Extended rotating disks around post-AGB stars

Context. There is a group of binary post-AGB stars that show a conspicu ous NIR excess, usually assumed to arise from hot dust in very compact possibly rotating disks. These stars are sur rounded by significantly fainter nebulae than the standard, well studied protoplanetary and planetary nebulae (PPNe, PNe), and, at l east some of them, have significantly lower luminosities. Aims. To identify and study extended rotating disks around these s tars, shedding light on the role of disks in the formation and shaping of planetary nebulae. Methods. We present high-sensitivity mm-wave observations of CO lin es in 24 objects of this type. The resulting CO lines are compared with profiles expected to arise from rotating disks , both from theoretical and observational grounds. We deriv e s mple formulae that allow determining the mass of the CO-emitting gas and estimating its extent; the reliability and uncertai nty of the methods are widely discussed. Results. CO emission is detected in most observed sources and the line profiles show that the emissions very probably come from disks in rotation. We derive typical values of the disk mass b etween 10−3 and 10−2 M⊙, about two orders of magnitude smaller than the (total) masses of standard PPNe. The high-detection rate (upper limits being in fact no t very significant) clearly confirm that the NIR excess of these stars arises from compact disks in rot ation, very probably the inner parts of those found here. Low -velocity outflows are also found in about eight objects, with moderate expansion velocities of ∼ 10 km s−1, to be compared with the velocities of about 100 km s −1 often found instandard PPNe. Except for two sources with complex profiles, the outflo wing gas in our objects represents a minor nebular component. Our simple estimates of the disk typical sizes yields values ∼ 0.5 – 1 arcsec, i.e. between 5 10 and 3 10 cm. Estimates of the linear momenta carried by the outflows, w hich can only be performed in a few well studied objects, also yield moderate values, compared with the line ar momenta that can be released by the stellar radiation pres sure (contrary, again, to the case of the very massive and fast bipolar outflow s in standard PPNe, that are strongly overluminous). The mass and dynamics of nebulae around various classes of post-AGB star s differ very significantly, and we can expect the formation of PNe w ith very different properties.