Sunspots are cool areas caused by strong surface magnetic fields that inhibit convection1,2. Moreover, strong magnetic fields can alter the average atmospheric structure3, 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 pole4. 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 optimal5. 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 artefacts6. 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 Sun7 is absent on ζ And, which hosts global spot patterns that cannot be produced by solar-type dynamos8.
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We thank E. Pedretti for his early efforts in imaging ζ And. This work is based upon observations obtained with the Georgia State University Center for the High Angular Resolution Astronomy Array at Mount Wilson Observatory. The CHARA Array is supported by the National Science Foundation under grant numbers AST-1211929 and AST-1411654. Institutional support has been provided from the GSU College of Arts and Sciences and the GSU Office of the Vice President for Research and Economic Development. The MIRC instrument at the CHARA Array was funded by the University of Michigan. The 2013 CHARA/MIRC observations were supported by a Rackham Graduate Student Research Grant from the University of Michigan. This imaging work was supported by the National Science Foundation (NSF) grant AST-1108963. The SURFING software was developed in part with funding from the Observatoire de Paris, Meudon. This research made use of the SIMBAD database, operated at CDS, Strasbourg, France and the Jean-Marie Mariotti Center SearchCal and Aspro2 services co-developed by FIZEAU and LAOG/IPAG. This work was supported by the Hungarian Scientific Research Fund (OTKA K-109276), the ‘Lendület-2009’ Young Researchers’ programme of the Hungarian Academy of Sciences, an STFC Rutherford Fellowship and Grant (ST/J004030/1, ST/K003445/1), and an ERC Starting Grant (grant agreement number 639889).
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Living Reviews in Solar Physics (2017)