The Morphology of Planetary Nebulae: Simulations with Time-Evolving Winds -Vikram V. Dwarkadas & Bruce Balick.
- 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 Plkanetary 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.
Other Links and Sources of Information
- If you are curious about the objects that prompted the planetary
nebula simulations, take a look at Bruce Balick's
homepage. He's got some beautiful images of PNe.
- Other HST images of PNe can also be found on Robin Ciardullo's
homepage.
- STScI has now set up a nice web page for planetary nebulae, make
sure you take a peek there.
Return to Research Page or
Home page
Vikram
Dwarkadas - vikram_at_oddjob.uchicago.edu