2009ApJ...692.1382D

Circumbinary disks have been hypothesized to exist around a number of binary post-asymptotic giant branch (AGB) stars. Although most of the circumbinary disks have been inferred through the near-infrared (IR) excess, a few of them are strong emitters of molecular emission. Here we present high-angular resolution observations of the emission of ¹²CO and its isotopomer ¹³CO J = 2-1 line from the circumstellar envelope around the binary post-AGB star IRAS 08544-4431, which is one of the most prominent members of this class of objects. We find that the envelope is resolved in our observations and two separate components can be identified: (a) a central extended and strong component with very narrow linewidth between 2 and 6 km/s; (b) a weak bipolar outflow with expansion velocity up to 8 km/s. The central compact component possesses low and variable ¹²CO/¹³CO J = 2-1 line ratio, indicating optically thick emission of the main isotope. We estimate a molecular gas mass of 0.0047 M_☉ for this component based on the optically thinner ¹³CO J = 2-1 line. We discuss the relation of the molecular envelope and the circumbinary disk inferred from near-IR excess and compare with other known cases where the distribution of molecular gas has been imaged at high angular resolution.