Resolving the stellar activity of the Mira AB binary with ALMA

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.

Resolving the extended atmosphere and the inner wind of Mira (oCeti) with long ALMA baselines

“…In general, our results are consistent with those reported by Matthews et al (2015). However, we did not find any evidence of the compact hotspot or reproduce the similar results of the visibility fitting as reported by Vlemmings et al (2015). We present our detailed continuum analysis of the visibility data in Appendix A.…”

“…The derived radius and effective temperature of the pseudo-continuum is R continuum = 3.60 × 10 13 cm (21.8 mas) and 2600 K, respectively. These values are comparable to the mean radii and brightness temperatures of the elliptical disks fitted by us (Appendix A), by Matthews et al (2015), and by Vlemmings et al (2015).…”

“…Matthews et al (2015) and Vlemmings et al (2015) have independently analysed the continuum data of the Mira AB system in both ALMA Bands 3 (96 GHz) and 6 (229 GHz) of this SV dataset. Additionally, Matthews et al (2015) include the continuum data from their Q-band (46 GHz) observation with the Karl G. Jansky VLA in 2014.…”

“…12), who adopted the von Neumann-Richtmyer pseudo-viscosity method as the artificial shock dissipation mechanism (von Neumann & Richtmyer 1950), the authors of early hydrodynamic models derived the dynamical structures around Mira-like variables. Bowen's models also considered the thermal relaxation of shocks via radiative cooling from neutral hydrogen atoms at high temperatures ( 6000 K) and from other species (which is represented by an assumed cooling coefficient) at low temperatures (see Sects.…”

Dawes Review 6: The Impact of Companions on Stellar Evolution

“…It also allows us to characterise further, and lend corroborating evidence to the phenomenon of wind Rochelobe overflow (Section 3, also Vlemmings et al 2015). Large Keplerian disks around stars can be used to infer a binary past (e.g., Bujarrabal et al, 2013) even when a binary companion is not seen.…”