Christopher J. Wareing

The interaction of planetary nebulae and their asymptotic giant branch progenitors with the interstellar medium

Interaction with the Interstellar Medium (ISM) cannot be ignored in understanding planetary nebula (PN) evolution and shaping. In an effort to understand the range of shapes observed in the outer envelopes of PNe, we have run a comprehensive set of three-dimensional hydrodynamic simulations, from the beginning of the asymptotic giant branch (AGB) superwind phase until the end of the post–AGB/PN phase. A ’triple-wind’ model is used, including a slow AGB wind, fast post–AGB wind and third wind reflecting the linear movement through the ISM. A wide range of stellar velocities, mass-loss rates and ISM densities have been considered. We find ISM interaction strongly affects outer PN structures, with the dominant shaping occuring during the AGB phase. The simulations predict four stages of PN– ISM interaction whereby the PN is initially unaffected (1), then limb-brightened in the direction of motion (2), then distorted with the star moving away from the geometric centre (3) and finally so distorted that the object is no longer recognisable as a PN and may not be classed as such (4). Parsec-size shells around PN are predicted to be common. The structure and brightness of ancient PNe is largely determined by the ISM interaction, caused by rebrightening during the second stage; this effect may address the current discrepancies in Galactic PN abundance. The majority of PNe will have tail structures. Evidence for strong interaction is found for all known planetary nebulae in globular clusters.

The interaction of planetary nebulae and their AGB progenitors with the interstellar medium

Interaction with the Interstellar Medium (ISM) cannot be ignored in understanding planetary nebula (PN) evolution and shaping. In an effort to understand the range of shapes observed in the outer envelopes of PNe, we have run a comprehensive set of three-dimensional hydrodynamic simulations, from the beginning of the asymptotic giant branch (AGB) superwind phase until the end of the post–AGB/PN phase. A ’triple-wind’ model is used, including a slow AGB wind, fast post–AGB wind and third wind reflecting the linear movement through the ISM. A wide range of stellar velocities, mass-loss rates and ISM densities have been considered. We find ISM interaction strongly affects outer PN structures, with the dominant shaping occuring during the AGB phase. The simulations predict four stages of PN– ISM interaction whereby the PN is initially unaffected (1), then limb-brightened in the direction of motion (2), then distorted with the star moving away from the geometric centre (3) and finally so distorted that the object is no longer recognisable as a PN and may not be classed as such (4). Parsec-size shells around PN are predicted to be common. The structure and brightness of ancient PNe is largely determined by the ISM interaction, caused by rebrightening during the second stage; this effect may address the current discrepancies in Galactic PN abundance. The majority of PNe will have tail structures. Evidence for strong interaction is found for all known planetary nebulae in globular clusters.

Detached shells as tracers of asymptotic giant branch-interstellar medium bow shocks

New Spitzer imaging observations have revealed the structure around the Mira variable star R Hya to be a one-sided parabolic arc 100 arcsec to the West stretching from North to South. We successfully model R Hya and its surroundings in terms of an interaction of the stellar wind from an asymptotic giant branch (AGB) star with the interstellar medium (ISM) the star moves through. Our three-dimensional hydrodynamic simulation reproduces the structure as a bow shock into the oncoming ISM. We propose this as another explanation of detached shells around such stars which should be considered alongside current theories of internal origin. The simulation predicts the existence of a tail of ram-pressure-stripped AGB material stretching downstream. Indications for such a tail behind R Hya are seen in IRAS maps.

It's a wonderful tail: The mass-loss history of Mira

Recent observations of the Mira AB binary system have revealed a surrounding arclike structure and a stream of material stretching 2° away in opposition to the arc. The alignment of the proper motion vector and the arclike structure shows the structures to be a bow shock and accompanying tail. We have successfully hydrodynamically modeled the bow shock and tail as the interaction between the asymptotic giant branch (AGB) wind launched from Mira A and the surrounding interstellar medium. Our simulations show that the wake behind the bow shock is turbulent; this forms periodic density variations in the tail similar to those observed. We investigate the possibility of mass-loss variations but find that these have limited effect on the tail structure. The tail is estimated to be approximately 450,000 yr old and is moving with a velocity close to that of Mira itself. We suggest that the duration of the high mass-loss phase on the AGB may have been underestimated. Finally, both the tail curvature and the rebrightening at large distance can be qualitatively understood if Mira recently entered the Local Bubble. This is estimated to have occurred 17 pc downstream from its current location.

