How spherical stars evolve to produce highly aspherical planetary nebulae
Washington, Feb. 13 : Researchers have said that only "strongly interacting" binary stars - or a star and a massive planet - can feasibly give rise to these powerful jets.
Eric Blackman, professor of physics and astronomy at Rochester, and his student, Scott Lucchini, wanted to determine whether the binaries can be widely separated and weakly interacting, or whether they must be close and strongly interacting.
By studying the jets from pre-planetary and planetary nebulae, Blackman and Lucchini were able to connect the energy and momentum involved in the accretion process with that in the jets; the process of accretion is what in effect provides the fuel for these jets.
As mass is accreted into one of the disks it loses gravitational energy. This is then converted into the kinetic energy and momentum of the outflowing jets, which is the mass that is expelled at a certain speed. Blackman and Lucchini determined the minimum power and minimum mass flows that these accretion processes needed to produce to account for the properties of the observed jets.
They then compared the requirements to specific existing accretion models, which have predicted specific power and mass flow rates.
They found that only two types of accretion models, both of which involve the most strongly interacting binaries, could create these jetted pre-planetary nebulae. In the first type of model, the "Roche lobe overflow," the companions are so close that the AGB stellar envelope gets pulled into a disk around the companion. In the second type of models, or "common envelope" models, the companion is even closer and fully enters the envelope of the AGB star so that the two objects have a "common" envelope.
From within the common envelope, very high accretion rate disks can either form around the companion from the AGB star material, or the companion can be shredded into a disk around the AGB star core. Both of these scenarios could provide enough energy and momentum to produce the jets that have been observed.
The study has been published in the Monthly Notices of the Royal Astronomical Society.
--ANI (Posted on 13-02-2014)
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