Garrelt Mellema

Evolution of clouds in radio galaxy cocoons

This letter presents a numerical study of the evolution of an emission line cloud of initial density 10 cm 3 , temper- ature 10 4 K, and size 200 pc, being overtaken by a strong shock wave. Whereas previous simple models proposed that such a cloud would either be completely destroyed, or simply shrink in size, our results show a dierent and more complex behaviour: due to rapid cooling, the cloud breaks up into many small and dense fragments, which can survive for a long time. We show that such rapid cooling behaviour is expected for a wide range of cloud and shock properties. This process applies to the evolution of emission line clouds being overtaken by the cocoon of a radio jet. The resulting small clouds would be Jeans unstable, and form stars. Our results thus give theoretical credibility to the process of jet induced star formation, one of the explanations for the alignment of the optical/UV and radio axis observed in high redshift radio galaxies.

Planetesimal and gas dynamics in binaries

Observations of extrasolar planets reveal that planets can be found in close binary systems, where the semi-major axis of the binary orbit is less than 20 AU. The existence of these planets challenges planet formation theory, because the strong gravitational perturbations due to the companion increase encounter velocities between planetesimals and make it difficult for them to grow through accreting collisions. We study planetesimal encounter velocities in binary systems, where the planetesimals are embedded in a circumprimary gas disc that is allowed to evolve under influence of the gravitational perturbations of the companion star. We find that the encounter velocities between planetesimals of different size strongly depend on the gas disc eccentricity. In all cases studied, inclusion of the full gas dynamics increases the encounter velocity compared to the case of a static, circular gas disc. Full numerical parameter exploration is still impossible, but we derive analytical formulae to estimate encounter velocities between bodies of different sizes given the gas disc eccentricity. The gas dynamical evolution of a protoplanetary disc in a binary system tends to make planetesimal accretion even more difficult than in a static, axisymmetric gas disc.

The radiation gas dynamics of planetary nebulae. 4. From the Owl to the Eskimo

We present the results of two-dimensional radiation-gasdynamic simulations of aspherical Planetary Nebulae (PNe) evolution. These simulations were constructed using the Generalized Interacting Stellar Winds (GISW) scenario of Balick (1987) where a fast, tenuous wind from the central star expands into a toroidal, slow, dense wind. We demonstrate that the GISW model can produce a wide range of aspherical flow patterns. We have constructed self-consistent synthetic observations of the models from forbidden line emissivities used in the energy loss term. We present integrated intensity and long-slit spectrum, (Position-Velocity) maps of the models projected at different angles on the sky. These synthetic observations are compared with real intensity and Position-Velocity maps of PNe. We find that there is a very good match between the synthetic and real observations in terms of morphologies, kinematics, and physical conditions. From the results of these simulations we conclude that the GISW scenario can account for most, if not all, PNe morphologies, thus confirming Balicks (1987) conjecture.

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 parallaxes.

Astrophysical gasdynamics confronts reality - The shaping of planetary nebulae

We present two-dimensional numerical simulations, which use techniques of radiation gasdynamics to simulate the structures of planetary nebulae (PNs). Our model incorporates realistic volume emissivities in order to fully account for the conversion of mechanical and thermal energy into radiation. The model also produces detailed predictions of observables such as projected structure (e.g., Hα and [N II] images) and kinematic patterns. Virtually the full range of PN morphologies are easily reproduced, as are the basic kinematics, ionization structures, and temperatures

Hydrodynamical Models of Outflow Collimation in Young Stellar Objects

In this paper 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 toroidal density distribution which has a moderate opening angle of θρ 90°. The results show that for all but low values of the equator-to-pole density contrast 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). Comparison of our simulations with analytical models demonstrates that the evolution seen in the simulations is a mix of wind-blown bubble and jet dynamics. The simulations reveal a number of time-dependent nonlinear features not anticipated in previous analytical studies. These include: 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.Animations from these simulations are available over the internet at http://www.msi.umn.edu/Projects/twj/jetcol.html.

