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NGC 7293 (Helix Nebula)



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The Discovery of a Companion to the Lowest Mass White Dwarf
We report the detection of a radial velocity companion to SDSSJ091709.55+463821.8, the lowest mass white dwarf currently known, withM~0.17 Msolar. The radial velocity of the white dwarf showsvariations with a semi-amplitude of 148.8+/-6.9 km s-1 and aperiod of 7.5936+/-0.0024 hr, which implies a companion mass ofM>=0.28 Msolar. The lack of evidence of a companion in theoptical photometry forces any main-sequence companion to be smaller than0.1 Msolar, hence a low-mass main-sequence star companion isruled out for this system. The companion is most likely another whitedwarf, and we present tentative evidence for an evolutionary scenariothat could have produced it. However, a neutron star companion cannot beruled out, and follow-up radio observations are required to search for apulsar companion.Observations reported here were obtained at the MMT Observatory, a jointfacility of the Smithsonian Institution and the University of Arizona.

Central Stars of Planetary Nebulae in the Magellanic Clouds: A Detailed Spectroscopic Analysis
We observed five central stars of planetary nebulae (CSPN) in the LMCand three in the SMC with FUSE, in the range 905-1187 Å. Weperformed a model-based analysis of these spectra in conjunction withHST spectra in the UV and optical range to determine stellar and nebularparameters. The signature of hot (T>~2000 K) circumstellar molecularhydrogen is found in the FUSE spectra of most objects. We also findevidence of X-rays in the wind of SMP LMC 76.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer and archival data. FUSE is operated for NASA bythe Johns Hopkins University under NASA contract NAS5-32985.

Determination of the Physical Conditions of the Knots in the Helix Nebula from Optical and Infrared Observations
We use new Hubble Space Telescope and archived images to clarify thenature of the ubiquitous knots in the Helix Nebula, which are variouslyestimated to contain a significant to majority fraction of the materialejected by its central star. We employ published far-infraredspectrophotometry and existing 2.12 μm images to establish that thepopulation distribution of the lowest rovibrational states ofH2 is close to the distribution of a gas in localthermodynamic equilibrium at 988+/-119 K. In addition, we presentcalculations that show that the weakness of the H2 0-0 S(7)line is not a reason for making the unlikely-to-be-true assumption thatH2 emission is caused by shock excitation. We derive a totalflux from the nebula in H2 lines and compare this with thepower available from the central star for producing this radiation. Weestablish that neither soft X-rays nor 912-1100 Å radiation hasenough energy to power the H2 radiation-only the stellarextreme-ultraviolet radiation shortward of 912 Å does. New imagesof the knot 378-801 in the H2 2.12 μm line reveal that the2.12 μm cusp lies immediately inside the ionized atomic gas zone.This property is shared by material in the ``tail'' region. TheH2 2.12 μm emission of the cusp confirms previousassumptions, while the tail's property firmly establishes that the tailstructure is an ionization-bounded radiation shadow behind the opticallythick core of the knot. A unique new image of a transitional region ofthe nebula's inner disk in the He II λ4686 line fails to show anyemission from knots that might have been found in the He++core of the nebula. We also re-examined high signal-to-noise ratioground-based telescope images of this same inner region and found noevidence of structures that could be related to knots.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.

