 |
| Artist’s impression of a giant flare on the surface of red giant Mira A. Behind the star, material is falling onto the star’s tiny companion Mira B (the average separation between the two stars is 0.472′′ corresponding to 43.4 AU at a distance of 92 pc) |
The following is the abstract from a study published in
Astronomy & Astrophysics on May 13, 2015 (
Resolving the stellar activity of the Mira AB binary with ALMA):
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 Tb ∼ 5300 K) and ∼4.0 × 3.6 AU in Band 6 (Tb ∼ 2500 K). Additionally, a bright hotspot ∼0.4 AU, with Tb ∼ 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.
