Rings and filaments: The remarkable detached CO shell of U Antliae^{⋆,⋆⋆,⋆⋆⋆}

¹ Department of Astrophysics, University of Vienna, Türkenschanzstr. 17, 1180 Vienna, Austria
e-mail: franz.kerschbaum@univie.ac.at
² Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
³ Koninklijke Sterrenwacht van België, Ringlaan 3, 1180 Brussels, Belgium
⁴ Laboratoire Lagrange, Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Bd de l’Observatoire, CS 34229, 06304 Nice Cedex 4, France
⁵ South African Astronomical Observatory, PO Box 9, 7935 Observatory, South Africa
⁶ Astronomy Department, University of Cape Town, 7701, Rondebosch, South Africa
⁷ Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles, Campus Plaine, Boulevard du Triomphe, 1050 Brussels, Belgium
⁸ Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden
⁹ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
¹⁰ National Institute for Theoretical Physics, Private Bag X1, 7602 Matieland, South Africa

Received: 20 February 2017
Accepted: 3 August 2017

Abstract

Aims. Our goal is to characterize the intermediate age, detached shell carbon star U Antliae morphologically and physically in order to study the mass-loss evolution after a possible thermal pulse.

Methods. High spatial resolution ALMA observations of unprecedented quality in thermal CO lines allow us to derive first critical spatial and temporal scales and constrain modeling efforts to estimate mass-loss rates for both the present day as well as the ejection period of the detached shell.

Results. The detached shell is remarkably thin, overall spherically symmetric, and shows a barely resolved filamentary substructure possibly caused by instabilities in the interaction zone of winds with different outflow velocities. The expansion age of the detached shell is of the order of 2700 yr and its overall width indicates a high expansion-velocity and high mass-loss period of only a few hundred years at an average mass-loss rate of ≈10^-5 M_⊙ yr^-1. The post-high-mass-loss-rate-epoch evolution of U Ant shows a significant decline to a substantially lower gas expansion velocity and a mass-loss rate amounting to 4 × 10^-8 M_⊙ yr^-1, at present being consistent with evolutionary changes as predicted for the period between thermal pulses.