X Herculis and TX Piscium: two cases of ISM interaction with stellar winds observed by Herschel^⋆

¹ Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles, CP 226, Boulevard du Triomphe, 1050 Bruxelles, Belgium
e-mail: svaneck@astro.ulb.ac.be; ajorisse@astro.ulb.ac.be
² University of Vienna, Department of Astronomy, Türkenschanzstraße 17, 1180 Wien, Austria
³ Dept. of Physics and Astronomy, University of Denver, Mail Stop 6900, Denver, CO 80208, USA
⁴ Onsala Space Observatory, 43992 Onsala, Sweden and Stockholm Observatory, AlbaNova University Centre, 10691 Stockholm, Sweden
⁵ Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan, 200D, 3001 Leuven, Belgium
⁶ Koninklijke Sterrenwacht van België, Ringlaan 3, 1180 Brussels, Belgium
⁷ National Radio Astronomy Observatory, PO Box 0, Socorro, NM 87801, USA
⁸ Institute of Computer Vision and Graphics, TU Graz, Infeldgasse 16/II, 8010 Graz, Austria

Received: 19 March 2011
Accepted: 24 May 2011

Abstract

The asymptotic giant branch (AGB) stars X Her and TX Psc have been imaged at 70 and 160 μm 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^-1 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.