Molecular and Atomic Excitation Stratification in the Outflow of the Planetary Nebula M27
High-resolution spectroscopy with FUSE and HST STIS of atomic andmolecular velocity stratification in the nebular outflow of M27challenge models of the abundance kinematics in planetary nebulae. Thesimple picture of a very high speed (~1000 km s-1),high-ionization, radiation-driven stellar wind surrounded by a slower(~10 km s-1) mostly molecular outflow, with low-ionizationand neutral atomic species residing at the wind interaction interface,is not supported. Instead, we find vibrationally excited H2intermixed with mostly neutral atomic species at a transition velocity(33 km s-1) between a fast (33-65 km s-1)low-ionization zone and a slow (<~33 km s-1)high-ionization zone. Lyα fluorescence of H2 has beendetected, but far-UV continuum fluorescence has not. The diffuse nebularmedium is inhospitable to molecules and dust. Maintaining a modestequilibrium abundance of H2 [N(H2)/N(HI)<<1] in the diffuse nebular medium requires a source ofH2, mostly likely the clumpy nebular medium. The stellar SEDshows no sign of reddening [E(B-V)<0.01], but paradoxicallyHα/Hβ indicates E(B-V)~0.1. The enhancement ofHα/Hβ in the absence of dust may result from a two-stepprocess of H2 ionization by Lyman continuum photons followedby dissociative recombination [H 2 + γ -->H+2 + e- --> H(1s) + H(nl)], whichultimately produces fluorescence of Hα and Lyα. In theoptically thin limit at the inferred radius of the velocity transition,we find that dissociation of H2 by stellar Lyman continuumphotons is an order of magnitude more efficient than spontaneousdissociation by far-UV photons. The importance of this H2destruction process in H II regions has been overlooked.

Discovery of photospheric argon in very hot central stars of planetary nebulae and white dwarfs
Context: We report the first discovery of argon in hot evolved stars andwhite dwarfs. We have identified the Ar VII 1063.55 Å line in someof the hottest known (T eff = 95 000-110 000 K) central starsof planetary nebulae and (pre-) white dwarfs of various spectral type. Aims: We determine the argon abundance and compare it to theoreticalpredictions from stellar evolution theory as well as from diffusioncalculations. Methods: We analyze high-resolution spectra taken withthe Far Ultraviolet Spectroscopic Explorer. We use non-LTEline-blanketed model atmospheres and perform line-formation calculationsto compute synthetic argon line profiles. Results: We find a solarargon abundance in the H-rich central star NGC 1360 and in theH-deficient PG 1159 star PG 1424+535. This confirms stellar evolutionmodeling that predicts that the argon abundance remains almostunaffected by nucleosynthesis. For the DAO-type central star NGC 7293and the hot DA white dwarfs PG 0948+534 and RE J1738+669 we find argonabundances that are up to three orders of magnitude smaller thanpredictions of calculations assuming equilibrium of radiative levitationand gravitational settling. For the hot DO white dwarf PG 1034+001 thetheoretical overprediction amounts to one dex. Conclusions: Our resultsconfirm predictions from stellar nucleosynthesis calculations for theargon abundance in AGB stars. The argon abundance found in hot whitedwarfs, however, is another drastic example that the current state ofequilibrium theory for trace elements fails to explain the observationsquantitatively.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by the Johns HopkinsUniversity under NASA contract NAS5-32985.

A Debris Disk around the Central Star of the Helix Nebula?
Excess emission from a pointlike source coincident with the central starof the Helix Nebula is detected with Spitzer at 8, 24, and 70 μm. At24 μm, the central source is superposed on an extended diffuseemission region. While the [O IV] 25.89 μm line contributes to thediffuse emission, a 10-35 μm spectrum of the central source shows astrong thermal continuum. The excess emission from the star most likelyoriginates from a dust disk with blackbody temperatures of 90-130 K.Assuming a simple optically thin debris disk model, the dust isdistributed in a ring between ~35 and ~150 AU from the central star,possibly arising from collisions of Kuiper Belt-like objects or thebreakup of comets from an Oort-like cloud that have survived from thepost-main-sequence evolution of the central star.

Optical Multicolor Photometry of Spectrophotometric Standard Stars
Photoelectric data on the Johnson-Kron-Cousins UBVRI broadbandphotometric system are provided for a set of stars that have been usedas spectrophotometric standard stars for the Hubble Space Telescope.

