We study the final stages of the common envelope (CE) evolution and find that a substantial fraction of the ejected mass does not reach the escape ve

We propose that the energy source of the outburst of V838 Mon and similar objects is an accretion event, i.e., gravitational energy rather than thermonuclear runaway. We show that the merger of two main sequence stars, of masses 1.5 Mo and 0.1-0.5 Mo can account for the luminosity, large radius, and low effective temperture of V838 Mon and similar objects. Subsequent cooling and gravitational contraction lead such objects to move along the Hayashi limit, as observed. By varying the masses and types of the merging stars, and by considering slowly expanding, rather than hydrostatic, envelopes, this model can account for a large range in luminosities and radii of such outburst events.

As stars which have planetary systems evolve along the red giant branch and expand, they interact with the close planets. The planets deposit angular momentum and energy into the red giant stars' envelopes, both of which are likely to enhance mass loss on the red giant branch. The enhanced mass loss causes the star to become bluer as it turns to the horizontal branch. I propose that the presence of planetary systems, through this mechanism, can explain some anomalies in horizontal branch morphologies. In particular, planetary systems may be related to the ``second parameter'', which determines the distribution of horizontal branch stars on the Hertzsprung-Russel diagram. The proposed scenario predicts that surviving massive planets or brown dwarfs orbit many of the extreme blue horizontal branch stars, at orbital periods of tens days.Comment: 21 pages, preprint, uses aasms4.st

We discuss the interaction of the slow wind blown by an asymptotic giant branch (AGB) star with a collimated fast wind (CFW) blown by its main-sequence or white dwarf companion, at orbital separations in the range of several AU a 200 AU. The CFW results from accretion of the AGB wind into an accretion disk around the companion. The fast wind is collimated by the accretion disk. We argue that such systems are the progenitors of bipolar planetary nebulae and bipolar symbiotic nebulae with a very narrow equatorial waist between the two polar lobes. The CFW wind will form two lobes along the symmetry axis and will further compress the slow wind near the equatorial plane, leading to the formation of a dense slowly expanding ring. Therefore, contrary to the common claim that a dense equatorial ring collimates the bipolar flow, we argue that in the progenitors of very narrow waist bipolar planetary nebulae, the CFW, through its interaction with the slow wind, forms the dense equatorial ring. Only later in the evolution, and after the CFW and slow wind cease, does the mass-losing star leave the AGB and blow a second, more spherical, fast wind. At this stage the flow structure becomes the one that is commonly assumed for bipolar planetary nebulae, i.e., collimation of the fast wind by the dense equatorial material. However, this results in the broadening of the waist in the equatorial plane and cannot by itself account for the presence of very narrow waists or jets. We conduct a population synthesis study of the formation of planetary nebulae in wide binary systems which quantitatively supports the proposed model. The population synthesis code follows the evolution of both stars and their arbitrarily eccentric orbit, including mass loss via stellar winds, for 5 × 10⁴ primordial binaries. We show the number of expected systems that blow a CFW is in accord with the number found from observations, to within the many uncertainties involved. Overall, we find that ~5% of all planetary nebulae are bipolars with very narrow waists. Our population synthesis not only supports the CFW model but more generally supports the binary model for the formation of bipolar planetary nebulae.

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