Vortices in the wakes of asymptotic giant branch stars

Vortices have been postulated at a range of size scales in the universe including at the stellar size-scale. Whilst hydrodynamically simulating the wind from an asymptotic giant branch (AGB) star moving through and sweeping up its surrounding interstellar medium (ISM), we have found vortices on the size scale of 10 pc to 10 pc in the wake of the star. These vortices appear to be the result of instabilities at the head of the bow shock formed upstream of the AGB star. The instabilities peel off downstream and form vortices in the tail of AGB material behind the bow shock, mixing with the surrounding ISM. We suggest such structures are visible in the planetary nebula Sh 2-188. Subject headings: hydrodynamics – turbulence – stars: AGB and post-AGB – circumstellar matter – ISM: structure – stars: mass-lossVortices have been postulated at a range of size scales in the universe, including at the stellar-size scale. While hydrodynamically simulating the wind from an asymptotic giant branch (AGB) star moving through and sweeping up its surrounding interstellar medium (ISM), we have found vortices on the size scale of 10-1 to 101 pc in the wake of the star. These vortices appear to be the result of instabilities at the head of the bow shock formed upstream of the AGB star. The instabilities peel off downstream and form vortices in the tail of AGB material behind the bow shock, mixing with the surrounding ISM. We suggest such structures are visible in the planetary nebula Sh 2-188.

The shaping of planetary nebula Sh 2–188 through interaction with the interstellar medium

Sh2-188 is an example of strong interaction between a planetary nebula (PN) and the interstellar medium (ISM). It shows a single arc-like structure, consisting of several filaments, which is postulated to be the result of motion through the ISM. We present new Hα images from the Isaac Newton Telescope Photometric Hα Survey of the Northern Galactic Plane (IPHAS) which reveal structure behind the filamentary limb. A faint, thin arc is seen opposite the bright limb, in combination forming a closed ring. Behind the faint arc a long wide tail is detected, doubling the size of the nebula. The nebula extends 15 arcmin on the sky in total. We have developed a ‘triplewind’ hydrodynamical model, comprising of the initial ‘slow’ asymptotic giant branch (AGB) wind and the later ‘fast’ stellar wind (the interacting stellar wind model), plus a third wind reflecting the motion through the ISM. Simulations at various velocities of the central star relative to the ISM indicate that a high velocity of 125 kms 1 is required to reproduce the observed structure. We find that the bright limb and the tail already formed during the AGB phase, prior to the formation of the PN. The closure of the ring arises from the slow–fast wind interaction. Most of the mass lost on the AGB has been swept downstream, providing a potential explanation of the missing mass problem in PNe. We report a proper motion for the central star of 30± 10 masyr 1 in the direction of the bright limb. Assuming the central star is moving at 125± 25 kms 1 , the distance to the nebula is estimated to be 850 +500 420 pc, consistent with a spectroscopic distance to the star. Expansion velocities measured from spectroscopic data of the bright filaments are consistent with velocities measured from the simulation. Sh2-188 is one of the largest PNe known, with an extent of 2.8 pc. The model shows that this size was already set during the AGB phase.