Trigonometric Parallaxes of Central Stars of Planetary Nebulae
Trigonometric parallaxes of 16 nearby planetary nebulae are presented,including reduced errors for seven objects with previous initial resultsand results for six new objects. The median error in the parallax is0.42 mas, and 12 nebulae have parallax errors of less than 20%. Theparallax for PHL 932 is found here to be smaller than was measured byHipparcos, and this peculiar object is discussed. Comparisons are madewith other distance estimates. The distances determined from theseparallaxes tend to be intermediate between some short distance estimatesand other long estimates; they are somewhat smaller than those estimatedfrom spectra of the central stars. Proper motions and tangentialvelocities are presented. No astrometric perturbations from unresolvedclose companions are detected.

Molecules in (proto)planetary Nebulae
This contribution reviews results on the structure and dynamics ofplanetary and protoplanetary nebulae from observations of molecular lineemission. Molecular line data have been particularly useful to study thevery fast evolution of these nebulae. Our knowledge on the molecularcontent of these objects, i.e., the molecule abundances, is alsosummarized.

Evolution of the Circumstellar Molecular Envelope. I. A BIMA CO Survey of Evolved Stars
This paper reports the results of a small imaging survey of eightevolved stars including two AGB stars (IRC +10216 and Mira), fiveproto-planetary nebula (PPN) candidates (AFGL 2688, IRAS 22272+5435, HD161796, 89 Her, and HD 179821), and a planetary nebula (PN, NGC 7027).We present high-resolution 12CO J=1-->0 maps of their fullmolecular envelopes made by combining BIMA Millimeter Array and NRAO 12m telescope observations. For the PPNe and PN, the neutral molecularenvelopes are compared with images taken at optical, near-IR, and mid-IRwavelengths. Drawing from the literature, we augmented our BIMA surveysample to 38 well-studied sources with CO emission maps. We classifiedthis sample of sources based on the kinematics and morphologies of theCO emission into three types: spherical/elliptical/shell sources, disksources, and structured outflow sources. Confirming previous studies, wefind strong evidence for the photodissociation of the molecular envelopeas an object evolves from the AGB to PN stages. While the spherical AGBstars follow theoretical expectations for mass-loss rate versus envelopesize, the post-AGB structured outflow sources have significantly highermass-loss rates than expected probably because of their recentsuperwinds. We find evidence that the structured outflows are clearlyyounger than the AGB wind. The disk sources have little correlationbetween mass-loss rate and envelope size because their properties aredetermined more by the properties of the central stars and diskevolution than by the mass-loss rate history that shapes the sphericaland structured-outflow sources.

Circumstellar Atomic Hydrogen in Evolved Stars
We present new results of a spectroscopic survey of circumstellar H I inthe direction of evolved stars made with the NançayRadiotelescope. The H I line at 21 cm has been detected in thecircumstellar shells of a variety of evolved stars: asymptotic giantbranch stars, oxygen-rich and carbon-rich stars, semiregular and Miravariables, and planetary nebulae. The emissions are generally spatiallyresolved, i.e., larger than 4', indicating shell sizes on the order of 1pc, which opens the possibility of tracing the history of mass loss overthe past ~104-105 yr. The line profiles aresometimes composite. The individual components generally have aquasi-Gaussian shape; in particular, they seldom show the double-hornprofile that would be expected from the spatially resolved opticallythin emission of a uniformly expanding shell. This probably implies thatthe expansion velocity decreases outward in the external shells (0.1-1pc) of these evolved stars. The H I line profiles do not necessarilymatch those of the CO rotational lines. Furthermore, the centroidvelocities do not always agree with those measured in the CO linesand/or the stellar radial velocities. The H I emissions may also beshifted in position with respect to the central stars. Without excludingthe possibility of asymmetric mass ejection, we suggest that these twoeffects could also be related to a nonisotropic interaction with thelocal interstellar medium. H I was detected in emission toward severalsources (ρ Per, α Her, δ2 Lyr, U CMi) thatotherwise have not been detected in any radio lines. Conversely, it wasnot detected in the two oxygen-rich stars with substantial mass-lossrate, NML Tau and WX Psc, possibly because these sources are young, withhydrogen in molecular form, and/or because the temperature of thecircumstellar H I gas is very low (<5 K).This paper is dedicated to the memory of Marie-Odile Mennessier(1940-2004).

