ages between 130 Myr and 700 Myr: EK Dra, π 1 UMa, HN Peg, k 1 Cet and BE Cet. In this study we also include the Pleiades-age ( 130 Myr) K0V star DX Leo. Our analysis is based on high precision photometric observations carried out as part of The Sun in Time project, aimed at a multiwavelength study of stars with solar-like global properties, but with different ages and thus at different stages of evolution. In the first paper of this series we presented the photometric observations and determined the existence of starspot cycles and their correlation with the global stellar properties. In the present paper we investigate the surface differential rotation (SDR). The periodogram analysis of the photometric data time series has allowed us to determine the rotational periods and to derive the following results: i) all the selected stars show variations of the rotational period. Such variations are definitely periodic and in phase with the starspot cycle for BE Cet and DX Leo. They are likely periodic and in phase also for π 1 UMa, EK Dra and HN Peg, but still need confirmation. By analogy with the solar butterfly diagram, the rotational period variations are interpretable in terms of surface differential rotation, that is, they are attributable to the existence of active latitude belts migrating during the activity cycle on a differentially rotating star; ii) BE Cet, π 1 UMa and EK Dra show a solar-like pattern of SDR, that is the rotational period steadily decreases along the activity cycle, jumping back to higher values at the beginning of the next cycle; on the contrary, DX Leo, k 1 Cet and HN Peg show an antisolar pattern; iii) the amplitude of the rotational period variations shows a power law dependence on the rotational period similar to that found in previous studies. Contrary to theoretical predictions, the cycle length is not correlated to the Dynamo number, it is indeed positively correlated to the SDR amplitude. More precisely, stars tend to concentrate along three different branches with the cycle length increasing with increasing ∆Ω/Ω. Moreover, we found that the SDR amplitude changes from cycle to cycle, which is reminiscent of a wave of excess rotation propagating in latitude; iiii) the apparently different solar and antisolar behaviours are probably due to different inclinations of the stellar rotation axis under which the star is seen. The long-term photometry of the young single star LQ Hya, although not included in the initial project, is also used in the present analysis to enlarge the investigated sample. We determined for LQ Hya three different starspot cycles and an antisolar pattern of SDR.

LO Peg: surface differential rotation, flares, and spot-topographic evolution

“…The first period is also found to be similar to the latitudinal spot migration period derived from SDR analysis, which could be similar to the 11 yr cycle of the solar butterfly diagram. This type of activity cycle was also observed in similar fast-rotating stars such as AB Dor (Collier Cameron & Donati, 2002; and LQ Hya (Messina & Guinan, 2003). In the SDR analysis, the decrease in photometric periods within most of the cycles is reminiscent of the sunspot cyclic behaviour, where the latitude of spot-forming region moves towards the equator, i.e., toward progressively faster rotating latitudes along an activity cycle, and spot-groups were present within ±45 • latitude of LO Peg.…”

“…It is interesting to note that the slope of the rotational period on LO Peg varies and therefore SDR amplitude ∆P (= Pmax − Pmin) changes from cycle to cycle. Similar behaviour was also observed in AB Dor (Collier Cameron & Donati, 2002), BE Cet, DX Leo, and LQ Hya (Messina & Guinan, 2003). This resembles either a wave of excess rotation on a time-scale of the order of decades, or a variation of the width of the latitude band in which spots occur.…”

“…Kitchatinov & Rüdiger (1999) found that n varies with both rotation rate and with spectral type. This power-law dependence is confirmed by observational data (Hall, 1991;Henry, Fekel & Hall, 1995), although the observational and theoretical values of n differ (see Messina & Guinan, 2003). Fig.…”

Stellar magnetic cycles

“…They found that the fastest rotating stars tended to display longer cycles, with the range of the observed cycle periods apparently converging from a large dispersion at ages of ≈ 100 − 200 Myr toward the solar cycle period at the age of the Sun. Messina & Guinan (2003) investigated the correlation between the period of the rotational modulation and the phase of the cycle in the above sample of stars finding a clearly significant correlation in two of them, and a less significant correlation in the other four. Both solar-like and anti-solar behaviours were observed, as in the case of the chromospheric cycles of the H&K project sample.…”

“…Assuming that α 0 = Ωl, Ω ∝ Ω, and η t = l 2 /τ c , where l is the mixing length at the base of the stellar convective envelope, we find D ∝ Ro −2 . Therefore, the Rossby number is a measure of the dynamo number which can be derived by combining observations of the stellar rotational modulation with a model of its internal structure (see, e.g., Messina & Guinan 2003).…”