Mass, linear momentum and kinetic energy of bipolar flows in protoplanetary nebulae

We have studied the CO emission from protoplanetary nebulae (PPNe). Our sam ple is composed of 37 objects and includes, we think, all well identified P PNe detected in CO, together with the two yellow hypergiants emitting in CO and one young PN. We present a summary of the existing CO data, including accurate new observations of the (CO)-C-12 and (CO)-C-13 J = 1-0 and J = 2- 1 lines in 16 objects. We identify in the nebulae a slowly expanding shell (represented in the spectra by a central core) and a fast out ow (correspon ding to the line wings), that in the well studied PPNe is known to be bipol ar. Excluding poor data, we end up with a sample of 32 sources (including t he 16 observed by us); fast flows are detected in 28 of these nebulae, bein g absent in only 4. We present a method to estimate from these data the mas s, "scalar" momentum and kinetic energy of the different components of the molecular outflows. We argue that the uncertainties of our method can hardl y lead to significant overestimates of these parameters, although underesti mates may be present in not well studied objects. The total nebular mass is often as high as similar to 1 M-., and the mass-loss rate, that (presumabl y during the last stages of the AGB phase) originated the nebula, had typic al values similar to 10(-4) M-. yr(-1). The momentum corresponding to this mass ejection process in most studied nebulae is accurately coincident with the maximum momentum that radiation pressure, acting through absorption by dust grains, is able to supply (under expected conditions). We estimate th at this high-efficiency process lasts about 1000-10 000 yr, after which the star has ejected a good fraction of its mass and the AGB phase ends. On th e other hand, the fast molecular outflows, that have probably been accelera ted by shock interaction with axial post-AGB jets, carry a significant frac tion of the nebular mass, with a very high momentum (in most cases between 10(37) and 10(40) g cm s(-1)) and very high kinetic energy (usually between 10(44) and 10(47) erg). In general, yellow hypergiants and post-AGB object s with low initial mass show nebular masses and momenta that are, respectiv ely, higher and lower than these values. We compare the momenta of the fast outflows with those that can be supplied by radiation pressure, taking int o account the expected short acceleration times and some effects that can i ncrease the momentum transfer. We find that in about 80% of PPNe, the fast molecular flows have too high momenta to be powered by radiation pressure. In some cases the momentum of the outflow is similar to 1000 larger than th at carried by radiation pressure; such high factors are difficult to explai n even under exceptional conditions. Wind interaction is the basic phenomen on in the PN shaping from the former AGB envelopes; we conclude that this i nteraction systematically takes place along a dominant direction and that t his process is not powered by radiation pressure. Due to the lack of theore tical studies, the possible momentum source remains a matter of speculation .