Two new evolved bipolar planetary nebulae in the solar neighbourhood
We present AAO/UKST Hα+[N II] narrow-band imagery and low- andmedium-resolution optical spectroscopy of RCW24 and RCW69. These nebulaewere previously classified as HII regions, but we now show them to betwo of the largest and nearest bipolar Type I PNe yet discovered.Distances were estimated using extinction-distance and kinematicmethods, and via a new Hα surface brightness-radius relation. Theadopted distances are 1.0 +/- 0.3kpc for RCW24 and 1.3 +/- 0.2kpc forRCW69. Both objects have enhanced nitrogen abundances, withlogN/O~=+0.44 for RCW24, and logN/O=+0.33 for RCW 69. Systemicvelocities and |z| distances are VLSR = +5 km s-1and |z| ~ 23pc for RCW 24, and VLSR = -33 km s-1and only |z| ~ 7pc for RCW 69. Both PNe originated from massiveprogenitors (>2.0-2.5Msolar), as deduced from theirchemical abundances, large ionized masses, small |z| distances, lowpeculiar velocities and relatively hot central stars. These two objectsform an important addition to the small sample of evolved bipolar PNe inthe solar neighbourhood.

Infrared Observations of the Helix Planetary Nebula
We have mapped the Helix (NGC 7293) planetary nebula (PN) with the IRACinstrument on the Spitzer Space Telescope. The Helix is one of theclosest bright PNs and therefore provides an opportunity to resolve thesmall-scale structure in the nebula. The emission from this PN in the5.8 and 8 μm IRAC bands is dominated by the pure rotational lines ofmolecular hydrogen, with a smaller contribution from forbidden lineemission such as [Ar III] in the ionized region. The IRAC images resolvethe ``cometary knots,'' which have been previously studied in this PN.The ``tails'' of the knots and the radial rays extending into the outerregions of the PN are seen in emission in the IRAC bands. IRS spectra onthe main ring and the emission in the IRAC bands are consistent withshock-excited H2 models, with a small (~10%) component fromphotodissociation regions. In the northeast arc, the H2emission is located in a shell outside the Hα emission.

Shadows and Photoevaporated Flows from Neutral Clumps Exposed to Two Ionizing Sources
Neutral clumps immersed in HII regions are frequently found in starformation regions. We investigate here the formation of tails of neutralgas, which are not reached by the direct ionizing flux coming from twomassive stars, using both an analytical approximation that allows us toestimate the shadow geometry behind the clumps for different initialgeometric configurations, and three-dimensional numerical simulations.We find a good agreement between both approaches to this theoreticallyinteresting problem. A particularly important application could be theproplyds that are found in the Trapezium cluster in Orion, which arebeing photoevaporated primordially by the O stars θ^{1} Ori C andθ^{2} Ori A.

Spitzer MIPS Imaging of NGC 650: Probing the History of Mass Loss on the Asymptotic Giant Branch
We present the far-infrared (IR) maps of a bipolar planetary nebula(PN), NGC 650, at 24, 70, and 160 μm, taken with the MultibandImaging Photometer for Spitzer (MIPS) on board the Spitzer SpaceTelescope. While the two-peak emission structure seen in all MIPS bandssuggests the presence of a near edge-on dusty torus, the distinctemission structure between the 24 μm map and the 70/160 μm mapsindicates the presence of two distinct emission components in thecentral torus. Based on the spatial correlation of these two far-IRemission components with respect to various optical line emission, weconclude that the 24 μm emission is largely due to the [O IV] line at25.9 μm arising from highly ionized regions behind the ionizationfront, whereas the 70 and 160 μm emissions are due to dust continuumarising from low-temperature dust in the remnant asymptotic giant branch(AGB) wind shell. The far-IR nebula structure also suggests that theenhancement of mass loss at the end of the AGB phase has occurredisotropically, but has ensued only in the equatorial directions whileceasing in the polar directions. The present data also show evidence forthe prolate spheroidal distribution of matter in this bipolar PN. TheAGB mass-loss history reconstructed in this PN is thus consistent withwhat has been previously proposed based on the past optical and mid-IRimaging surveys of the post-AGB shells.

