2021A&A...654A..18V

Context. Observation of CO emission around asymptotic giant branch (AGB) stars is the primary method to determine gas mass-loss rates. While radiative transfer models have shown that molecular levels of CO can become mildly inverted, causing maser emission, CO maser emission has yet to be confirmed observationally.
Aims. High-resolution observations of the CO emission around AGB stars now have the brightness temperature sensitivity to detect possible weak CO maser emission.
Methods. We used high angular resolution observations taken with the Atacama Large Millimeter/submillimeter Array (ALMA) to observe the small-scale structure of CO J = 3-2 emission around the oxygen-rich AGB star W Hya.
Results. We find CO maser emission amplifying the stellar continuum with an optical depth τ ≃-0.55. The maser predominantly amplifies the limb of the star because CO J = 3-2 absorption from the extended stellar atmosphere is strongest towards the centre of the star.
Conclusions. The CO maser velocity corresponds to a previously observed variable component of high-frequency H₂O masers and with the OH maser that was identified as the amplified stellar image. This implies that the maser originates beyond the acceleration region and constrains the velocity profile since we find the population inversion primarily in the inner circumstellar envelope. We find that inversion can be explained by the radiation field at 4.6 µm and that the existence of CO maser emission is consistent with the estimated mass-loss rates for W Hya. However, the pumping mechanism requires a complex interplay between absorption and emission lines in the extended atmosphere. Excess from dust in the circumstellar envelope of W Hya is not sufficient to contribute significantly to the required radiation field at 4.6 µm. The interplay between molecular lines that cause the pumping can be constrained by future multi-level CO observations.