Faraday Rotation in the Tail of the Planetary Nebula DeHt 5

We present 1420 MHz polarization images of a 5x5 degree region around the planetary nebula (PN) DeHt 5. The images reveal narrow Faraday-rotation structures on the visible disk of DeHt 5, as well as two wider, tail-like, structures "behind" DeHt 5. Though DeHt 5 is an old PN known to be interacting with the interstellar medium (ISM), a tail has not previously been identified for this object. The innermost tail is approximately 3 pc long and runs away from the north-east edge of DeHt 5 in a direction roughly opposite that of the sky-projected space velocity of the white dwarf central star, WD 2218+706. We believe this tail to be the signature of ionized material ram-pressure stripped and deposited downstream during a >74,000 yr interaction between DeHt 5 and the ISM. We estimate the rotation measure (RM) through the inner tail to be -15 +/- 5 rad/m^2, and, using a realistic estimate for the line-of-sight component of the ISM magnetic field around DeHt 5, derive an electron density in the inner tail of n_e = 3.6 +/- 1.8 cm^-3. Assuming the material is fully ionized, we estimate a total mass in the inner tail of 0.68 +/- 0.33 solar masses, and predict that 0.49 +/- 0.33 solar masses was added during the PN-ISM interaction. The outermost tail consists of a series of three roughly circular components, which have a collective length of approximately 11.0 pc. This tail is less conspicuous than the inner tail, and may be the signature of the earlier interaction between the WD 2218+706 asymptotic giant branch (AGB) progenitor and the ISM. The results for the inner and outer tails are consistent with hydrodynamic simulations, and may have implications for the PN missing-mass problem as well as for models which describe the impact of the deaths of intermediate-mass stars on the ISM.Comment: 30 pages (single-column preprint format), 5 figures. Accepted for publication in the Astrophysical Journa

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.Comment: 22 pages, 16 figures, accepted by MNRAS (consists of 14 page journal paper and 8 page online-only appendix). Email C Wareing for high quality PDF versio

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

Recent observations of the Mira AB binary system have revealed a surrounding arc-like structure and a stream of material stretching 2 degrees away in opposition to the arc. The alignment of the proper motion vector and the arc-like structure shows the structures to be a bow shock and accompanying tail. We have successfully hydrodynamically modelled 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 possiblity of mass-loss variations, but find that these have limited effect on the tail structure. The tail is estimated to be approximately 450,000 years 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 occured 17 pc downstream from its current location.Comment: 12 pages, 3 colour figures, accepted by ApJ Part II (Letters

Vortices in the wakes of AGB 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^-1 pc to 10^1 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.Comment: ApJL accepted, preprint form, 13 pages including 4 pages of figure

Sh2-188: a model for a speedy PN

Sh2-188 is thought to be an ancient planetary nebula in the galactic disk. It appears to be one-sided with recent observations revealing structure behind the filamentary limb. We postulate that Sh2-188 is interacting with the ISM and simulate it in terms of a ``triple-wind'' model comprising of the usual ``fast'' and ``slow'' interacting stellar winds plus the wind due to motion through the ISM. We have run simulations at various velocities of the central star relative to the ISM and find that a high velocity of 125 km/s best approximates the observed structure. We also suggest that Sh2-188 is younger than previously thought and that much of the mass lost on the AGB has been swept downstream.Comment: 5 pages, 3 figures, Proceedings of the "Stellar end products" workshop, 13-15 April 2005, Granada, Spain, ed. M.A. Perez-Torres. To appear in Vol. 77 (Jan 2006) of MmSAI. Reference corrected, start of 2nd paragraph in discussion corrected, image scaling correcte

An HI shell-like structure associated with nova V458 Vulpeculae?

We report the radio detection of a shell-like HI structure in proximity to, and probably associated with, the nova V458 Vul. High spectral resolution observation with the Giant Metrewave Radio Telescope has made it possible to study the detailed kinematics of this broken and expanding shell. Unlike the diffuse Galactic HI emission, this is a single velocity component emission with significant clumping at ~ 0.5' scales. The observed narrow line width of ~ 5 km/s suggests that the shell consists of mostly cold gas. Assuming a distance of 13 kpc to the system, as quoted in the literature, the estimated HI mass of the nebula is about 25 M_sun. However, there are some indications that the system is closer than 13 kpc. If there is a physical association of the HI structure and the nova system, the asymmetric morphology and the off-centred stellar system indicates past strong interaction of the mass loss in the asymptotic giant branch phase with the surrounding interstellar medium. So far, this is the second example, after GK Per, of a large HI structure associated with a classical nova.Comment: 6 pages, 2 table, 3 figures. Accepted for publication in MNRAS Letters. The definitive version will be available at http://www.blackwell-synergy.com

Probing the Magnetized Interstellar Medium Surrounding the Planetary Nebula Sh 2-216

