Publications and Simulations

The Shaping of Planetary Nebulae: Asymmetry in the External Wind

          - Vikram V Dwarkadas, Roger Chevalier and John Bondin 1996, ApJ, 457, 773

The Morphology of Planetary Nebulae: Simulations with Time-Evolving Winds -

Vikram V. Dwarkadas & Bruce Balick, 1998, ApJ, 497, 267

Abstract:We have carried out simulations of planetary nebulae (PNs) within the general framework of the interacting stellar winds models, wherein the fast wind from a central star sweeps up the slow wind emitted in a previous epoch. Our two-dimensional simulations take into account the evolution of the velocity and mass-loss rate of the fast wind. This leads to considerably more structure on smaller scales than was seen in those nebulae where the fast wind velocity is held constant. The nebula evolves through an initial momentum-conserving phase before entering the more commonly encountered energy-conserving stage. Both stages are prone to instabilities, which may be the precursor of knots, filaments, and other microstructures that are now commonly seen in Hubble Space Telescope images of PNs. In particular, we note the occurrence of the nonlinear thin shell instability in the early stages, and the formation of Rayleigh-Taylor filaments in the energy-conserving stage. The growth of small-scale structure in the momentum-conserving stage is sensitive to the ratio of the initial wind momenta; the lifetime of the nebula in this stage depends also on the evolution of the wind properties. The overall size scale of the system is determined mainly by the evolution of the fast wind properties. If the evolution is not taken into account when computing kinematic ages, then the ages may be underestimated.

Simulations: Movies of simulations described in this paper

Instabilities and the Formation of Small-Scale Structures in Planetary Nebulae

Vikram V. Dwarkadas, 2000, in Asymmetrical Planetary Nebulae II: From Origins to Microstructures, ASP Conference Series, Vol. 199. Edited by J. H. Kastner, N. Soker, and S. Rappaport, 2000, p. 379

Abstract: Recent observations have revealed the presence of a variety of small- scale "micro-structures" in Planetary Nebulae images. I discuss numerical hydrodynamics models of planetaries and outline the formation and growth of instabilities that may be seen in these models. It is possible that these may be the forerunners of some of the observed small-scale structures.

Radiatively Driven Winds and the Formation of Bipolar Planetary Nebulae

Vikram V. Dwarkadas & Stanley Owocki, IAU Symposium on Planetary Nebulae held in Canberra, Australia, Nov 2001

Abstract: We study the role of radiatively driven winds in the formation of PNe. The modulation of these winds due to stellar rotation, and Von Zeipel gravity darkening, are both considered. Rotation produces a wind that is faster at the poles but denser at the equator. The Von Zeipel effect, which states that the stellar flux is proportional to the effective surface gravity, results in the inverse density distribution, giving a wind that is both faster and denser at the poles. Both these effects produce bipolar nebulae for large rotational velocities of the star.

Stellar Rotation and the Formation of Asymmetric Nebulae

Vikram V. Dwarkadas, to appear in "Asymmetrical Planetary Nebulae III" editors M. Meixner, J. Kastner, N. Soker, & B. Balick (ASP Conf. Series), 2004

Abstract: We illustrate how rotation of the central star can give rise to latitudinal variations in the wind properties from the star. Interaction of these winds with the surrounding medium can produce asymmetrical planetary nebulae.