The tails in the Helix Nebula NGC 7293
Aims.We have examined a stream-source model for the production of thecometary tails observed in the Helix Nebula NGC 7293 in which atransonic or moderately supersonic stream of ionized gas overruns asource of ionized gas. We have compared the velocity structurescalculated with the available observational data. We have alsoinvestigated the suggestion that faint striations visible in the nebulargas are the decaying tails of now destroyed cometary globules.Methods: .We have selected relevant results from extensive hydrodynamiccalculations made with the COBRA code. Results: .The velocitiescalculated are in good agreement with the observational data on tailvelocities and are consistent with observations of the nebularstructure. The results also are indicative of a stellar atmosphereorigin for the cometary globules. Tail remnants persist for timescaleslong enough for their identification with striations to be plausible.

An Extended FUSE Survey of Diffuse O VI Emission in the Interstellar Medium
We present a survey of diffuse O VI emission in the interstellar medium(ISM) obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE).Spanning 5.5 yr of FUSE observations, from launch through 2004 December,our data set consists of 2925 exposures along 183 sight lines, includingall of those with previously published O VI detections. The data wereprocessed using an implementation of CalFUSE version 3.1 modified tooptimize the signal-to-noise ratio and velocity scale of spectra from anaperture-filling source. Of our 183 sight lines, 73 show O VIλ1032 emission, 29 at >3 σ significance. Six of the 3σ features have velocities |vLSR|>120 kms-1, while the others have |vLSR|<=50 kms-1. Measured intensities range from 1800 to 9100 LU (lineunit; 1 photon cm-2 s-1 sr-1), with amedian of 3300 LU. Combining our results with published O VI absorptiondata, we find that an O VI-bearing interface in the local ISM yields anelectron density ne=0.2-0.3 cm-3 and a path lengthof 0.1 pc, while O VI-emitting regions associated with high-velocityclouds in the Galactic halo have densities an order of magnitude lowerand path lengths 2 orders of magnitude longer. Although the O VIintensities along these sight lines are similar, the emission isproduced by gas with very different properties.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by Johns HopkinsUniversity under NASA contract NAS5-32985.

The transparency of the Universe limited by Lyα clouds
The brightnesses of supernovae are commonly understood to indicate thatcosmological expansion is accelerating due to dark energy. However theentire discussion presumes a perfectly transparent universe because noeffects of reddening associated with the interstellar extinction law areseen. We note that with two kinds of dark matter (baryonic andnon-baryonic) strongly dominating the known mass of the universe, it isseriously premature to assume that these dark matter components have notreduced the transmission of the universe for cosmological sources. Weshow that the long-known Lyα clouds, if nucleated by thepopulation of baryonic dark matter primordial planetoids indicated byquasar microlensing, would act as spherical lenses and achromaticallyfade cosmologically distant sources. We attempt to estimate the amountof this cosmological fading, but ultimately the calculation is limitedby lack of a satisfactory model for the tenuous outer parts of aprimordial planetoid. We also consider the effects of such cosmologicalfading on the light of quasars.

The Dynamical Evolution of Planetary Nebulae after the Fast Wind
We explore the dynamics of ionization-bounded planetary nebulae afterthe termination of the fast stellar wind. When the stellar wind becomesnegligible, the hot, shocked bubble depressurizes, and the thermalpressure of the photoionized region, at the inner edge of the swept-upshell, becomes dominant. At this stage the shell tends to fragment,creating clumps with comet-like tails and long, photoionized trails inbetween, while the photoionized material expands back toward the centralstars as a rarefaction wave. Once the photoionized gas fills the innercavity, it develops a kinematical pattern of increasing velocity fromthe center outward, with a typical range of velocities starting from thesystemic velocity to ~50 km s-1 at the edges. The HelixNebula is a clear example of a planetary nebula at this lateevolutionary stage.

