X Her and TX Psc: Two cases of ISM interaction with stellar winds observed by Herschel

TL;DR

This study uses Herschel observations to analyze how the stellar winds of AGB stars X Her and TX Psc interact with the interstellar medium, revealing complex structures shaped by wind-ISM interactions.

Contribution

It provides detailed imaging evidence of wind-ISM interaction structures around X Her and TX Psc, highlighting the influence of relative stellar and ISM velocities.

Findings

Detection of axisymmetric extended structures aligned with star motion

Identification of upstream compact blobs possibly due to instabilities

Observation of circular rings indicating termination shocks

Abstract

The asymptotic giant branch (AGB) stars X Her and TX Psc have been imaged at 70 and 160 microns with the PACS instrument onboard the Herschel satellite, as part of the large MESS (Mass loss of Evolved StarS) Guaranteed Time Key Program. The images reveal an axisymmetric extended structure with its axis oriented along the space motion of the stars. This extended structure is very likely to be shaped by the interaction of the wind ejected by the AGB star with the surrounding interstellar medium (ISM). As predicted by numerical simulations, the detailed structure of the wind-ISM interface depends upon the relative velocity between star+wind and the ISM, which is large for these two stars (108 and 55 km/s for X Her and TX Psc, respectively). In both cases, there is a compact blob upstream whose origin is not fully elucidated, but that could be the signature of some instability in the wind-ISM shock. Deconvolved images of X Her and TX Psc reveal several discrete structures along the outermost filaments, which could be Kelvin-Helmholtz vortices. Finally, TX Psc is surrounded by an almost circular ring (the signature of the termination shock?) that contrasts with the outer, more structured filaments. A similar inner circular structure seems to be present in X Her as well, albeit less clearly.