(Sub)millimeter emission lines of molecules in born-again stars^⋆

¹ Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
e-mail: daniel.tafoya@chalmers.se
² Institute of Astronomy and Astrophysics, Academia Sinica (ASIAA), 10617 Taipei, Taiwan, ROC
³ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁴ Instituto de Astronomía, Universidad Católica del Norte, Av. Angamos 0610, Casilla 1280 Antofagasta, Chile
⁵ Institut für Astro- und Teilchenphysik, Universität Innsbruck, Technikerstr. 25/8, 6020 Innsbruck, Austria
⁶ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK
⁷ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Cologne, Germany
⁸ Instituto de Ciencia de Materiales de Madrid, Sor Juana Inés de la Cruz 3, 28049 Cantoblanco, Madrid, Spain
⁹ Observatorio Astronómico Nacional, Apdo. 112, 28803 Alcalá de Henares Madrid, Spain

Received: 16 November 2016
Accepted: 20 December 2016

Abstract

Context. Born-again stars provide a unique possibility to study the evolution of the circumstellar envelope of evolved stars in human timescales. Up until now, most of the observations of the circumstellar material in these stars have been limited to studying the relatively hot gas and dust. In other evolved stars, the emission from rotational transitions of molecules, such as CO, is commonly used to study the cool component of their circumstellar envelopes. Thus, the detection and study of molecular gas in born-again stars is of great importance when attempting to understand their composition and chemical evolution. In addition, the molecular emission is an invaluable tool for exploring the physical conditions, kinematics, and formation of asymmetric structures in the circumstellar envelopes of these evolved stars. However, up until now, all attempts to detect molecular emission from the cool material around born-again stars have failed.

Aims. We searched for emission from rotational transitions of molecules in the hydrogen-deficient circumstellar envelopes of born-again stars to explore the chemical composition, kinematics, and physical parameters of the relatively cool gas.

Methods. We carried out observations using the APEX and IRAM 30 m telescopes to search for molecular emission toward four well-studied born-again stars, V4334 Sgr, V605 Aql, A30, and A78, that are thought to represent an evolutionary sequence.

Results. For the first time, we detected emission from HCN and H¹³CN molecules toward V4334 Sgr, and CO emission in V605 Aql. No molecular emission was detected above the noise level toward A30 and A78. The detected lines exhibit broad linewidths ≳150 km s^-1, which indicates that the emission comes from gas ejected during the born-again event, rather than from the old planetary nebula. A first estimate of the H¹²CN/H¹³CN abundance ratio in the circumstellar environment of V4334 Sgr is ≈3, which is similar to the value of the ¹²C/¹³C ratio measured from other observations. We derived a rotational temperature of T_rot = 13 ± 1 K, and a total column density of N_HCN = 1.6 ± 0.1 × 10¹⁶ cm^-2 for V4334 Sgr. This result sets a lower limit on the amount of hydrogen that was ejected into the wind during the born-again event of this source. For V605 Aql, we obtained a lower limit for the integrated line intensities I_¹²CO/I_¹³CO> 4.