Impulsive Ejection of Gas In Bipolar Planetary Nebulae

We simulate the formation of bipolar planetary nebulae (PNe) through very short impulsive mass ejection events from binary systems, where the asymptotic giant branch (AGB) star ejects a mass shell that is accelerated by jets launched from a compact companion. The acceleration process takes place at very short distances from the binary system, such that the photon-diffusion time is long enough to prevent rapid cooling of the shocked jets' material. When the shocked jets' gas density is lower than the shell density the flow becomes Rayleigh-Taylor unstable and dense clumps are formed in the flow. At later times a PN with clumpy lobes that have a linear distance-velocity relation will be observed. This process might account for the formation of bipolar PNe with clumpy lobes, such as NGC 6302. The energy radiated during the months to years duration of such an event will appear as an intermediate-luminosity optical transient (ILOT)Comment: Accepted for Publication in MNRA

Numerical simulations of wind-equatorial gas interaction in eta Carinae

We perform three-dimensional gas-dynamical simulations and show that the asymmetric morphology of the blue and red-shifted components of the outflow at hundreds of astronomical units (AU) from the massive binary system eta Carinae can be accounted for from the collision of the free primary stellar wind with the slowly expanding dense equatorial gas. Owing to the very complicated structure of the century-old equatorial ejecta, that is not fully spatially resolved by observations, we limit ourselves to modelling the equatorial dense gas by one or two dense spherical clouds. Because of that we reproduce the general qualitative properties of the velocity maps, but not the fine details. The fine details of the velocity maps can be matched by simply structuring the dense ejecta in an appropriate way. The blue and red-shifted components are formed in the post-shock flow of the primary wind, on the two sides of the equatorial plane, respectively. The fast wind from the secondary star plays no role in our model, as for most of the orbital period in our model the primary star is closer to us. The dense clouds are observed to be closer to us than the binary system is, and so in our model the primary star faces the dense equatorial ejecta for the majority of the orbital period.Comment: MNRAS, in pres

X-Ray Emission from Planetary Nebulae Calculated by 1D Spherical Numerical Simulations

We calculate the X-ray emission from both constant and time evolving shocked fast winds blown by the central stars of planetary nebulae (PNs) and compare with observations. Using spherically symmetric numerical simulations with radiative cooling, we calculate the flow structure, and the X-ray temperature and luminosity of the hot bubble formed by the shocked fast wind. We find that a constant fast wind gives results that are very close to those obtained from the self-similar solution. We show that in order for a fast shocked wind to explain the observed X-ray properties of PNs, rapid evolution of the wind is essential. More specifically, the mass loss rate of the fast wind should be high early on when the speed is ~300-700 km/s, and then it needs to drop drastically by the time the PN age reaches ~1000 yr. This implies that the central star has a very short pre-PN (post-AGB) phase.Comment: accepted to MNRA

Forming H-shaped and barrel-shaped nebulae with interacting jets

We conduct three-dimensional hydrodynamical simulations of two opposite jets launched from a binary stellar system into a previously ejected shell and show that the interaction can form barrel-like and H-like shapes in the descendant nebula. Such features are observed in planetary nebulae and supernova remnants. Under our assumption the dense shell is formed by a short instability phase of the giant star as it interacts with a stellar companion, and the jets are then launched by the companion as it accretes mass through an accretion disk from the giant star. We find that the H-shaped and barrel-shaped morphological features that the jets form evolve with time, and that there are complicated flow patterns, such as vortices, instabilities, and caps moving ahead along the symmetry axis. We compare our numerical results with images of 12 planetary nebulae, and show that jet-shell interaction that we simulate can account for the barrel-like or H-like morphologies that are observed in these PNe.Comment: 28 pages, 12 figures. Submitte

Accretion onto the Companion of Eta Carinae During the Spectroscopic Event: II. X-Ray Emission Cycle

We calculate the X-ray luminosity and light curve for the stellar binary system Eta Carinae for the entire orbital period of 5.54 years. By using a new approach we find, as suggested before, that the collision of the winds blown by the two stars can explain the X-ray emission and temporal behavior. Most X-ray emission in the 2-10 \kev band results from the shocked secondary stellar wind. The observed rise in X-ray luminosity just before minimum is due to increase in density and subsequent decrease in radiative cooling time of the shocked fast secondary wind. Absorption, particularly of the soft X-rays from the primary wind, increases as the system approaches periastron and the shocks are produced deep inside the primary wind. However, absorption can not account for the drastic X-ray minimum. The 70 day minimum is assumed to result from the collapse of the collision region of the two winds onto the secondary star. This process is assumed to shut down the secondary wind, hence the main X-ray source. We show that this assumption provides a phenomenological description of the X-ray behavior around the minimum.Comment: The Astrophysical Journal, in pres

A Model for the Formation of Large Circumbinary Disks Around Post AGB Stars

We propose that the large, radius of ~1000 AU, circumbinary rotating disks observed around some post-asymptotic giant branch (post-AGB) binary stars are formed from slow AGB wind material that is pushed back to the center of the nebula by wide jets. We perform 2D-axisymmetrical numerical simulations of fast and wide jets that interact with the previously ejected slow AGB wind. In each system there are two oppositely launched jets, but we use the symmetry of the problem and simulate only one jet. A large circularization-flow (vortex) is formed to the side of the jet which together with the thermal pressure of the shocked jet material accelerate cold slow-wind gas back to the center from distances of ~1000-10000 AU. We find for the parameters we use that up to 0.001 Mo is back-flowing to the center. We conjecture that the orbital angular momentum of the disk material results from the non-axisymmetric structure of jets launched by an orbiting companion. This conjecture will have to be tested with 3D numerical codes.Comment: New Astronomy, in pres