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

TL;DR

This study reports the detection of high-excitation OH emission near AGB stars using ALMA, revealing insights into shocked gas and magnetic fields in their extended atmospheres.

Contribution

It presents the first detection and analysis of high-excitation OH lines close to AGB stars, suggesting their use as probes for shocked gas and magnetic fields.

Findings

OH lines are produced close to the stars and are optically thin.

Rotational temperatures of ~2500 K and high column densities were derived.

OH line frequencies differ from predictions, offering a way to refine molecular models.

Abstract

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