2023A&A...674A.125B

Context. Water (H₂O) and the hydroxyl radical (OH) are major constituents of the envelope of O-rich late-type stars. Transitions involving energy levels that are rotationally or vibrationally highly excited (energies ≳4000 K) have been observed in both H₂O and OH. These and more recently discovered transitions can now be observed at a high sensitivity and angular resolution in the inner wind close to the stellar photosphere with the Atacama Large Millimeter/submillimeter Array (ALMA).
Aims. Our goals are: (1) to identify and map the emission and absorption of H₂O in several vibrational states, and of OH in Λ-doubling transitions with similar excitation energies; and (2) to determine the physical conditions and kinematics in gas layers close to the extended atmosphere in a sample of asymptotic giant branch stars (AGBs) and red supergiants (RSGs).
Methods. Spectra and maps of H₂O and OH lines observed in a 27 GHz aggregated bandwidth and with an angular resolution of ∼0."02-1."0 were obtained at two epochs with the main ALMA array. Additional observations with the Atacama Compact Array (ACA) were used to check for time variability of water transitions. Radiative transfer models of H₂O were revisited to characterize masing conditions. Up-to-date chemical models were used for comparison with the observed OH/H₂O abundance ratio.
Results. Ten rotational transitions of H₂O with excitation energies ∼4000-9000 K were observed in vibrational states up to (υ₁,υ₂,υ₃) = (0,1,1). All but one are new detections in space, and from these we have derived accurate rest frequencies. Hyperfine split Λ-doubling transitions in υ = 0, J = 27/2 and 29/2 levels of the ²Π_3/2 state, as well as J = 33/2 and 35/2 of the ²Π_1/2 state of OH with excitation energies of ∼4780-8900 K were also observed. Four of these transitions are new detections in space. Combining our measurements with earlier observations of OH, the υ = 0 and υ = 1 Λ-doubling frequencies have been improved. Our H₂O maps show compact emission toward the central star and extensions up to twelve stellar radii or more. The 268.149 GHz emission line of water in the υ₂ = 2 state is time variable, tends to be masing with dominant radiative pumping, and is widely excited in AGBs and RSGs. The widespread but weaker 262.898 GHz water line in the υ₂ = 1 state also shows signs of maser emission. The OH emission is weak and quasithermally excited. Emission and absorption features of H₂O and OH reveal an infall of matter and complex kinematics influenced by binarity. From the OH and H₂O column densities derived with nonmasing transitions in a few sources, we obtain OH/H₂O abundance ratios of ~(0.7-2.8) × 10^–2.