Chemical abundances for Hf 2-2, a planetary nebula with the strongest-known heavy-element recombination lines
We present high-quality optical spectroscopic observations of theplanetary nebula (PN) Hf2-2. The spectrum exhibits many prominentoptical recombination lines (ORLs) from heavy-element ions. Analysis ofthe HI and HeI recombination spectrum yields an electron temperature of~900 K, a factor of 10 lower than given by the collisionally excited[OIII] forbidden lines. The ionic abundances of heavy elements relativeto hydrogen derived from ORLs are about a factor of 70 higher than thosededuced from collisionally excited lines (CELs) from the same ions, thelargest abundance discrepancy factor (adf) ever measured for a PN. Bycomparing the observed OIIλ4089/λ4649 ORL ratio totheoretical value as a function of electron temperature, we show thatthe OII ORLs arise from ionized regions with an electron temperature ofonly ~630 K. The current observations thus provide the strongestevidence that the nebula contains another previously unknown componentof cold, high-metallicity gas, which is too cool to excite anysignificant optical or ultraviolet CELs and is thus invisible via suchlines. The existence of such a plasma component in PNe provides anatural solution to the long-standing dichotomy between nebular plasmadiagnostics and abundance determinations using CELs on the one hand andORLs on the other.

On the Hydrodynamic Interaction of Shock Waves with Interstellar Clouds. II. The Effect of Smooth Cloud Boundaries on Cloud Destruction and Cloud Turbulence
The effect of smooth cloud boundaries on the interaction of steadyplanar shock waves with interstellar clouds is studied using a localadaptive mesh refinement technique with an axisymmetric Godunovhydrodynamic scheme. A three-dimensional calculation is also done toconfirm the two-dimensional results. We find that smooth cloudboundaries significantly affect cloud morphology and retard clouddestruction. After shock passage, a sharp density jump forms due tovelocity gradients generated in the smooth cloud boundary. We refer tothis density jump as a ``slip surface'' because the velocity is shearedparallel to its surface. The formation of a slip surface leads tocomplete cloud destruction because of the Kelvin-Helmholtz andRayleigh-Taylor instabilities. We construct analytic models of clouddrag and vorticity generation that compare well with the numericalresults. Small shreds formed by the instabilities have significantvelocity dispersions of 10%-20% of the ambient shock velocity. Theycould be related to the small cold H I clouds recently observed byStanimirović & Heiles. The dependence of the velocitydispersion on region size, the so-called line width-size relation, isfound to be time-dependent. In the early stages, the line width-sizerelation is more or less flat because of the significant small-scalefluctuations generated by the Kelvin-Helmholtz instability. In the laterstages, the small-scale fluctuations tend to damp, leading to a linewidth that increases with size. The possibility of gravitationalinstability triggered by shock compression is discussed. We show thatgravitational collapse can be induced in an initially uniform cloud by aradiative shock (γ<4/3) only if it is not too strong andnonthermal motions are weak.

