Magnetically Driven Winds from Post–Asymptotic Giant Branch Stars: Solutions for High‐Speed Winds and Extreme Collimation (original) (raw)
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Shaping Proto–Planetary and Young Planetary Nebulae with Collimated Fast Winds
The Astrophysical Journal, 2003
Using two-dimensional hydrodynamical simulations, we investigate the interaction of a collimated fast wind (CFW) interacting with a spherical asymptotic giant branch (AGB) wind as the mechanism for shaping proto-planetary nebulae and young planetary nebulae. In particular, we compare our simulations to the observations of an evolved PPN with multiple, highly collimated lobes, CRL 618. We characterize our model CFW by three parameters: opening angle, velocity and mass-loss rate, and explore the dependence of the properties of the shell on the first two. For given opening angle and velocity, the mass-loss rate is chosen to give a shell velocity of about 150 km s −1 at the tip, similar to that seen in CRL 618. In our simulations, the shell dynamics is found to depend on the velocity of the fast wind: we obtain a momentum-driven shell for a 300 km s −1 fast wind and a ballistic bow-shock driven shell for a 1000 km s −1 fast wind. The shell driven by the collimated fast wind is highly collimated, even though the AGB wind is spherical. Time variations in the velocity of the fast wind produce a series of internal shock pairs interacting with the inner surface of the shell. Due to radial expansion, the density of the internal shocks decreases with distance.
Shaping of Planetary Nebulae by Magnetic Fields
Publications of the Astronomical Society of the Pacific, 2008
Superwind production triggered by internal MHD effects in Red Giants is considered. Huge vortices of gas are predicted in and above the atmosphere during the peaked phase of ejection. Examination of the results clearly indicates a strong departure from the idealized spherical photosphere for these objects. Incidence on nonsphericity commonly observed in many planetary nebulae is discussed.
The Astrophysical Journal, 1997
Three-dimensional, magnetohydrodynamical simulations of the formation and evolution of planetary nebulae are discussed, and we confirm that planetary nebula jets and ansae can be obtained by magnetic collimation of their central, post-asymptotic giant branch, fast winds. Jets and ansae form at the polar regions as a result of the magnetic tension produced by the magnetized winds. Exterior density distributions anisotropic with latitude (e.g., accretion disks, wind-compressed disks, etc.) are not required inside this framework. We find that the expansion velocity of the jets and ansae coincides with the wind velocity of the post-asymptotic giant branch phase. We propose that the formation of "attached" and "detached" ansae involves, as a simplified model, two and three winds, respectively. We also show that the formation of rotating jets and point-symmetric nebular shapes can be the result of magnetic collimation around a precessing star. If the precession is caused by a tidal force, only a wide binary system is required.
Stellar Evolution from AGB to Planetary Nebulae
Proceedings of the International Astronomical Union
Planetary nebulae are formed by an interacting winds process where the remnant of the AGB wind is compressed and accelerated by a later-developed fast wind from the central star. One-dimensional dynamical models have successfully explained the multi-shell (bubble, shell, crown, haloes) structures and the kinematics of planetary nebulae. However, the origin of the diverse asymmetric morphology of planetary nebulae is still not understood. Recent observations in the visible, infrared, and the submillimeter have suggested that the AGB mass loss becomes aspherical in the very late stages, forming an expanding torus around the star. A fast, highly collimated wind then emerges in the polar directions and carves out a cavity in the AGB envelope to form a bipolar nebula. Newly discovered structures such as concentric arcs, 2-D rings, multiple lobes, and point-symmetric structures suggest that both the slow and fast winds may have temporal and directional variations, and precession can play ...
Magnetic fields in Planetary Nebulae: paradigms and related MHD frontiers
Proceedings of the International Astronomical Union, 2008
Many, if not all, post AGB stellar systems swiftly transition from a spherical to a powerful aspherical pre-planetary nebula (pPNE) outflow phase before waning into a PNe. The pPNe outflows require engine rotational energy and a mechanism to extract this energy into collimated outflows. Just radiation and rotation are insufficient but a symbiosis between rotation, differential rotation and large scale magnetic fields remains promising. Present observational evidence for magnetic fields in evolved stars is suggestive of dynamically important magnetic fields, but both theory and observation are rife with research opportunity. I discuss how magnetohydrodynamic outflows might arise in pPNe and PNe and distinguish different between approaches that address shaping vs. those that address both launch and shaping. Scenarios involving dynamos in single stars, binary driven dynamos, or accretion engines cannot be ruled out. One appealing paradigm involves accretion onto the primary post-AGB wh...
