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Context. Late-type stars rotate differentially owing to anisotropic turbulence in their outer convection zones. The rotation is called solar-like (SL) when the equator rotates fastest and anti-solar (AS) otherwise. Hydrodynamic simulations show a transition from SL to AS rotation as the influence of rotation on convection is reduced, but the opposite transition occurs at a different point in the parameter space. The system is bistable, i.e., SL and AS rotation profiles can both be stable. Aims. We study the effect of a dynamo-generated magnetic field on the large-scale flows, particularly on the possibility of bistable behaviour of differential rotation. Methods. We solve the hydromagnetic equations numerically in a rotating spherical shell that typically covers ±75• latitude (wedge geometry) for a set of different radiative conductivities controlling the relative importance of convection. We analyse the resulting differential rotation, meridional circulation, and magnetic field and compare the corresponding modifications of the Reynolds and Maxwell stresses. Results. In agreement with earlier findings, our models display SL rotation profiles when the rotational influence on convection is strong and a transition to AS when the rotational influence decreases. We find that dynamo-generated magnetic fields help to produce SL differential rotation compared to the hydrodynamic simulations. We do not observe any bistable states of differential rotation. In the AS cases we find coherent single-cell meridional circulation, whereas in SL cases we find multi-cellular patterns. In both cases, we obtain poleward circulation near the surface with a magnitude close to that observed in the Sun. In the slowly rotating cases, we find activity cycles, but no clear polarity reversals, whereas in the more rapidly rotating cases irregular variations are obtained. Moreover, both differential rotation and meridional circulation have significant temporal variations that are similar in strength to those of the Sun. Conclusions. Purely hydrodynamic simulations of differential rotation and meridional circulation are shown to be of limited relevance as magnetic fields, self-consistently generated by dynamo action, significantly affect the flows.

Abstract. We present the first evidence that a single active dwarf of solar type can show a long-lived, nonaxisymmetric spot distribution -active longitudes on opposite hemispheres, similar to evolved, rapidly rotating RS CVn-type binary stars. We analyse new as well as published photometric observations of the young active dwarf LQ Hya, spanning almost 20 years. We find that activity of the star has three activity cycles: a 5.2-yr "flip-flop" cycle, a 7.7-yr period in the amplitude modulation of the brightness and an approximately 15-yr period in variations of the mean brightness. The two shorter cycles are related to the alternating active longitudes and are similar to cycles observed in RS CVn-type stars. The 15-yr cycle reflects periodic changes of the mean spottedness of the star and resembles the solar 11-year cycle. The spot rotation period (about 1.6 days) changes during the 15-yr cycle, indicating the presence of small differential rotation. The lengths of the three cycles are related as 3:2:1, with the repetition of the spot configuration after 15 years. We discuss the possibility that the observed spot cycles represent two different magnetic dynamo modes operating in LQ Hya: an axisymmetric mode, as in the Sun, and a nonaxisymmetric higher order mode with two cycles in spot patterns. Our results suggest that young stars exhibit their cycles in spot distribution, as seen in LQ Hya. This is in contrast to the conclusion based on the analysis of Ca  H&K emission from plages. The results suggest also that the Vaughan-Preston gap represents a transition from a multiple-mode dynamo to a single-mode dynamo.

Context. Solar magnetic activity shows both smooth secular changes, such as the modern Grand Maximum, and quite abrupt drops that are denoted as grand minima, such as the Maunder Minimum. Direct numerical simulations (DNS) of convection-driven dynamos offer one way of examining the mechanisms behind these events. Aims.In this work, we analyze a solution of a solar-like DNS that was evolved for roughly 80 magnetic cycles of 4.9 years and where epochs of irregular behavior are detected. The emphasis of our analysis is to find physical causes for such behavior. Methods. The DNS employed is a semi-global (wedge-shaped) magnetoconvection model. For the data analysis tasks we use Ensemble Empirical Mode Decomposition and phase dispersion methods, as they are well suited for analyzing cyclic (non-periodic) signals. Results. A special property of the DNS is the existence of multiple dynamo modes at different depths and latitudes. The dominant mode is solar-like (equatorward migration at low latitudes and poleward at high latitudes). This mode is accompanied by a higher frequency mode near the surface and at low latitudes, showing poleward migration, and a low-frequency mode at the bottom of the convection zone. The low-frequency mode is almost purely antisymmetric with respect to the equator, while the dominant mode has strongly fluctuating mixed parity. The overall behavior of the dynamo solution is extremely complex, exhibiting variable cycle lengths, epochs of disturbed and even ceased surface activity, and strong short-term hemispherical asymmetries. Surprisingly, the most prominent suppressed surface activity epoch is actually a global magnetic energy maximum; during this epoch the bottom toroidal magnetic field obtains a maximum, demonstrating that the interpretation of grand minima-type events is non-trivial. The hemispherical asymmetries are seen only in the magnetic field, while the velocity field exhibits considerably weaker asymmetry. Conclusions. We interpret the overall irregular behavior as being due to the interplay of the different dynamo modes showing different equatorial symmetries, especially the smoother part of the irregular variations being related to the variations of the mode strengths, evolving with different and variable cycle lengths. The abrupt low-activity epoch in the dominant dynamo mode near the surface is related to a strong maximum of the bottom toroidal field strength, which causes abrupt disturbances especially in the differential rotation profile via the suppression of the Reynolds stresses.

