No Sun-like dynamo on the active star ζ Andromedae from starspot asymmetry

Infrared interferometry imaging of the old, magnetically active star ζ Andromedae reveals an asymmetric distribution of starspots, unlike the north–south starspot symmetry observed on the Sun, meaning the underlying dynamo mechanisms must be different. Star spots on ζ Andromedae As observed on the Sun, strong magnetic fields suppress convection to create localized cooler regions that appear as sunspots or starspots. It is difficult to observe starspots on stars other than the Sun, but here Rachael Roettenbacher and colleagues present infrared interferometry images of the old, magnetically active star ζ Andromedae that reveal starspot distribution in detail. They find an asymmetric distribution, in contrast to the north–south symmetry observed on the Sun, meaning that the underlying dynamo mechanisms in the two bodies must be different. Sunspots are cool areas caused by strong surface magnetic fields that inhibit convection 1 , 2 . Moreover, strong magnetic fields can alter the average atmospheric structure 3 , degrading our ability to measure stellar masses and ages. Stars that are more active than the Sun have more and stronger dark spots than does the Sun, including on the rotational pole 4 . Doppler imaging, which has so far produced the most detailed images of surface structures on other stars, cannot always distinguish the hemisphere in which the starspots are located, especially in the equatorial region and if the data quality is not optimal 5 . This leads to problems in investigating the north–south distribution of starspot active latitudes (those latitudes with more starspot activity); this distribution is a crucial constraint of dynamo theory. Polar spots, whose existence is inferred from Doppler tomography, could plausibly be observational artefacts 6 . Here we report imaging of the old, magnetically active star ζ Andromedae using long-baseline infrared interferometry. In our data, a dark polar spot is seen in each of two observation epochs, whereas lower-latitude spot structures in both hemispheres do not persist between observations, revealing global starspot asymmetries. The north–south symmetry of active latitudes observed on the Sun 7 is absent on ζ And, which hosts global spot patterns that cannot be produced by solar-type dynamos 8 .