Four decades of photometry of XX Trianguli, `the most spotted star' in the sky

Abstract

Over the past 40 years the brightness variations of XX Tri, a single line RS CVn type binary system with a synchronized K-giant primary, has exceeded one magnitude in the V band. Although these changes are primarily caused by starspots, an additional activity-related mechanism may also be behind the long-term trend of overall brightness increase. By compiling the most complete photometric data set so far, we attempt to examine how the nature of seasonal-to-decadal changes can be linked to global magnetism. The long-term brightening of XX Tri was accompanied by a gradual increase in the effective temperature, which resulted in a blueing shift in the Herzsprung-Russell diagram. In the long term, a constant cycle of about 4 years is most strongly present in the entire data. Besides, we also found a modulation of about 11 years, and a slowly decreasing cycle of about 5.7-5.2 years. From the seasonal datasets we found that the most dominant rotation-related periods are scattered around the orbital period. From this we infer a solar-type surface differential rotation, although the surface shear is significantly smaller than that of the Sun. The 4-year cycle indicates flip-flop-like behavior: during this time, the 2-3 active longitudes usually present on the stellar surface are rearranged. The magnitude-range changes in the long term cannot be interpreted solely as changes in the number and size of spots; the unspotted brightness of XX Tri has also increased over the decades. This should alert users of photometric spot models to reconsider the basic concept of constant unspotted brightness in similar cases.

11pages, accepted for publication in A&A

References in corpus (12)

• No Sun-like dynamo on the active star $\zeta$ Andromedae from starspot asymmetry
• Spots, plages, and flares on lambda Andromedae and II Pegasi
• Spot evolution on the red giant star XX Triangulum. A starspot-decay analysis based on time-series Doppler imaging
• Antisolar differential rotation of the K1-giant sigma Geminorum revisited
• Solar activity in the past and the chaotic behaviour of the dynamo
• Rotation--differential rotation relationships for late-type single and binary stars from Doppler imaging
• Time-series Doppler imaging of the red giant HD 208472. Active longitudes and differential rotation
• Li-rich K giants, dust excess, and binarity
• Antisolar differential rotation with surface lithium enrichment on the single K-giant V1192 Orionis
• Magnitude-range brightness variations of overactive K giants
• Convective Mixing: The Formation Channel of Li-rich Giants
• First Doppler image and starspot-corrected orbit for $\lambda$ Andromedae: A multifaceted activity analysis

Cited by in corpus (1)

• Magnetic activity on the young Sun: a case study of EK Draconis