CO observations in rotating circumbinary post-AGB disks

¹ Observatorio Astronómico Nacional (OAN-IGN), Alfonso XII 3, 28014 Madrid, Spain
² Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium
³ Observatorio Astronómico Nacional (OAN-IGN), Apartado 112, 28803 Alcalá de Henares, Madrid, Spain
⁴ Centro de Desarrollos Tecnológicos, Observatorio de Yebes (IGN), 19141 Yebes, Guadalajara, Spain

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 16 July 2025
Accepted: 28 October 2025

Abstract

Context. There is a group of post-asymptotic giant branch (post-AGB) stars that are part of a binary system and that show a significant infrared excess. It is by now well established that these systems have disks around the stellar system with either Keplerian or quasi-Keplerian dynamics, and they have outflows consisting of gas escaping from the rotating disk. These binary post-AGB stars can be categorized into two subclasses depending on the predominance of specific kinematic components: disk-dominated and outflow-dominated sources.

Aims. We present the survey of such sources observed in CO using the IRAM-30 m telescope, in which we aim to identify the molecular gas in these circumbinary-disk-containing post-AGB nebulae. We aim to analyze the mass distribution of the disk objects studied in CO.

Methods. We present high-sensitivity millimeter-wave observations of the CO line emission from ten binary post-AGB stars. Using the derived formulation and observational data, we calculated the mass of the total gas in the CO-emitting regions of these nebulae. The logarithmic distribution of nebular masses was analyzed using a normal model that incorporates censored data, enabling a more comprehensive analysis and yielding more accurate and representative results.

Results. CO emission is detected in six post-AGB nebulae from our sample. We have significantly increased the sample of observed sources in CO lines. Some of these objects exhibit very weak molecular emission. Within the sample of disk-containing sources, the total gas mass spans a range of 10⁻⁴ to 10⁻¹ M_⊙. The results derived from this work, along with those from a previous CO single-dish survey and interferometric data, show that this class of binary post-AGB stars exhibits a large range of nebular masses (including both the rotating disk and the expanding component), with variations exceeding a factor of 600. The typical nebular mass of these objects, accounting for both detections and non-detections through censored data analysis, is approximately 2.0 × 10⁻³ M_⊙.