Hybrid characteristics: 3D radiative transfer for parallel adaptive mesh refinement hydrodynamics
We have developed a three-dimensional radiative transfer method designedspecifically for use with parallel adaptive mesh refinementhydrodynamics codes. This new algorithm, which we call hybridcharacteristics, introduces a novel form of ray tracing that can neitherbe classified as long, nor as short characteristics, but which appliesthe underlying principles, i.e. efficient execution throughinterpolation and parallelizability, of both.
Primary applicationsof the hybrid characteristics method are radiation hydrodynamicsproblems that take into account the effects of photoionization andheating due to point sources of radiation. The method is implemented inthe hydrodynamics package FLASH. The ionization, heating, and coolingprocesses are modelled using the DORIC ionization package. Uponcomparison with the long characteristics method, we find that our methodcalculates the column density with a similarly high accuracy andproduces sharp and well defined shadows. We show the quality of the newalgorithm in an application to the photoevaporation of multipleover-dense clumps.
We present several test problems demonstratingthe feasibility of our method for performing high resolutionthree-dimensional radiation hydrodynamics calculations that span a largerange of scales. Initial performance tests show that the ray tracingpart of our method takes less time to execute than other parts of thecalculation (e.g. hydrodynamics and adaptive mesh refinement), and thata high degree of efficiency is obtained in parallel execution. Althoughthe hybrid characteristics method is developed for problems involvingphotoionization due to point sources, and in its current implementationignores the effects of diffuse radiation and scattering, the algorithmcan be easily adapted to the case of more general radiation fields.

The structure of planetary nebulae: theory vs. practice
Context.This paper is the first in a short series dedicated to thelong-standing astronomical problem of de-projecting the bi-dimensional,apparent morphology of a three-dimensional mass of gas. Aims.Wefocus on the density distribution in real planetary nebulae (and alltypes of expanding nebulae). Methods. We introduce some basictheoretical notions, discuss the observational methodology, and developan accurate procedure for determining the matter radial profile withinthe sharp portion of nebula in the plane of the sky identified by thezero-velocity-pixel-column (zvpc) of high-resolution spectral images.Results. The general and specific applications of the method (andsome caveats) are discussed. Moreover, we present a series of evolutivesnapshots, combining illustrative examples of both model and trueplanetary nebulae. Conclusions. The zvpc radial-densityreconstruction - added to tomography and 3D recovery developed at theAstronomical Observatory of Padua (Italy) - constitutes a very usefultool for looking more closely at the spatio-kinematics, physicalconditions, ionic structure, and evolution of expanding nebulae.

The Origin and Physical Properties of the Cometary Knots in NGC 7293
On the basis that the cometary knots observed in the Helix Nebula formas a result of larger ``parent clouds'' breaking up due toRayleigh-Taylor instability induced by radiative acceleration of theclouds, we compute characteristics of the cometary knots and of theparent clouds as well. Present observations place constraints on thepositions, velocities, and sizes of the parent clouds. Requiring theclouds to produce cometary knots that are stable places furtherconstraints on the properties of parent clouds. We formulate thoseconstraints and show how they further limit the predicted properties ofthe cometary knots and bring them into agreement with their observedproperties.

Morphological Analysis of the Centimeter-Wave Continuum in the Dark Cloud LDN 1622
The spectral energy distribution of the dark cloud LDN 1622, as measuredby Finkbeiner using WMAP data, drops above 30 GHz and is suggestive of aBoltzmann cutoff in grain rotation frequencies, characteristic ofspinning dust emission. LDN 1622 is conspicuous in the 31 GHz image weobtained with the Cosmic Background Imager, which is the firstcentimeter-wave resolved image of a dark cloud. The 31 GHz emissionfollows the emission traced by the four IRAS bands. The normalizedcross-correlation of the 31 GHz image with the IRAS images is higher by6.6 σ for the 12 and 25 μm bands than for the 60 and 100 μmbands: C12+25=0.76+/-0.02, andC60+100=0.64+/-0.01. The mid-IR-centimeter-wave correlationin LDN 1622 is evidence for very small grain (VSG) or continuum emissionat 26-36 GHz from a hot molecular phase. In dark clouds and theirphoton-dominated regions (PDRs), the 12 and 25 μm emission isattributed to stochastic heating of the VSGs. The mid-IR andcentimeter-wave dust emissions arise in a limb-brightened shellcoincident with the PDR of LDN 1622, where the incident UV radiationfrom the Ori OB 1b association heats and charges the grains, as isrequired for spinning dust.

