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

Abstract

Context. High angular resolution (sub)millimetre observations of asymptotic giant branch (AGB) stars, now possible with the Atacama Large Millimeter/submillimeter Array (ALMA), allow direct imaging of these objects' photospheres. The physical properties of the molecular material around these regions, which until now has only been studied by imaging of maser emission and spatially unresolved absorption spectroscopy, can be probed with radiative transfer modelling and compared to hydrodynamical model predictions. The prototypical Mira variable, o Cet (Mira), was observed as a Science Verification target in the 2014 ALMA Long Baseline Campaign, offering the first opportunity to study these physical conditions in detail. Aims. With the longest baseline of 15 km, ALMA produces clearly resolved images of the continuum and molecular line emission/absorption at an angular resolution of ∼30 mas at 220 GHz. Models are constructed for Mira's extended atmosphere to investigate the physics and molecular abundances therein. Methods. We imaged the data of 28 SiO = 0, 2 J = 5−4 and H 2 O v 2 = 1 J Ka,Kc = 5 5,0 −6 4,3 transitions and extracted spectra from various lines of sight towards Mira's extended atmosphere. In the course of imaging the emission/absorption, we encountered ambiguities in the resulting images and spectra that appear to be related to the performance of the CLEAN algorithm when applied to a combination of extended emission, and compact emission and absorption. We addressed these issues by a series of tests and simulations. We derived the gas density, kinetic temperature, molecular abundance, and outflow/infall velocities in Mira's extended atmosphere by modelling the SiO and H 2 O lines. Results. We resolve Mira's millimetre continuum emission and our data are consistent with a radio photosphere with a brightness temperature of 2611 ± 51 K. In agreement with recent results obtained with the Very Large Array, we do not confirm the existence of a compact region (<5 mas) of enhanced brightness. Our modelling shows that SiO gas starts to deplete beyond 4 R and at a kinetic temperature of 600 K. The inner dust shells are probably composed of grain types other than pure silicates. During this ALMA observation, Mira's atmosphere generally exhibited infall motion with a shock front of velocity 12 km s −1 outside the radio photosphere. Despite the chaotic nature of Mira's atmosphere, the structures predicted by the hydrodynamical model, codex, can reproduce the observed spectra in astonishing detail, while some other models fail when confronted with the new data. Conclusions. For the first time, millimetre-wavelength molecular absorption against the stellar continuum has been clearly imaged. Combined with radiative transfer modelling, the ALMA data successfully demonstrates the ability to reveal the physical conditions of the extended atmospheres and inner winds of AGB stars in unprecedented detail. Long-term monitoring of oxygen-rich evolved stars will be the key to understanding the unsolved problem of dust ...

The close circumstellar environment of Betelgeuse

“…2. It is interesting to note that the circumstellar emission of Betelgeuse exhibits a comparable structure to the ALMA long-baseline observations of the close environment of Mira reported by Wong et al (2016). Images made at lower resolution and better surface brightness sensitivity from the combined TE and TC data also show extended emission in different molecular emission lines; this will be discussed in a forthcoming paper.…”

Recurring OH Flares towards o Ceti: I. location and structure of the 1990s’ and 2010s’ events

“…Fig. 14) fall within a circle of radius 0.3 ′′ , comparable with the SiO emission presented in Wong et al (2016). Even the weak OH feature at 1 ′′ observed in 1995, seems to have a counterpart in the extended SiO emission.…”

“…Even the weak OH feature at 1 ′′ observed in 1995, seems to have a counterpart in the extended SiO emission. Also, we incidentally note that the excited H2O transition at 232.67 GHz, mapped by ALMA, with an excitation energy Eup/k ∼ 3462 K (Wong et al 2016), hence tracing the warm H2O layer, is confined within a much smaller radius of ≤ 0.1 ′′ from O Ceti than the OH flaring region.…”

“…Note that independent inference of the systemic velocity from the inner part of the CSE (few R * ) via SiO emission, adopted to be 45.7 ± 0.7 km s −1 (Cotton et al 2006) or more recently of 46.7 km s −1 by Wong et al (2016), based on complex asymmetrical profiles, produce a similar range for the stellar velocity of o Ceti.…”

An observational study of dust nucleation in Mira (o Ceti)

“…Most recently, also the millimeter photosphere of Mira has been resolved giving a radius of R mm,ph = 23 mas near visual phase 0.5 (i.e. when the atmosphere is more extended than at maximum) (Vlemmings et al 2015;Matthews et al 2015;Wong et al 2016). Reid & Menten (2007) measured a radius of the radio photosphere at 43 GHz of 26 mas at phase 0.05.…”