New Candidate Planetary Nebulae in the IPHAS Survey: the Case of Planetary Nebulae with ISM interaction

We present the results of the search for candidate Planetary Nebulae interacting with the interstellar medium (PN–ISM) in the framework of the INT Photometric Hα Survey (IPHAS) and located in the right ascension range 18–20 h. The detection capability of this new Northern survey, in terms of depth and imaging resolution, has allowed us to overcome the detection problem generally associated to the low surface brightness inherent to PNe-ISM. We discuss the detection of 21 IPHAS PN–ISM candidates. Thus, different stages of interaction were observed, implying various morphologies i.e. from the unaffected to totally disrupted shapes. The majority of the sources belong to the so-called WZO2 stage which main characteristic is a brightening of the nebula’s shell in the direction of motion. The new findings are encouraging as they would be a first step into the reduction of the scarcity of observational data and they would provide new insights into the physical processes occurring in the rather evolved PNe.

Forward and inverse cascades in decaying two-dimensional electron magnetohydrodynamic turbulence

Electron magnetohydrodynamic (EMHD) turbulence in two dimensions is studied via high-resolution numerical simulations with a normal diffusivity. The resulting energy spectra asymptotically approach a k−5/2 law with increasing RB, the ratio of the nonlinear to linear time scales in the governing equation. No evidence is found of a dissipative cutoff, consistent with nonlocal spectral energy transfer. Dissipative cutoffs found in previous studies are explained as artificial effects of hyperdiffusivity. Relatively stationary structures are found to develop in time, rather than the variability found in ordinary or MHD turbulence. Further, EMHD turbulence displays scale-dependent anisotropy with reduced energy transfer in the direction parallel to the uniform background field, consistent with previous studies. Finally, the governing equation is found to yield an inverse cascade, at least partially transferring magnetic energy from small to large scales.

VLT/near-infrared integral field spectrometer observations of molecular hydrogen lines in the knots of the planetary nebula NGC 7293 (the Helix Nebula)

Knots are commonly found in nearby planetary nebulae (PNe) and star-forming regions. Within PNe, knots are often found to be associated with the brightest parts of the nebulae and understanding the physics involved in knots may reveal the processes dominating in PNe. As one of the closest PNe, the Helix Nebula (NGC 7293) is an ideal target to study such small-scale (∼300 au) structures. We have obtained infrared integral spectroscopy of a comet-shaped knot in the Helix Nebula using the Spectrograph for INtegral Field Observations in the Near Infrared (SINFONI) on the Very Large Telescope at high spatial resolution (50-125 mas). With spatially resolved 2-μm spectra, we find that the H 2 rotational temperature within the cometary knots is uniform. The rotational-vibrational temperature of the cometary knot (situated in the innermost region of the nebula, 2.5 arcmin away from the central star) is 1800 K, higher than the temperature seen in the outer regions (5-6 arcmin from the central star) of the nebula (900 K), suggesting that the excitation temperature varies across the nebula. The obtained intensities are reasonably well fitted with 27 km s- 1 C-type shock model. This ambient gas velocity is slightly higher than the observed [He II] wind velocity of 13 km s -1 . The gas excitation can also be reproduced with a photon-dominant region (PDR) model, but this requires an order of magnitude higher ultraviolet radiation. Both models have limitations, highlighting the need for models that treat both hydrodynamical physics and the PDR.

Cascades in decaying three-dimensional electron magnetohydrodynamic turbulence

Decaying electron magnetohydrodynamic (EMHD) turbulence in three dimensions is studied via high-resolution numerical simulations. The resulting en- ergy spectra asymptotically approach a k −2 law with increasing RB , the ratio of the nonlinear to linear time scales in the governing equation, consistent with theoretical predictions. No evidence is found of a dissipative cutoff, consistent with non-local spectral energy transfer and recent studies of 2D EMHD turbulence. Dissipative cutoffs found in previous studies are explained as artificial effects of hyperdiffusiv- ity. In another similarity to 2D EMHD turbulence, relatively stationary structures are found to develop in time, rather than the variability found in ordinary or MHD turbulence. Further, cascades of energy in 3D EMHD turbulence are found to be suppressed in all directions under the influence of a uniform background field. Energy transfer is further reduced in the direction parallel to the field, displaying scale-dependent anisotropy. Finally, the governing equation is found to yield a weak inverse cascade, at least partially transferring magnetic energy from small to large scales.