Rings and filaments: The remarkable detached CO shell of U Antliae

TL;DR

This study uses high-resolution ALMA observations to analyze the morphology and physical properties of U Antliae's detached CO shell, revealing insights into its structure, age, and mass-loss evolution after a thermal pulse.

Contribution

First detailed spatial and physical characterization of U Antliae's detached shell using ALMA, constraining mass-loss rates and shell dynamics post-thermal pulse.

Findings

Detached shell is thin, spherical, with filamentary substructure.

Shell expansion age is approximately 2700 years.

Current mass-loss rate is about 4×10⁻⁸ M☉/yr, significantly lower than during shell formation.

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 years 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 $\approx$ 10$^{-5}$ $M_\odot$ 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$\times$10$^{-8}$ $M_\odot$ yr$^{-1}$, at present being consistent with evolutionary changes as predicted for the period between thermal pulses.