Morphology and kinematics of the gas envelope of Mira Ceti

Abstract

Observations of 12CO(3-2) emission of the circumbinary envelope of Mira Ceti, made by Atacama Large Millimetre/sub-millimetre Array, are analysed. The observed Doppler velocity distribution is made of three components: a blueshifted south-eastern arc, which can be described as a ring in slow radial expansion, ∼1.7 km s-1, making an angle of ∼50° with the plane of the sky and born some 2000 years ago; a few arcs, probably born at the same epoch as the blueshifted arc, all sharing Doppler velocities redshifted by approximately 3±2 km s-1 with respect to the main star; thirdly, a central region dominated by the circumbinary envelope, displaying two outflows in the south-western and north-eastern hemispheres. At short distances from the star, up to ∼1.5 arcsec, these hemispheres display very different morphologies: the south-western outflow covers a broad solid angle, expands radially at a rate between 5 and 10 km s-1 and is slightly redshifted; the north-eastern outflow consists of two arms, both blueshifted, bracketing a broad dark regionwhere emission is suppressed. At distances between ∼1.5 and ∼2.5 arcsec the asymmetry between the two hemispheres is significantly smaller and detached arcs, particularly spectacular in the north-eastern hemisphere are present. Close to the stars, we observe a mass of gas surrounding Mira B, with a size of a few tens of au, and having Doppler velocities with respect to Mira B reaching ±1.5 km s-1, which we interpret as gas flowing from Mira A towards Mira B.