J. Koller

VORTICES IN THE CO-ORBITAL REGION OF AN EMBEDDED PROTOPLANET

We present global two-dimensional inviscid disk simulations with an embedded planet, emphasizing the nonlinear dynamics in its co-orbital region. We find that the potential vorticity of the flow in this region is not conserved because of the presence of two spiral shocks produced by the planet. As the system evolves, the potential vorticity profile develops extrema (inflection points) that eventually render the flow unstable. Vortices are produced in association with the potential vorticity minima. Born in the separatrix region, these vortices experience close encounters with the planet, consequently exerting strong torques on the planet. The existence of these vortices have important implications for understanding the migration rates of low-mass planets.

The Anomalous Infrared Emission of Abell 58

We present a new model to explain the excess in mid- and near-infrared emission of the central, hydrogen-poor dust knot in the planetary nebula (PN) Abell 58. Current models disagree with ISO measurements because they apply an average grain size and equilibrium conditions only. We investigate grain size distributions and temperature fluctuations affecting infrared emission using a new radiative transfer code and discuss in detail the conditions requiring an extension of the classical description. The peculiar infrared emission of V605 Aql, the central dust knot in Abell 58, has been modeled with our code. V605 Aql is of special interest as it is one of only three stars ever observed to move from the evolutionary track of a central PN star back to the post-asymptotic giant branch state.

The evolution of V4334 Sgr (Sakurai's object) in the Mid-Infrared

Since Sakurais object (V4334 Sgr) underwent a late helium flash in 1995, its fast evolution has been monitored with great eort. We present TIMMI and TIMMI 2 photometric observations covering a spectral range from 5 to 20m with a total of 8 filters. We found drastic changes in the spectral energy distribution indicating a drop of temperature in the dust envelope from 1000 to 600 K in half a year only. Combined with optical observations reported elsewhere, we conclude that the phase of dust formation has probably stopped in mid-2001 and that the dusty envelope has been expanding only, ever since. An expansion velocity of about 1000 km s 1 is required to explain the rapid cooling. Other optical and near-infrared observations support the existence of such high velocity flows in the envelope of V4334 Sgr. An estimate for the bolometric luminosity of 2000 L is derived for a distance of 1 kpc. Taking into account theoretical models a distance of 2 1 kpc to V4334 Sgr is suggested. As a consequence of its fast final helium flash evolution, Sakurais object may start to destroy its surrounding dust in the not too distant future.

Modeling the Dust Shell of V4334 Sgr

The central star V4334 Sgr (Sakurais Object) of the planetary nebula PN G010.4+04.4 underwent in 1995–1996 the rare event of a very late helium flash.It is only one of two such events during the era of modern astronomy (the other event was V605 Aql = Nova Aql 1919). All other prominentobjects of that type originate from events several thousands of years ago (e.g. A30, A78). Hence, only snapshots can be modeled for those objects.V4334 Sgr allows for the first time a dynamic consideration of the formation of the dust shell from the beginning.We present here a model which is able to describe the complete photometric behaviorof the object, including the fine structure dips of the optical light curve during the first two years of the mass loss and the dust formation.