The Dynamical Evolution of Planetary Nebulae after the Fast Wind
The Astrophysical Journal, 2006
In this paper 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 towards the central stars as a rarefaction wave. Once that the photoionized gas fills the inner cavity, it develops a kinematical pattern of increasing velocity from the center outwards 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 Magnetohydrodynamic Model for the AGB Star: Preplanetary Nebula Symbiosis
Publications of the Astronomical Society of the Pacific, 2010
In this article, a magnetohydrodynamic (MHD) model is presented that is aimed at understanding the anisotropic mass loss by an AGB star and subsequent bipolar planetary nebula formation. The main challenge in developing such a model was to simultaneously treat the dynamo engine in the core region of the AGB progenitor, magnetic buoyancy, meridional circulation, and gas ejection in a self-consistent manner. A previous article did not incorporate the dynamo and the feedback of the produced magnetic field on differential rotation.
Physics of Plasmas, 2001
The early evolution of hydrogen ϩ ͑H II͒ regions is controlled by the properties of the star-forming cloud cores. The observed density distributions in some young H II regions indicate that the power-law stratifications can be steeper than r Ϫ2. Ionization fronts can overrun these gradients and the ionized outflows are strongly accelerated along these steep density distributions. Thus, photoionized regions can either reach pressure equilibrium inside the inner parts of the high-pressure cores ͓with sizes and densities similar to those observed in ultra compact ͑UC͒ H II regions͔, or create bright H II regions with extended emission. The density inhomogeneities engulfed within the ionization fronts create corrugations in the front, which in turn drive instabilities in the ionization-shock ͑I-S͒ front. These instabilities grow on short time scales and lead to the fragmentation of the dense shells generated by the shock fronts. Thus, new clumps are continuously created from the fragmented shell, and the resulting finger-like structures can explain the existence of elephant trunks and cometary-like globules in most H II regions. In the case of planetary nebulae ͑PNe͒, wind asymmetries and magnetic fields from rotating stars, along with precession of the rotation axis, can create the wide range of observed PNe morphologies and collimated outflows ͑jets͒. Magnetic collimation and jet formation in PNe become very efficient after the flow has passed through the reverse shock of the PN.
The planetary nebula NGC 1360, a test case of magnetic collimation and evolution after the fast wind
2000
The central star of this nebula has an observed intense magnetic field and the fast wind is no longer present, indicating that a back flow process has probably developed. Long-slit, spatially resolved echelle spectra have been obtained across the main body of NGC 1360 and over its system of bipolar jets. Deep images of the knotty structures of the jets have also been obtained. The data allow a detailed study of the structure and kinematics of this object and the results are modeled considering the effects of a magnetic collimation process in the development of the nebula and then switching off the fast stellar wind to follow its evolution to its current state. The model is able to successfully reproduce many key features of NGC 1360 under these premises. Subject headingg s: ISM: jets and outflows -planetary nebulae: generalplanetary nebulae: individual ( NGC 1360) -stars: magnetic fields
MAGNETIC NESTED-WIND SCENARIOS FOR BIPOLAR OUTFLOWS: PREPLANETARY AND YSO NEBULAR SHAPING
The Astrophysical Journal, 2009
We present results of a series of magnetohydrodynamic (MHD) and hydrodynamic (HD) 2.5D simulations of the morphology of outflows driven by nested wide-angle winds -i.e. winds which eminate from a central star as well as from an orbiting accretion disk. While our results are broadly relevent to nested wind systems we have tuned the parameters of the simulations to touch on issues in both Young Stellar Objects and Planetary Nebula studies. In particular our studies connect to open issues in the early evolution of Planetary Nebulae. We find that nested MHD winds exhibit marked morphological differences from the single MHD wind case along both dimensions of the flow. Nested HD winds on the other hand give rise mainly to geometric distortions of an outflow that is topologically similar to the flow arising from a single stellar HD wind. Our MHD results are insensitive to changes in ambient temperature between ionized and un-ionized circumstellar environments. The results are sensitive to the relative mass-loss rates, and to the relative speeds of the stellar and disk winds. We also present synthetic emission maps of both nested MHD and HD simulations. We find that nested MHD winds show knots of emission appearing on-axis that do not appear in the HD case.