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

Sunspots are cool areas caused by strong surface magnetic fields that inhibit convection. Moreover, strong magnetic fields can alter the average atmospheric structure, 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. 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. 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. 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 is absent on ζ And, which hosts global spot patterns that cannot be produced by solar-type dynamos.

HD 181068: a red giant in a triply eclipsing compact hierarchical triple system.

Hierarchical triple systems comprise a close binary and a more distant component. They are important for testing theories of star formation and of stellar evolution in the presence of nearby companions. We obtained 218 days of Kepler photometry of HD 181068 (magnitude of 7.1), supplemented by ground-based spectroscopy and interferometry, which show it to be a hierarchical triple with two types of mutual eclipses. The primary is a red giant that is in a 45-day orbit with a pair of red dwarfs in a close 0.9-day orbit. The red giant shows evidence for tidally induced oscillations that are driven by the orbital motion of the close pair. HD 181068 is an ideal target for studies of dynamical evolution and testing tidal friction theories in hierarchical triple systems.

Differential path considerations in optical stellar interferometry.

A formulation of the differential-air-path problem for a large-baseline optical interferometer is presented. Because air is a dispersive medium, each wavelength has a different optical path length. This can be corrected to a large extent if an extra piece of glass is placed in each arm of the interferometer. The problem then becomes analogous to the doublet-achromat design, in which, in this case, the air path takes the place of one piece of glass. Atmospheric refraction and field-of-view considerations also amount to a change in the differential paths within the interferometer. All three effects can therefore be considered to be aspects of the same problem. The focus of this work has been for a proposed array to be developed by the Center for High Angular Resolution Astronomy (CHARAArray), although the results are applicable to any optical array.