Aims. We study the time variations in the cycles of 20 active stars based on decade-long photometric or spectroscopic observations. Methods. A method of time-frequency analysis, as discussed in a companion paper, is applied to the data. Results. Fifteen stars definitely show multiple cycles, but the records of the rest are too short to verify a timescale for a second cycle. The cycles typically show systematic changes. For three stars, we found two cycles in each of them that are not harmonics and vary in parallel, indicating a common physical mechanism arising from a dynamo construct. The positive relation between the rotational and cycle periods is confirmed for the inhomogeneous set of active stars. Conclusions. Stellar activity cycles are generally multiple and variable.
We have used two robotic telescopes to obtain time-series high-resolution optical echelle spectroscopy and VI and/or by photometry for a sample of 60 active stars, mostly binaries. Orbital solutions are presented for 26 double-lined systems and for 19 single-lined systems, seven of them for the first time but all of them with unprecedented phase coverage and accuracy. Eighteen systems turned out to be single stars. The total of 6609 R = 55 000échelle spectra are also used to systematically determine effective temperatures, gravities, metallicities, rotational velocities, lithium abundances and absolute Hα-core fluxes as a function of time. The photometry is used to infer unspotted brightness, V − I and/or b − y colors, spot-induced brightness amplitudes and precise rotation periods. An extra 22 radial-velocity standard stars were monitored throughout the science observations and yield a new barycentric zero point for our STELLA/SES robotic system. Our data are complemented by literature data and are used to determine rotation-temperature-activity relations for active binary components. We also relate lithium abundance to rotation and surface temperature. We find that 74 % of all known rapidly-rotating active binary stars are synchronized and in circular orbits but 26 % (61 systems) are rotating asynchronously of which half have Prot > P orb and e > 0. Because rotational synchronization is predicted to occur before orbital circularization active binaries should undergo an extra spin-down besides tidal dissipation. We suspect this to be due to a magnetically channeled wind with its subsequent braking torque. We find a steep increase of rotation period with decreasing effective temperature for active stars, Prot ∝ T −7 eff , for both single and binaries, main sequence and evolved. For inactive, single giants with Prot > 100 d, the relation is much weaker, Prot ∝ T −1.12 eff . Our data also indicate a period-activity relation for Hα of the form RHα ∝ P −0.24 rot for binaries and RHα ∝ P −0.14 rot for singles. Its power-law difference is possibly significant. Lithium abundances in our (field-star) sample generally increase with effective temperature and are paralleled with an increase of the dispersion. The dispersion for binaries can be 1-2 orders of magnitude larger than for singles, peaking at an absolute spread of 3 orders of magnitude near T eff ≈ 5000 K. On average, binaries of comparable effective temperature appear to exhibit 0.25 dex less surface lithium than singles, as expected if the depletion mechanism is rotation dependent. We also find a trend of increased Li abundance with rotational period of form log n(Li) ∝ −0.6 log Prot but again with a dispersion of as large as 3-4 orders of magnitude.
Abstract. We analyse photometric observations of the young active dwarf AB Dor, spanning more than 20 years. Similar to the young solar analog LQ Hya, AB Dor shows long-lived, nonaxisymmetric spot distribution -active longitudes in opposite hemispheres. The active longitudes migrate nonlinearly in the fixed reference frame, because of the differential rotation and changes of the mean spot latitudes. At least two activity cycles are found in the data. One cycle originates from repeating switches of the activity between the two active longitudes in about (2-3)-year intervals. This results in the flip-flop cycle of about 5.5 years, which includes two consecutive switches. The 5.5-yr cycle also modulates variations of the minimum stellar brightness and the peak-to-peak amplitude, that suggests a periodic redistribution of the spot area between the opposite longitudes and supports the reality of the flip-flop cycle. The other cycle is clearly seen in variations of the mean and maximum stellar brightness on the time-scale of 20 years and is reminiscent of the 11-year sunspot cycle.
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