# Water isotopologues in the circumstellar envelopes of M-type AGB stars

## TL;DR

This study uses radiative transfer modeling of water isotopologue emission lines to determine oxygen isotope ratios in M-type AGB stars, constraining their initial masses and stellar evolution.

## Contribution

It provides the first detailed isotopologue abundance analysis for a sample of M-type AGB stars using optically thick water lines and Herschel data.

## Key findings

- Low $^{17}$O/$^{18}$O ratios indicate low initial stellar masses (~1.0-1.5 M$_\\odot$).
- Ortho-to-para ratios are close to 3, consistent with warm formation.
- Results align with stellar evolution models predicting low-mass AGB stars.

## Abstract

AIM: In this study we examine rotational emission lines of two isotopologues of water: H$_2$$^{17}$O and H$_2$$^{18}$O. By determining the abundances of these molecules, we aim to use the derived isotopologue --- and hence oxygen isotope --- ratios to put constraints on the masses of a sample of M-type AGB stars that have not been classified as OH/IR stars. METHODS: We use detailed radiative transfer analysis based on the accelerated lambda iteration method to model the circumstellar molecular line emission of H$_2$$^{17}$O and H$_2$$^{18}$O for IK Tau, R Dor, W Hya, and R Cas. The emission lines used to constrain our models come from Herschel/HIFI and Herschel/PACS observations and are all optically thick, meaning that full radiative transfer analysis is the only viable method of estimating molecular abundance ratios. RESULTS: We find generally low values of the $^{17}$O/$^{18}$O ratio for our sample, ranging from 0.15 to 0.69. This correlates with relatively low initial masses, in the range $\sim1.0$ to 1.5 M$_\odot$ for each source, based on stellar evolutionary models. We also find ortho-to-para ratios close to 3, which are expected from warm formation predictions. CONCLUSIONS: The $^{17}$O/$^{18}$O ratios found for this sample are at the lower end of the range predicted by stellar evolutionary models, indicating that the sample chosen had relatively low initial masses.