We present optical I-band light curves of the gravitationally lensed double QSO B1600]434 from observations obtained at the Nordic Optical Telescope (NOT) between 1998 April and 1999 November. The photometry has been performed by simultaneous deconvolution of all the data frames, involving a numerical lens galaxy model. Four methods have been applied to determine the time delay between the two QSO components, giving a mean estimate of *t \ 51^4 days (95% conÐdence level). This is the fourth optical time delay ever measured. Adopting a ) \ 0.3, " \ 0 universe and using the mass model of Maller et al., this time delay estimate yields a Hubble parameter of km s~1 Mpc~1 (95% H 0 \ 52~8 14 conÐdence level), where the errors include time delay as well as model uncertainties. There are timedependent o †sets between the two (appropriately shifted) light curves that indicate the presence of external variations due to microlensing.

Context. FK Comae Berenices is a rapidly rotating magnetically active star, the light curve of which is modulated by cool spots on its surface. It was the first star where the "flip-flop" phenomenon was discovered. Since then, flip-flops in the spot activity have been reported in many other stars. Follow-up studies with increasing length have shown, however, that the phenomenon is more complex than was thought right after its discovery. Aims. Therefore, it is of interest to perform a more thorough study of the evolution of the spot activity in FK Com. In this study, we analyse 15 years of photometric observations with two different time series analysis methods, with a special emphasis on detecting flip-flop type events from the data. Methods. We apply the continuous period search and carrier fit methods on long-term standard Johnson-Cousins V-observations from the years 1995−2010. The observations were carried out with two automated photometric telescopes, Phoenix-10 and Amadeus T7 located in Arizona. Results. We identify complex phase behaviour in 6 of the 15 analysed data segments. We identify five flip-flop events and two cases of phase jumps, where the phase shift is Δφ < 0.4. In addition we see two mergers of spot regions and two cases where the apparent phase shifts are caused by spot regions drifting with respect to each other. Furthermore we detect variations in the rotation period corresponding to a differential rotation coefficient of |k| > 0.031. Conclusions. The flip-flop cannot be interpreted as a single phenomenon, where the main activity jumps from one active longitude to another. In some of our cases the phase shifts can be explained by differential rotation: two spot regions move with different angular velocity and even pass each other. Comparison between the methods show that the carrier fit utility is better in retrieving slow evolution especially from a low amplitude light curve, while the continuous period search is more sensitive in case of rapid changes.

Context. Debate over the existence of branches in the stellar activity-rotation diagrams continues. Application of modern time series analysis tools to study the mean cycle periods in chromospheric activity index is lacking. Aims. We develop such models, based on Gaussian processes, for one-dimensional time series and apply it to the extended Mount Wilson Ca H&K sample. Our main aim is to study how the previously commonly used assumption of strict harmonicity of the stellar cycles as well as handling of the linear trends affect the results. Methods. We introduce three methods of different complexity, starting with Bayesian harmonic regression model, followed by Gaussian process (GP) regression models with periodic and quasi-periodic covariance functions. We also incorporate a linear trend as one of the components. We construct rotation to magnetic cycle period ratio -activity (RCRA) diagrams and apply a Gaussian mixture model to learn the optimal number of clusters explaining the data. Results. We confirm the existence of two populations in the RCRA diagram; this finding is robust with all three methods used. We find only one significant trend in the inactive population, namely that the cycle periods get shorter with increasing rotation, leading to a positive slope in the RCRA diagram. This is in contrast with earlier studies, that postulate the existence of trends of different types in both of the populations. Our data is consistent with only two activity branches (inactive, transitional) instead of three (inactive, active, transitional) such that the active branch merges together with the transitional one. The retrieved stellar cycles are uniformly distributed over the R HK activity index, indicating that the operation of stellar large-scale dynamos carries smoothly over the Vaughan-Preston gap. At around the solar activity index, however, indications of a disruption in the cyclic dynamo action are seen. Conclusions. Our study shows that stellar cycle estimates from time series the length of which is short in comparison to the searched cycle itself depend significantly on the model applied. Such model-dependent aspects include the improper treatment of linear trends, while the assumption of strict harmonicity can result in the appearance of double cyclicities that seem more likely to be explained by the quasi-periodicity of the cycles. In the case of quasi-periodic GP models, which we regard the most physically motivated ones, only 15 stars were found with statistically significant cycles against red noise model. The periodicities found have to, therefore, be regarded as suggestive.

Abstract. We present a short re-evaluation of a recently published time delay estimate for the gravitational lens system HE 1104-1805 with emphasis on important methodological aspects: bias of the statistics, inconsistency of the methods and use of the purposeful selection of data points (or so-called "cleaning") at the preprocessing stage. We show how the inadequate use of simple analysis methods can lead to too strong conclusions. Our analysis shows that there are indications for the time delay in HE 1104-1805 to be between −0.9 and −0.7 years, but still with a large uncertainty.