CO observations in rotating circumbinary post-AGB disks

TL;DR

This paper addresses the molecular gas content in circumbinary disks around binary post-AGB stars, a class that exhibits rotating disks and extended outflows. The authors perform a CO survey with IRAM-30m and Yebes-40m, derive nebular masses from CO line areas under optically thin assumptions, and treat non-detections with censored statistics, revealing a wide mass distribution with a typical mass near $2\times10^{-3}$ M$_\odot$. The results show six detections out of ten new targets and, when combined with previous work, demonstrate a mass range spanning $10^{-4}$ to $10^{-1}$ M$_\odot$ and a high-mass tail, indicating diverse disk/outflow configurations and evolutionary states. The study also finds no evidence for molecular species beyond CO in these systems, suggesting pervasive molecular paucity possibly due to photodissociation or outflow-driven gas depletion, and it establishes a robust statistical framework (log-normal with censored data) to characterize the underlying mass distribution of circumbinary-pPNe disks. Together, these findings provide a valuable census of the molecular gas budget in circumbinary disks and set the stage for interferometric follow-up to separate disk and outflow contributions and to search for additional molecular tracers.

Abstract

There is a group of post-AGB stars that are part of a binary system and that show a significant NIR excess. These systems are known to host disks with Keplerian or quasi-Keplerian dynamics and to drive outflows 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. We present the survey of such sources observed in CO using the IRAM-30m 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. We present high-sensitivity mm-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. 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 1E-4 to 1E-1 Mo. 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 2E-3 Mo.

Figures (9)

• Figure 1: Newly detected $^{12}$CO $J=$ 2 -- 1 line profile in HD 213985. The x axis indicates velocity with respect to $V_{\text{LSR}}$ and the y axis represents the detected flux measured in janskys.
• Figure 2: Newly detected $^{12}$CO $J=$ 2 -- 1 line profile in RV Tau. Axes as in Fig. \ref{['fig:213985_co']}.
• Figure 3: Newly detected CO line profiles in HD 52961. Axes as in Fig. \ref{['fig:213985_co']}.
• Figure 4: Newly detected $^{12}$CO $J=$ 2 -- 1 line profile in U Mon. Axes as in Fig. \ref{['fig:213985_co']}.
• Figure 5: Newly detected CO line profiles in HR 4049. Axes as in Fig. \ref{['fig:213985_co']}.