Resolving the stellar activity of the Mira AB binary with ALMA^⋆

¹ Department of Earth and Space SciencesChalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
e-mail: wouter.vlemmings@chalmers.se
² Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden
³ ESO Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
⁴ Université de Bordeaux 1, LAB, UMR 5804, 33270 Floirac, France
⁵ CNRS, LAB, UMR 5804, 33270 Floirac, France
⁶ Smithsonian Astrophysical Observatory, MS 70, 60 Garden Street, Cambridge, MA 02138, USA

Received: 25 March 2015
Accepted: 18 April 2015

Abstract

Aims. We present the size, shape, and flux densities at millimeter continuum wavelengths, based on ALMA science verification observations in Band 3 (~94.6 GHz) and Band 6 (~228.7 GHz), from the binary Mira A (o Ceti) and Mira B.

Methods. The Mira AB system was observed with ALMA at a spatial resolution down to ~25 mas. 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 89–100 GHz) and between Bands 3 and 6 were also derived.

Results. 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 ~3.8 × 3.2 AU in Band 3 (with brightness temperature T_b ~ 5300 K) and ~4.0 × 3.6 AU in Band 6 (T_b ~ 2500 K). Additionally, a bright hotspot ~0.4 AU, with T_b ~ 10 000 K, 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 ~2.4 AU.

Conclusions. 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.