ALMA observations of the vibrationally-excited rotational CO transition $v=1, J=3-2$ towards five AGB stars

TL;DR

This study reports ALMA detections of vibrationally-excited CO lines in five AGB stars, revealing details about the warm molecular layer and its dynamics, including infall and variability, opening new avenues for studying stellar envelopes.

Contribution

First detection of vibrationally-excited rotational CO lines in multiple AGB stars, providing insights into the inner circumstellar regions with a simple modeling approach.

Findings

Detected vibrationally-excited CO lines in five AGB stars.

Observed inverse P-Cygni profiles indicating infall.

Found variability in line profiles for some stars.

Abstract

We report the serendipitous detection with ALMA of the vibrationally-excited pure-rotational CO transition $v=1, J=3-2$ towards five asymptotic giant branch (AGB) stars, $o$ Cet, R Aqr, R Scl, W Aql, and $\pi^1$ Gru. The observed lines are formed in the poorly-understood region located between the stellar surface and the region where the wind starts, the so-called warm molecular layer. We successfully reproduce the observed lines profiles using a simple model. We constrain the extents, densities, and kinematics of the region where the lines are produced. R Aqr and R Scl show inverse P-Cygni line profiles which indicate infall of material onto the stars. The line profiles of $o$ Cet and R Scl show variability. The serendipitous detection towards these five sources shows that vibrationally-excited rotational lines can be observed towards a large number of nearby AGB stars using ALMA. This opens a new possibility for the study of the innermost regions of AGB circumstellar envelopes.