ALMA observations of the “fresh” carbon-rich AGB star TX Piscium

The discovery of an elliptical detached shell^⋆

¹ Department for Astrophysics, University of Vienna, Türkenschanzstrasse 17, 1180, Vienna
e-mail: magdalena.brunner@univie.ac.at
² Department of Space, Earth and Environment, Chalmers University of Technology, 43992 Onsala, Sweden
³ NASA Goddard Space Flight Center, Code 667, Greenbelt, MD 20771, USA
⁴ Department of Physics, The Catholic University of America, Washington DC 20064, USA

Received: 15 June 2018
Accepted: 29 October 2018

Abstract

Aims. The carbon-rich asymptotic giant branch (AGB) star TX Piscium (TX Psc) has been observed multiple times during multiple epochs and at different wavelengths and resolutions, showing a complex molecular CO line profile and a ring-like structure in thermal dust emission. We investigate the molecular counterpart in high resolution, aiming to resolve the ring-like structure and identify its origin.

Methods. Atacama Large Millimeter/submillimeter Array (ALMA) observations have been carried out to map the circumstellar envelope (CSE) of TX Psc in CO(2–1) emission and investigate the counterpart to the ring-like dust structure.

Results. We report the detection of a thin, irregular, and elliptical detached molecular shell around TX Psc, which coincides with the dust emission. This is the first discovery of a non-spherically symmetric detached shell, raising questions about the shaping of detached shells.

Conclusions. We investigate possible shaping mechanisms for elliptical detached shells and find that in the case of TX Psc, stellar rotation of 2 km s⁻¹ can lead to a non-uniform mass-loss rate and velocity distribution from stellar pole to equator, recreating the elliptical CSE. We discuss the possible scenarios for increased stellar momentum, enabling the rotation rates needed to reproduce the ellipticity of our observations, and come to the conclusion that momentum transfer of an orbiting object with the mass of a brown dwarf would be sufficient.