ATOMIUM: Probing the inner wind of evolved O-rich stars with new, highly excited H2O and OH lines

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 & GSIM;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 Lambda-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 similar to 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 similar to 4000-9000 K were observed in vibrational states up to (& upsilon;(1),& upsilon;(2),& upsilon;(3)) = (0,1,1). All but one are new detections in space, and from these we have derived accurate rest frequencies. Hyperfine split Lambda-doubling transitions in & upsilon; = 0, J = 27/2 and 29/2 levels of the (2)& pi;(3/2) state, as well as J = 33/2 and 35/2 of the (2)& pi;(1/2) state of OH with excitation energies of similar to 4780-8900 K were also observed. Four of these transitions are new detections in space. Combining our measurements with earlier observations of OH, the & upsilon; = 0 and & upsilon; = 1 Lambda-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 & upsilon;(2) = 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 & upsilon;(2) = 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 similar to(0.7-2.8) x 10(-2).

Automated summary

ALMA can observe highly excited rotational and vibrational transitions of water and hydroxyl radical near stars, and map their distributions. It was found that water shows compact emission towards the central stars and extends up to twelve stellar radii or more in AGB and RSG stars. The emission and absorption features of hydroxyl radical reveal infall of matter and complex kinematics influenced by binarity.