Magnitude-range brightness variations of overactive K giants

Abstract

Context. Decades-long, phase-resolved photometry of overactive spotted cool stars has revealed that their long-term peak-to-peak light variations can be as large as one magnitude. Such brightness variations are too large to be solely explained by rotational modulation and/or a cyclic, or pseudo-cyclic, waxing and waning of surface spots and faculae as we see in the Sun. Aims. We study three representative, overactive spotted K giants (IL Hya, XX Tri, and DM UMa) known to exhibit V-band light variations between 0. m 65-1. m 05. Our aim is to find the origin of their large brightness variation. Methods. We employ long-term phase-resolved multicolor photometry, mostly from automatic telescopes, covering 42 yr for IL Hya, 28 yr for XX Tri, and 34 yr for DM UMa. For one target, IL Hya, we present a new Doppler image from NSO data taken in late 1996. Effective temperatures for our targets are determined from all well-sampled observing epochs and are based on a V − I C color-index calibration.Results. The effective temperature change between the extrema of the rotational modulation for IL Hya and XX Tri is in the range 50-200 K. The bolometric flux during maximum of the rotational modulation, i.e., the least spotted states, varied by up to 39% in IL Hya and up to 54% in XX Tri over the course of our observations. We emphasize that for IL Hya it is just about half of the total luminosity variation that can be explained by the photospheric temperature (spots/faculae) changes, while for XX Tri it is even about one third. The long-term, 0. m 6 V-band variation of DM UMa is more difficult to explain because little or no B − V color index change is observed on the same timescale. Placing the three stars with their light and color variations into H-R diagrams, we find that their overall luminosities are generally too low compared to predictions from current evolutionary tracks. Conclusions. A change in the stellar radius due to strong and variable magnetic fields during activity cycles likely plays a role in explaining the anomalous brightness and luminosity of our three targets. At least for IL Hya, a radius change of about 9% is suggested from m bol and T eff , and is supported by independent v sin i measurements.

Spot evolution on the red giant star XX Triangulum

“…Its errors are internal errors based on the root mean square (rms) of the entire time series. The above effective temperature is close to what Oláh et al (2014) obtained from VI photometry during maximum photometric brightness, but the metallicity differs by a factor of two.…”

“…We took interstellar absorption into account with 0. m 1 per 100 pc (Strassmeier 1999). We note that Oláh et al (2014) estimated the reddening of XX Tri from all-sky infrared imaging and found a color excess…”

“…XX Tri reached its maximum brightness in 2009 and is in a declining state since then. Removing this overall trend, Oláh et al (2014) found a second shorter-period variation of approximately 6 yr or an integer multiple of it. No clear explanation for this period, if real, could be given.…”

“…So far, stellar activity cycles were inferred from long-term chromospheric Ca ii H&K or photospheric V-band variations (e.g., Baliunas et al 1995;Oláh et al 2007) or from repeated detections of a "flip-flop" phenomenon (see Hackman et al 2013;Korhonen & Järvinen 2007). Derived timescales for RS CVn stars range between 2-50 yr. Just recently, Oláh et al (2014) investigated 28 yr of photometry of XX Tri along with two other over-active K giants and found a long-term sinusoidal brightness trend with a length comparable to the length of the data set, i.e., ≈28 yr. Our diffusivity-based cycle prediction of ≈26 yr matches this observation surprisingly well despite the comparable shortness of the photometric coverage. XX Tri reached its maximum brightness in 2009 and is in a declining state since then.…”

“…During the season 1998/99 a record amplitude of ≈0. m 6 in V was observed in comparison to an amplitude of <0. m 4 during the seasons from 2006/07 to 2011/12 (see Oláh et al 2014). …”