W. Steffen

Shape: A 3D Modeling Tool for Astrophysics

We present a flexible interactive 3D morpho-kinematical modeling application for astrophysics. Compared to other systems, our application reduces the restrictions on the physical assumptions, data type, and amount that is required for a reconstruction of an objects morphology. It is one of the first publicly available tools to apply interactive graphics to astrophysical modeling. The tool allows astrophysicists to provide a priori knowledge about the object by interactively defining 3D structural elements. By direct comparison of model prediction with observational data, model parameters can then be automatically optimized to fit the observation. The tool has already been successfully used in a number of astrophysical research projects.

Shadows behind Neutral Clumps in Photoionized Regions

Shadows behind neutral clumps in photoionized regions can have a neutral core surrounded by gas that is photoionized by the diffuse flux produced by the nebula. We present a simple analytic model describing the configuration of such shadows. We also present numerical gasdynamic simulations of the relaxation to the final, steady state. These models have clear applications (e.g., to the cometary knots in the Helix Nebula) but can also be applied in other astrophysical contexts.

Evidence for an outflow from the Seyfert galaxy NGC

New observations using narrow band imaging, long-slit spectroscopy and MERLIN observations of the nuclear region of the Seyfert galaxy NGC 4051 have been made. An edge brightened, triangular region of ionized gas extending 420 pc from the centre of the galaxy has been detected. Long-slit spectra of this ionised gas, taken at 1.5 ′′ from the core, show the [O iii] 5007-u emission line to consist of two velocity components, both blue-shifted from the systemic radial velocity, with velocity widths of 140 kms −1 and separated by 120 kms −1 . This region is co-spatial with weak extended radio emission and is suggestive of a centrally driven outflow. The [O iii] 5007-u line spectrum and image of this region have been modelled as an outflowing conical structure at 50 ◦ to the line of sight with a half opening angle of 23 ◦ . In addition to the extended structure, high resolution MERLIN observations of the 18-cm nuclear radio emission reveal a compact (1 ′′ ) radio triple source in PA 73 ◦ . This source is coincident with the HST-imaged emission line structure. These high resolution observations are consistent with a more compact origin of activity (i.e. a Seyfert nucleus) than a starburst region.

ALMA data suggest the presence of spiral structure in the inner wind of CW Leonis

Context. Evolved low-mass stars lose a significant fraction of their mass through a stellar wind. While the overall morphology of the stellar wind structure during the Asymptotic Giant Branch (AGB) phase is thought to be roughly spherically symmetric, the morphology changes dramatically during the post-AGB and planetary nebula phase during which often bipolar and multi-polar structures are observed. Aims. We aim to study the inner wind structure of the closest well-known AGB star CW Leo. Different diagnostics probing different geometrical scales have pointed toward a non-homogeneous mass-loss process for this star: dust clumps are observed at milli-arcsec scale, a bipolar structure is seen at arcsecond-scale and multi-concentric shells are detected beyond 1′′. Methods. We present the first ALMA Cycle 0 band 9 data around 650 GHz (450μm) tracing the inner wind of CW Leo. The fullresolution data have a spatial resolution of 0′′ .42×0′′ .24, allowing us to study the morpho-kinematical structure of CW Leo within ∼6′′. Results. We have detected 25 molecular emission lines in four spectral windows. The emission of all but one line is spatially resolved. The dust and molecular lines are centered around the continuum peak position, assumed to be dominated by stellar emission. The dust emission has an asymmetric distribution with a central peak flux density of ∼2 Jy. The molecular emission lines trace different regions in the wind acceleration region and suggest that the wind velocity increases rapidly from about 5 R? almost reaching the terminal velocity at ∼11 R?. The images prove that vibrational lines are excited close to the stellar surface and that SiO is a parent molecule. The channel maps for the brighter lines show a complex structure; specifically for the CO J=6-5 line different arcs are detected within the first few arcseconds. The curved structure present in the position-velocity (PV) map of the CO J=6-5 line can be explained by a spiral structure in the inner wind of CW Leo, probably induced by a binary companion. From modeling the ALMA data, we deduce that the potential orbital axis for the binary system lies at a position angle of ∼10–20◦ to the North-East and that the spiral structure is seen almost edge-on. We infer an orbital period of 55 yr and a binary separation of 25 au (or ∼8.2 R?). We tentatively estimate that the companion is an unevolved low-mass main-sequence star. Conclusions. A scenario of a binary-induced spiral shell can explain the correlated structure seen in the ALMA PV images of CW Leo. Moreover, this scenario can also explain many other observational signatures seen at different spatial scales and in different wavelength regions, such as the bipolar structure and the almost-concentric shells. The ALMA data hence provide us for the first time with the crucial kinematical link between the dust clumps seen at milli-arcsecond scale and the almost concentric arcs seen at arcsecond scale.