Collimation of astrophysical jets by inertial confinement

MANY astrophysical objects, from young stars, Herbig–Haro objects and planetary nebulae up to active galactic nuclei, can be very simply modelled as isotropic sources of high-energy tenuous gas embedded in dense toroidal clouds. Here we describe numerical simulations showing how such an arrangement can in general circumstances give rise to a well collimated jet, as is observed in many of these systems. Our model is a two-dimensional generalization of the interacting-winds description of planetary nebulae. Where the two winds come into contact, a discontinuity is formed, which is dragged out by the fast outflowing gas into a chimney along the polar axis. High-energy gas rushes up this channel and flows out around the top, creating a hot backflow which keeps the chimney in place. The inner shock, enclosing the source of the fast wind, also aids in collimation, and ionization cones such as those observed in active galactic nuclei may also form.

Outflow collimation in young stellar objects

In this paper we explore the eect of radiative losses on purely hydrodynamic jet collimation models applicable to Young Stellar Objects (YSOs). In our models aspherical bubbles form from the interaction of a central YSO wind with an aspherical circumprotostellar density distribution. Building on a previous non-radiative study (Frank & Mellema 1996) we demonstrate that supersonic jets are a natural and robust consequence of aspherical wind-blown bubble evolution. The simulations show that the addition of radiative cooling makes the hydrodynamic collimation mechanisms studied by Frank & Mellema (1996) more eective. We nd a number of time-dependent processes contributing to the collimation whose relative strength depends on the age of the system and parameters characterising the wind and the environment. As predicted by Frank & Mellema (1996) the flow-focusing at an oblique inner shock becomes more eective when radiative cooling is included. An unexpected result of this is the production of cool ( T< 10 4 K), dense (n 10 4 cm 3 ) jets forming through conical converging flows at the poles of the bubbles. For steady winds the formation of these jets occurs early in the bubble evolution. At later times we nd that the dynamical and cooling time scales for the jet material become similar and the jet beam increases in temperature (T 10 6 K). The duration of the cool jet phase depends on the mass loss rate, _ Mw, and velocity, Vw, of the wind. High values of _ Mw and low values of Vw produce longer cool jet phases. Since observations of YSO jets show considerable variability in the jet beam we present a simple one-dimensional (1-D) model for the evolution of a variable wind interacting with an accreting environment. We nd that the accretion ram pressure can halt the expansion of the bubble on time scales comparable to the periodicity of the wind and length scales less than 100 AU, the approximate observed scale for YSO jet collimation. These models indicate that, in the presence of a varying protostellar wind, the hydrodynamic collimation processes studied in our simulations can produce cool jets with sizes and time scales consistent with observations.

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 dierent 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 bubbles momentum and kinetic energy to the input wind momentum and kinetic energy, can give misleading information about the dynamics of the bubble.

Prospects of observing a quasar HII region during the Epoch of Reionization with redshifted 21cm

We present a study of the impact of a bright quasar on the redshifted 21cm signal during the Epoch of Reionization (EoR). Using three different cosmological radiative transfer simulations, we investigate if quasars are capable of substantially changing the size and morphology of the H II regions they are born in. We choose stellar and quasar luminosities in a way that is favourable to seeing such an effect. We find that even the most luminous of our quasar models is not able to increase the size of its native H II region substantially beyond those of large H II regions produced by clustered stellar sources alone. However, the quasar H II region is found to be more spherical. We next investigate the prospects of detecting such H II regions in the redshifted 21cm data from the Low Frequency Array (LOFAR) by means of a matched filter technique. We find that H II regions with radii ~ 25 comoving Mpc or larger should have a sufficiently high detection probability for 1200 hours of integration time. Although the matched filter can in principle distinguish between more and less spherical regions, we find that when including realistic system noise this distinction can no longer be made. The strong foregrounds are found not to pose a problem for the matched filter technique. We also demonstrate that when the quasar position is known, the redshifted 21cm data can still be used to set upper limits on the ionizing photon rate of the quasar. If both the quasar position and its luminosity are known, the redshifted 21 cm data can set new constrains on quasar lifetimes.