2016MNRAS.463L..74K

We report the serendipitous detection with the Atacama Large Millimeter/submillimeter Array (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 π¹ 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 on to 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.