Jet-Cloud Interactions and the Brightening of the Narrow-Line Region in Seyfert Galaxies

We study the kinematical and brightness evolution of emission-line clouds in the narrow-line region (NLR) of Seyfert galaxies during the passage of a jet. We derive a critical density above which a cloud remains radiative after compression by the jet cocoon. The critical density depends mainly on the cocoon pressure. Supercritical clouds increase in emission-line brightness, while subcritical clouds generally are highly overheated, reducing their luminosity below that of the intercloud medium. As a result of the pressure stratification in the bow shock of the jet, a cylindrical structure of nested shells develops around the jet. The most compact and brightest compressed clouds surround the cloud-free channel of the radio jet. To support our analytical model, we present a numerical simulation of a supersonic jet propagating into a clumpy NLR. The position-velocity diagram of the simulated Hα emission shows total line widths of the order of 500 km s−1 with large-scale variations in the radial velocities of the clouds due to the stratified pressure in the bow shock region of the jet. Most of the luminosity is concentrated in a few dense clouds surrounding the jet. These morphological and kinematic signatures are all found in the well-observed NLR of NGC 1068 and other Seyfert galaxies.

THE OUTFLOWS AND THREE-DIMENSIONAL STRUCTURE OF NGC 6337: A PLANETARY NEBULA WITH A CLOSE BINARY NUCLEUS

NGC 6337 is a member of the rare group of planetary nebulae where a close binary nucleus has been identified. The nebulas morphology and emission line profiles are both unusual, particularly the latter. We present a thorough mapping of spatially resolved, long-slit echelle spectra obtained over the nebula that allows a detailed characterization of its complex kinematics. This information, together with narrowband imagery, is used to produce a three-dimensional (3D) model of the nebula using the code SHAPE. The 3D model yields a slowly expanding toroid with large density fluctuations in its periphery that are observed as cometary knots. A system of bipolar expanding caps of low ionization is located outside the toroid. In addition, an extended high velocity and tenuous bipolar collimated outflow is found emerging from the core and sharply bending in opposite directions, a behavior that cannot be accounted for by pure magnetic launching and collimation unless the source of the outflow is precessing or rotating, as could be expected from a close binary nucleus.

HYDRODYNAMICAL VELOCITY FIELDS IN PLANETARY NEBULAE

Based on axisymmetric hydrodynamical simulations and three-dimensional (3D) reconstructions with Shape, we investigate the kinematical signatures of deviations from homologous (Hubble-type) outflows in some typical shapes of planetary nebulae (PNs). We find that, in most situations considered in our simulations, the deviations from a Hubble-type flow are significant and observable. The deviations are systematic and a simple parameterization of them considerably improves morphokinematical models of the simulations. We describe such extensions to a homologous expansion law that capture the global velocity structure of hydrodynamical axisymmetric nebulae during their wind-blown phase. It is the size of the poloidal velocity component that strongly influences the shape of the position-velocity diagrams that are obtained, not so much the variation of the radial component. The deviations increase with the degree of collimation of the nebula and they are stronger at intermediate latitudes. We describe potential deformations which these deviations might produce in 3D reconstructions that assume Hubble-type outflows. The general conclusion is that detailed morphokinematical observations and modeling of PNs can reveal whether a nebula is still in a hydrodynamically active stage (windy phase) or whether it has reached ballistic expansion.

The Dynamical Evolution of Planetary Nebulae after the Fast Wind

We explore the dynamics of ionization-bounded planetary nebulae after the termination of the fast stellar wind. When the stellar wind becomes negligible, the hot, shocked bubble depressurizes, and the thermal pressure of the photoionized region, at the inner edge of the swept-up shell, becomes dominant. At this stage the shell tends to fragment, creating clumps with comet-like tails and long, photoionized trails in between, while the photoionized material expands back toward the central stars as a rarefaction wave. Once the photoionized gas fills the inner cavity, it develops a kinematical pattern of increasing velocity from the center outward, with a typical range of velocities starting from the systemic velocity to ~50 km s-1 at the edges. The Helix Nebula is a clear example of a planetary nebula at this late evolutionary stage.

A wind–shell interaction model for multipolar planetary nebulae

We explore the formation of multipolar structures in planetary and pre-planetary nebulae from the interaction of a fast post-AGB wind with a highly inhomogeneous and filamentary shell structure assumed to form during the final phase of the high density wind. The simulations were performed with a new hydrodynamics code integrated in the interactive framework of the astrophysical modeling package SHAPE. In contrast to conventional astrophysical hydrodynamics software, the new code does not require any programming intervention by the user for setting up or controlling the code. Visualization and analysis of the simulation data has been done in SHAPE without external software. The key conclusion from the simulations is that secondary lobes in planetary nebulae, such as Hubble 5 and K3-17, can be formed through the interaction of a fast low-density wind with a complex high density environment, such as a filamentary circumstellar shell. The more complicated alternative explanation of intermittent collimated outflows that change direction, in many cases may therefore not be necessary. We consider that the wind-shell interaction scenario is more likely since the bow-shock shape expected from a strongly cooling bow-shock from jets is different from that of the observed bubbles. Furthermore, the timescales of the wind-wind interaction suggest that the progenitor star was rather massive.

ON THE VELOCITY STRUCTURE IN CLUMPY PLANETARY NEBULAE

Hydrodynamical, axisymmetric simulations and analytic calculations are presented for the interaction of a fast tenuous stellar wind with a spherical distribution of high-density clouds or clumps embedded in a stationary intercloud medium with an approximately inverse square density fall-off. Two distinct cases are studied. In the first case, clumps are formed by dynamic and thermal instabilities in the shocked wind. In the second case clumps, are assumed to have been formed by inhomogeneities in the AGB wind, prior to the onset of the fast wind. We find that the velocity distribution of these two cases is very different. However, in both cases the ensemble of clumps develops a positive velocity gradient with distance, and the kinematics might provide information on the evolutionary stage of the nebula.