We present 1420 MHz polarization images of a 2.5 X 2.5 degree region around the planetary nebula (PN) Sh 2-216. The images are taken from the Canadian Galactic Plane Survey (CGPS). An arc of low polarized intensity appears prominently in the north-east portion of the visible disk of Sh 2-216, coincident with the optically identified interaction region between the PN and the interstellar medium (ISM). The arc contains structural variations down to the ~1 arcminute resolution limit in both polarized intensity and polarization angle. Several polarization-angle "knots" appear along the arc. By comparison of the polarization angles at the centers of the knots and the mean polarization angle outside Sh 2-216, we estimate the rotation measure (RM) through the knots to be -43 +/- 10 rad/m^2. Using this estimate for the RM and an estimate of the electron density in the shell of Sh 2-216, we derive a line-of-sight magnetic field in the interaction region of 5.0 +/- 2.0 microG. We believe it more likely the observed magnetic field is interstellar than stellar, though we cannot completely dismiss the latter possibility. We interpret our observations via a simple model which describes the ISM magnetic field around Sh 2-216, and comment on the potential use of old PNe as probes of the magnetized ISM.Comment: 25 pages, 4 figures. Accepted for publication in the Astrophysical Journa

HI and CO in the circumstellar environment of the oxygen-rich AGB star RX Lep

Circumstellar shells around AGB stars are built over long periods of time that may reach several million years. They may therefore be extended over large sizes (~1 pc, possibly more), and different complementary tracers are needed to describe their global properties. In the present work, we combined 21-cm HI and CO rotational line data obtained on an oxygen-rich semi-regular variable, RX Lep, to describe the global properties of its circumstellar environment. With the SEST, we detected the CO(2-1) rotational line from RX Lep. The line profile is parabolic and implies an expansion velocity of ~4.2 km/s and a mass-loss rate ~1.7 10^-7 Msun/yr (d = 137 pc). The HI line at 21 cm was detected with the Nancay Radiotelescope on the star position and at several offset positions. The linear shell size is relatively small, ~0.1 pc, but we detect a trail extending southward to ~0.5 pc. The line profiles are approximately Gaussian with an FWHM ~3.8 km/s and interpreted with a model developed for the detached shell around the carbon-rich AGB star Y CVn. Our HI spectra are well-reproduced by assuming a constant outflow (Mloss = 1.65 10^-7 Msun/yr) of ~4 10^4 years duration, which has been slowed down by the external medium. The spatial offset of the HI source is consistent with the northward direction of the proper motion, lending support to the presence of a trail resulting from the motion of the source through the ISM, as already suggested for Mira, RS Cnc, and other sources detected in HI. The source was also observed in SiO (3 mm) and OH (18 cm), but not detected. The properties of the external parts of circumstellar shells around AGB stars should be dominated by the interaction between stellar outflows and external matter for oxygen-rich, as well as for carbon-rich, sources, and the 21-cm HI line provides a very useful tracer of these regions.Comment: 15 pages, 9 figures, accepted for publication in A&

The interface between the stellar wind and interstellar medium around R Cassiopeiae revealed by far-infrared imaging

The circumstellar dust shells of intermediate initial-mass (about 1 to 8 solar masses) evolved stars are generated by copious mass loss during the asymptotic giant branch phase. The density structure of their circumstellar shell is the direct evidence of mass loss processes, from which we can investigate the nature of mass loss. We used the AKARI Infrared Astronomy Satellite and the Spitzer Space Telescope to obtain the surface brightness maps of an evolved star R Cas at far-infrared wavelengths, since the temperature of dust decreases as the distance from the star increases and one needs to probe dust at lower temperatures, i.e., at longer wavelengths. The observed shell structure and the star's known proper motion suggest that the structure represents the interface regions between the dusty wind and the interstellar medium. The deconvolved structures are fitted with the analytic bow shock structure to determine the inclination angle of the bow shock cone. Our data show that (1) the bow shock cone of 1 - 5 x 10^-5 solar masses (dust mass) is inclined at 68 degrees with respect to the plane of the sky, and (2) the dust temperature in the bow shock cone is raised to more than 20 K by collisional shock interaction in addition to the ambient interstellar radiation field. By comparison between the apex vector of the bow shock and space motion vector of the star we infer that there is a flow of interstellar medium local to R Cas whose flow velocity is at least 55.6 km/s, consistent with an environment conducive to dust heating by shock interactions.Comment: 7 pages, 2 figures, accepted for publication in Astronomy and Astrophysic

Krylov iterative methods applied to multidimensional S[sub n] calculations in the presence of material discontinuities

We show that a Krylov iterative meihod, preconditioned with DSA, can be used to efficiently compute solutions to diffusive problems with discontinuities in material properties. We consider a lumped, linear discontinuous discretization of the S N transport equation with a 'partially consistent' DSA preconditioner. The Krylov method can be implemented in terms of the original S N source iteration coding with little modification. Results from numerical experiments show that replacing source iteration with a preconditioned Krylov method can efficiently solve problems that are virtually intractable with accelerated source iteration. Key Words: Krylov iterative methods, discrete ordinates, deterministic transport methods, diffusion synthetic acceleratio