Herschel observations of extreme OH/IR stars - the isotopic ratios of oxygen as a sign-post for the stellar mass

TL;DR

This study uses Herschel observations of heavily reddened AGB stars to analyze water isotopologues, revealing that the stars are likely intermediate-mass with masses above 5 solar masses, which impacts galactic chemical evolution.

Contribution

It introduces a method to estimate stellar mass lower limits using water isotopologue ratios in Herschel spectra of obscured AGB stars.

Findings

Detected 16O and 17O water isotopologues in the stars.

Absence of H2^{18}O lines supports hot-bottom burning scenario.

Stars are inferred to have masses >= 5 solar masses.

Abstract

Aim: The late stages of stellar evolution are mainly governed by the mass of the stars. Low- and intermediate-mass stars lose copious amounts of mass during the asymptotic giant branch (AGB) which obscure the central star making it difficult to study the stellar spectra and determine the stellar mass. In this study, we present observational data that can be used to determine lower limits to the stellar mass. Method: Spectra of nine heavily reddened AGB stars taken by the Herschel Space Observatory display numerous molecular emission lines. The strongest emission lines are due to H2O. We search for the presence of isotopologues of H2O in these objects. Result: We detected the 16O and 17O isotopologues of water in these stars, but lines due to H2^{18}O are absent. The lack of 18O is predicted by a scenario where the star has undergone hot-bottom burning which preferentially destroys 18O relative to 16O and 17O. From stellar evolution calculations, this process is thought to occur when the stellar mass is above 5 Msun for solar metallicity. Hence, observations of different isotopologues of H2O can be used to help determine the lower limit to the initial stellar mass. Conclusion: From our observations, we deduce that these extreme OH/IR stars are intermediate-mass stars with masses of >= 5 Msun. Their high mass-loss rates of ~ 1.0e-4 Msun/yr may affect the enrichment of the interstellar medium and the overall chemical evolution of our Galaxy.