Water content and wind acceleration in the envelope around the oxygen-rich AGB star IK Tauri as seen byHerschel/HIFI

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 highresolution HIFI spectrometer onboard Herschel. We report on the first detection of H 16 2 O and the rarer isotopologues H 17 2 O and H 18 2 O in both the ortho and para states. We deduce a total water content (relative to molecular hydrogen) of 6.6 × 10 −5 , and an ortho-to-para ratio of 3:1. These results are consistent with the formation of H 2 O 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 12 CO, 13 CO, 28 SiO, 29 SiO, 30 SiO, 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.

A HIFI view on circumstellar H2O in M-type AGB stars: radiative transfer, velocity profiles, and H2O line cooling

“…The velocity profiles derived here and by Decin et al (2010c) are consistent with measurements of maser lines in the CSE of IK Tau (Bains et al 2003). The momentum equation for IK Tau on the other hand gives β = 0.6 (Decin et al 2010c). Khouri et al (2014a) adopt a β = 1.5 to describe the velocity profile in W Hya, and find evidence that higher values may fit the high-J transitions of CO better.…”

“…All measurements indicate that the wind follows a more shallow profile than generally predicted by dynamical models solving the coupled momentum equations of the dust and gas (e.g., Decin et al 2006;Ramstedt et al 2008;Decin et al 2010c). Note however, that Ramstedt et al (2008) derive a velocity profile for IK Tau that is consistent with the results by Bains et al (2003), showing that it is possible to derive velocity profiles from dynamical models that are consistent with the observed, shallow profiles.…”

“…For R Cas we derive β = 2.5, and for the remaining three sources β = 1.5. For IK Tau the momentum equation gives a steeper profile with β = 0.6 (Decin et al 2010c). Although comparatively shallow velocity profile seem to be preferred, the β is not very well constrained by the observed CO and H 2 O lines (Fig.…”

“…R Dor is a SRb variable, and the remaining sources are Mira-type variables. The HIFI observations of IK Tau were modelled by Decin et al (2010c) using a different radiative-transfer code. We include IK Tau here for a comparison of the modelling approaches.…”

“…We include IK Tau here for a comparison of the modelling approaches. The HIFI observations of W Hya were modelled in detail by Khouri et al (2014a,b) using the same code as Decin et al (2010c). Mira has a very complex circumstellar environment due to binary interaction (Ramstedt et al 2014), and the remaining sources are OH/IR stars with high mass-loss rates.…”