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We report the detection and investigate the properties of high-excitationlambda-doubling line emission of hydroxyl (OH) detected towards threeasymptotic giant branch (AGB) stars (W Hya, R Dor, and IK Tau) using ALMA. TheOH lines are produced very close to the central stars and seem optically thinand with no maser effect. We analyse the molecular excitation using populationdiagrams and find rotational temperatures of $\sim 2500$ K and column densitiesof $\sim 10^{19}$ cm$^{-2}$ for both W Hya and R Dor. For W Hya, we observeemission from vibrationally excited H2O arising from the same region as the OHemission. Moreover, CO $v = 1; J = 3 - 2$ emission also shows a brightness peakin the same region. Considering optically thin emission and the rotationaltemperature derived for OH, we find a CO column density $\sim 15$ times higherthan that of OH, within an area of ($92 \times 84$) mas$^2$ centred on the OHemission peak. These results should be considered tentative because of thesimple methods employed. The observed OH line frequencies differ significantlyfrom the predicted transition frequencies in the literature, and provide thepossibility of using OH lines observed in AGB stars to improve the accuracy ofthe Hamiltonian used for the OH molecule. We predict stronger OHlambda-doubling lines at millimetre wavelengths than those we detected. Theselines will be a good probe of shocked gas in the extended atmosphere and arepossibly even suitable as probes of the magnetic field in the atmospheres ofclose-by AGB stars through the Zeeman effect.

Detection of highly excited OH towards AGB stars: A new probe of shocked gas in the extended atmospheres