Water content and wind acceleration in the envelope around the oxygen-rich AGB star IK Tau as seen by Herschel/HIFI

TL;DR

This study uses Herschel/HIFI observations to detect water and other molecules around the AGB star IK Tau, revealing insights into its wind acceleration and chemical composition.

Contribution

First detection of multiple water isotopologues in IK Tau, showing water forms in thermodynamical equilibrium without non-equilibrium processes.

Findings

Water abundance is 6.6x10^{-5} relative to H_2.

Ortho-to-para ratio of water is 3:1.

Wind acceleration in the inner envelope is slower than previously thought.

Abstract

During their asymptotic giant branch, evolution low-mass stars lose a significant fraction of their mass through an intense wind, enriching the interstellar medium with products of nucleosynthesis. We observed the nearby oxygen-rich asymptotic giant branch star IK Tau using the high-resolution HIFI spectrometer onboard Herschel. We report on the first detection of H_2^{16}O and the rarer isotopologues H_2^{17}O and H_2^{18}O in both the ortho and para states. We deduce a total water content (relative to molecular hydrogen) of 6.6x10^{-5}, and an ortho-to-para ratio of 3:1. These results are consistent with the formation of H_2O in thermodynamical chemical equilibrium at photospheric temperatures, and does not require pulsationally induced non-equilibrium chemistry, vaporization of icy bodies or grain surface reactions. High-excitation lines of 12CO, 13CO, 28SiO, 29SiO, 30SiO, HCN, and SO have also been detected. From the observed line widths, the acceleration region in the inner wind zone can be characterized, and we show that the wind acceleration is slower than hitherto anticipated.