On expansion parallax distances for planetary nebulae

The distances to individual wind-driven bubbles such as Planetary Nebulae (PNe) can be determined using expansion parallaxes: the angular expansion velocity in the sky is compared to the radial velocity of gas measured spectroscopically. Since the one is a pattern velocity, and the other a matter velocity, these are not necessarily the same. Using the jump conditions for both shocks and ionization fronts, I show that for typical PNe the pattern velocity is 20 to 30% larger than the material velocity, and the derived distances are therefore typically 20 to 30% too low. I present some corrected distances and suggest approaches to be used when deriving distances using expansion parallaxesComment: 7 pages, 6 figures, accepted for publication in A&

Where's the Doughnut? LBV bubbles and Aspherical Fast Winds

In this paper we address the issue of the origin of LBV bipolar bubbles. Previous studies have explained the shapes of LBV nebulae, such as $\eta$ Car, by invoking the interaction of an isotropic fast wind with a previously deposited, slow aspherical wind (a ``slow torus''). In this paper we focus on the opposite scenario where an aspherical fast wind expands into a previously deposited isotropic slow wind. Using high resolution hydrodynamic simulations, which include the effects of radiative cooling, we have completed a series of numerical experiments to test if and how aspherical fast winds effect wind blown bubble morphologies. Our experiments explore a variety of models for the latitudinal variations of fast wind flow parameters. The simulations demonstrate that aspherical fast winds can produce strongly bipolar outflows. In addition the properties of outflows recover some important aspects of LBV bubbles which the previous "slow torus" models can not.Comment: 23 pages, 6 figures, to appear the Astrophysical Journa

The post-AGB evolution of AGB mass loss variations

We present new numerical hydrodynamical modelling of the evolution of Asymptotic Giant Branch (AGB) mass loss fluctuations during the post-AGB/Planetary Nebula phase. These models show that after ionization, the observable effects of the mass loss fluctuations disappear in a few thousand years, consistent with the fact that only few PNe have been found to be surrounded by `rings'. We derive the observational characteristics of these rings, and compare them to reported observations of the rings around NGC 6543, finding a good match of emission properties and line shapes. We predict small variations in the observable electron temperatures.Comment: 12 pages, 13 figures, accepted by A&

Stellar wind bubbles around WR and [WR] stars

We study the dynamics of stellar wind bubbles around hydrogen-deficient stars using numerical simulations with time- and ion dependent cooling. We consider two types of hydrogen-deficient stars, massive WR stars, producing Ring Nebulae, and low mass [WR] stars, producing Planetary Nebulae. We show that for the Planetary Nebulae, the different cooling properties of the hydrogen-deficient wind lead to a later transition from momentum- to energy-driven flow, which could explain the observed turbulence of these nebulae. We find that Ring Nebulae should all be energy-driven, and show how comparing the bubble's momentum and kinetic energy to the input wind momentum and kinetic energy, can give misleading information about the dynamics of the bubble.Comment: 9 pages, 3 figures, to be published in A&

Growing and moving low-mass planets in non-isothermal disks

We study the interaction of a low-mass planet with a protoplanetary disk with a realistic treatment of the energy balance by doing radiation-hydrodynamical simulations. We look at accretion and migration rates and compare them to isothermal studies. We used a three-dimensional version of the hydrodynamical method RODEO, together with radiative transport in the flux-limited diffusion approach. The accretion rate, as well as the torque on the planet, depend critically on the ability of the disk to cool efficiently. For densities appropriate to 5 AU in the solar nebula, the accretion rate drops by more than an order of magnitude compared to isothermal models, while at the same time the torque on the planet is positive, indicating outward migration. It is necessary to lower the density by a factor of 2 to recover inward migration and more than 2 orders of magnitude to recover the usual Type I migration. The torque appears to be proportional to the radial entropy gradient in the unperturbed disk. These findings are critical for the survival of protoplanets, and they should ultimately find their way into population synthesis models.Comment: Accepted for publication in Astronomy and Astrophysic

Hydrodynamical Models of Outflow Collimation in YSOs

We explore the physics of time-dependent hydrodynamic collimation of jets from Young Stellar Objects (YSOs). Using parameters appropriate to YSOs we have carried out high resolution hydrodynamic simulations modeling the interaction of a central wind with an environment characterized by a moderate opening angle toroidal density distribution. The results show that the the wind/environment interaction produces strongly collimated supersonic jets. The jet is composed of shocked wind gas. Using analytical models of wind blown bubble evolution we show that the scenario studied here should be applicable to YSOs and can, in principle, initiate collimation on the correct scales (R ~ 100 AU). The simulations reveal a number of time-dependent non-linear features not anticipated in previous analytical studies including: a prolate wind shock; a chimney of cold swept-up ambient material dragged into the bubble cavity; a plug of dense material between the jet and bow shocks. We find that the collimation of the jet occurs through both de Laval nozzles and focusing of the wind via the prolate wind shock. Using an analytical model for shock focusing we demonstrate that a prolate wind shock can, by itself, produce highly collimated supersonic jets.Comment: Accepted by ApJ, 31 pages with 12 figures (3 JPEG's) now included, using aasms.sty, Also available in postscript via a gzipped tar file at ftp://s1.msi.umn.edu/pub/afrank/SFIC1/SFIC.tar.g

Interaction of Infall and Winds in Young Stellar Objects

The interaction of a stellar or disk wind with a collapsing environment holds promise for explaining a variety of outflow phenomena observed around young stars. In this paper we present the first simulations of these interactions. The focus here is on exploring how ram pressure balance between wind and ambient gas and post-shock cooling affects the shape of the resulting outflows. In our models we explore the role of ram pressure and cooling by holding the wind speed constant and adjusting the ratio of the inflow mass flux to the wind mass flux (Mdot_a/Mdot_w) Assuming non-spherical cloud collapse, we find that relatively strong winds can carve out wide, conical outflow cavities and that relatively weak winds can be strongly collimated into jet-like structures. If the winds become weak enough, they can be cut off entirely by the infalling environment. We identify discrepancies between results from standard snowplow models and those presented here that have important implications for molecular outflows. We also present mass vs. velocity curves for comparison with observations.Comment: 35 pages, 11 figures (PNG and EPS

Numerical Simulations of HH 555

We present 3D gasdynamic simulations of the Herbig Haro object HH 555. HH 555 is a bipolar jet emerging from the tip of an elephant trunk entering the Pelican Nebula from the adjacent molecular cloud. Both beams of HH 555 are curved away from the center of the H II region. This indicates that they are being deflected by a side-wind probably coming from a star located inside the nebula or by the expansion of the nebula itself. HH 555 is most likely an irradiated jet emerging from a highly embedded protostar, which has not yet been detected. In our simulations we vary the incident photon flux, which in one of our models is equal to the flux coming from a star 1 pc away emitting 5x10^48 ionizing (i. e., with energies above the H Lyman limit) photons per second. An external, plane-parallel flow (a ``side-wind'') is coming from the same direction as the photoionizing flux. We have made four simulations, decreasing the photon flux by a factor of 10 in each simulation. We discuss the properties of the flow and we compute Halpha emission maps (integrated along lines of sight). We show that the level of the incident photon flux has an important influence on the shape and visibility of the jet. If the flux is very high, it causes a strong evaporation of the neutral clump, producing a photoevaporated wind traveling in the direction opposite to the incident flow. The interaction of the two flows creates a double shock ``working surface'' around the clump protecting it and the jet from the external flow. The jet only starts to curve when it penetrates through the working surface.Comment: 14 pages, 4 figures, accepted by Ap