etoka2017 fig9Using the Nancay Radio Telescope in France, the Medicina and Effelsberg single-dish radio telescopes in Italy and Germany, and the European EVN and British eMERLIN radio interferometers, a group of European astronomers studied a recent multi-year flare of the hydroxyl (OH) and water vapor (H2O) masers towards the famous variable star Mira. The study was led by Sandra Etoka during her stay as research scientist at Hamburger Sternwarte. The masers emitting at a wavelength of 18 cm (OH) and 1.3 cm (H2O) originate in the circumstellar shell of the star about 40 astronomical units (AU) apart. One AU is the distance between Earth and Sun and 40 AU corresponds to a distance between the orbits of Saturn and Uranus. The star Mira (Latin for the 'wonderful') is a pulsating variable star with a period of about 1 year. The pulsation leads to mass loss about 1000 times stronger than the solar wind, which forms the low-density shell of gas and dust. Mira's light and radio emission needs about 300 years to reach Earth.

The authors of the study analyzed radio interferometer images of the OH maser flare, as shown in the figure. The images cover a region of the size of the solar system, and have an extremely high resolution, which corresponds to a size of ~4 AU at the distance of Mira. The interferometer takes images at different velocities, here between 46 and 48 km/s. The spread of the maser in velocity is then used to construct a 3D model of the emission region. Mira has a companion star called "Mira B" with a separation of 70 AU only and possibly disturbs the emission regions. There was however no clear evidence found that the maser flares are triggered by Mira B, instead the flares may arise at random on the time scale of many years.

S. Etoka, E. Gerard, A.M.S. Richards, D. Engels, J. Brand, T. Le Bertre, 2017,  Recurring OH Flares towards o Ceti: I. location and structure of the 1990s’ and 2010s’ events