The variable mass loss of the AGB star WX Piscium as traced by the CO J = 1-0 through 7-6 lines and the dust emission

Abstract

Context. Low and intermediate mass stars lose a significant fraction of their mass through a dust-driven wind during the Asymptotic Giant Branch (AGB) phase. Recent studies show that winds from late-type stars are far from being smooth. Mass-loss variations occur on different time scales, from years to tens of thousands of years. The variations appear to be particularly prominent towards the end of the AGB evolution. The occurrence, amplitude and time scale of these variations are still not well understood. Aims. The goal of our study is to gain insight into the structure of the circumstellar envelope (CSE) of WX Psc and map the possible variability of the late-AGB mass-loss phenomenon. Methods. We have performed an in-depth analysis of the extreme infrared AGB star WX Psc by modeling (1) the CO J = 1−0 through 7−6 rotational line profiles and the full spectral energy distribution (SED) ranging from 0.7 to 1300 µm. We hence are able to trace a geometrically extended region of the CSE. Results. Both mass-loss diagnostics bear evidence of the occurrence of mass-loss modulations during the last ∼2000 yr. In particular, WX Psc went through a high mass-loss phase ( u M ∼ 5 × 10 −5 M� /yr) some 800 yr ago. This phase lasted about 600 yr and was followed by a long period of low mass loss ( u M ∼ 5 × 10 −8 M� /yr). The present day mass-loss rate is estimated to be ∼6 × 10 −6 M� /yr. Conclusions. The AGB star WX Psc has undergone strong mass-loss rate variability on a time scale of several hundred years during the last few thousand years. These variations are traced in the strength and profile of the CO rotational lines and in the SED. We have consistently simulated the behaviour of both tracers using radiative transfer codes that allow for non-constant mass-loss rates.