2015A&A...577L...4V

We present the size, shape, and flux densities at millimeter continuum wavelengths, based on ALMA science verification observations in Band 3 (∼94.6GHz) and Band 6 (∼228.7GHz), from the binary Mira A (omicron Ceti) and Mira B. The Mira AB system was observed with ALMA at a spatial resolution down to ∼25mas. The extended atmosphere of Mira A and the wind around Mira B sources were resolved, and we derived the sizes of Mira A and of the ionized region around Mira B. The spectral indices within Band 3 (between 89-100GHz) and between Bands 3 and 6 were also derived. The spectral index of Mira A is found to change from 1.71±0.05 within Band 3 to 1.54±0.04 between Bands 3 and 6. The spectral index of Mira B is 1.3±0.2 in Band 3, in good agreement with measurements at longer wavelengths; however, it rises to 1.72±0.11 between the bands. For the first time, the extended atmosphere of a star is resolved at these frequencies, and for Mira A the diameter is ∼3.8x3.2AU in Band 3 (with brightness temperature T_b∼5300K) and ∼4.0x3.6AU in Band 6 (T_b∼2500K). Additionally, a bright hotspot ∼0.4AU, with T_b∼10000K, is found on the stellar disk of Mira A. The size of the ionized region around the accretion disk of Mira B is found to be ∼2.4AU. The emission around Mira B is consistent with emission from a partially ionized wind of gravitationally bound material from Mira A close to the accretion disk of Mira B. The Mira A atmosphere does not fully match predictions with brightness temperatures in Band 3 significantly higher than expected, potentially owing to shock heating. The hotspot is very likely due to magnetic activity and could be related to the previously observed X-ray flare of Mira A.