Dust around Type Ia Supernovae
An explanation is given of the low value ofRλ≡Aλ/E(B-V), the ratio ofabsolute to selective extinction deduced from Type Ia supernovaobservations. The idea involves scattering by dust clouds located in thecircumstellar environment or at the highest velocity shells of thesupernova ejecta. The scattered light tends to reduce the effectiveRλ in the optical but has an opposite effect in theultraviolet. The presence of circumstellar dust can be tested byultraviolet to near-infrared observations and by multiepochspectropolarimetry of Type Ia supernovae.

Planetary Nebulae and Stellar Kinematics in the Flattened Elliptical Galaxy NGC 1344
We present photometric and kinematic information obtained by measuring197 planetary nebulae (PNs) discovered in the flattened Fornaxelliptical galaxy NGC 1344 (also known as NGC 1340) with an on-band,off-band, and grism+on-band filter technique. We build the PN luminosityfunction (PNLF) and use it to derive a distance modulus m-M=31.4+/-0.18,which is slightly smaller than, but in good agreement with, the surfacebrightness fluctuation distance. The PNLF also provides an estimate ofthe specific PN formation rate: (6+/-3)×10-12 PNsyr-1 L-1solar. If we combine thepositional information from the on-band image with PN positions measuredon the grism+on-band image, we can measure the radial velocities of 195PNs, some of them distant more than three effective radii from thecenter of NGC 1344. We complement this data set with stellar kinematicsderived from integrated spectra along the major and minor axes andparallel to the major axis of NGC 1344. The line-of-sight velocitydispersion profile indicates the presence of a dark matter halo aroundthis galaxy.Some of the data presented herein were obtained at the European SouthernObservatory, Chile, in programs ESO 67.B-0231 and 68.B-0173A.

Echelle Spectroscopy of a Gamma-Ray Burst Afterglow at z = 3.969: A New Probe of the Interstellar and Intergalactic Media in the Young Universe
We present an echelle spectrum of the Swift GRB 050730, obtained 4 hrafter the burst using the MIKE spectrograph on the Magellan Claytelescope when the afterglow was at R=17.7. The spectrum reveals aforest of absorption features superposed on a simple power-law shapedcontinuum, best described asfν(λ)~λα withα=1.88+/-0.01 over λ=7000-9000 Å. We identify thegamma-ray burst (GRB) host at zGRB=3.96855 based on thehydrogen Lyman absorption series, narrow absorption lines due to heavyions such as O I, C II, Si II, S II, Ni II, Fe II, C IV, Si IV, and N V,and fine-structure transitions such as O I*, O I**, Si II*, C II*, andFe II*. Together these transitions allow us to study the properties ofthe interstellar medium (ISM) in the GRB host. The principal results areas follows. (1) We estimate a neutral hydrogen column density of logN(HI)=22.15+/-0.05 in the host. (2) The associated metal lines exhibitmultiple components over a velocity range of ~80 km s-1, with>90% of the neutral gas confined in 20 km s-1. (3)Comparisons between different ionic transitions show that the host haslittle or no dust depletion and has 1/100 solar metallicity. (4) Theabsorbing gas has much higher density than that of intervening dampedLyα absorption (DLA) systems. In addition, we report theidentification of an intervening DLA system at zDLA=3.56439with logN(H I)=20.3+/-0.1 and <5% solar metallicity, a Lyman limitsystem at zLLS=3.02209 with logN(H I)=19.9+/-0.1, a strong MgII absorber at zMgII=2.25313, and a pair of Mg II absorbersat zMgII=1.7731, 57 km s-1 apart. We demonstratethat rapid echelle spectroscopy of GRB afterglows helps to reveal awealth of information in the ISM and the intergalactic medium along thesight line, which, when followed up with late-time deep imaging, willallow us to uncover a sample of distant galaxies with known ISMproperties to constrain galaxy formation models.

The 3-D Structure of the Helix Nebula
The 3-D structure of the Helix Nebula has been addressed multiple timesand is slowly yielding to application of monochromatic emission lineimaging, high resolution spectroscopy, and photoionization theory. Theinner structure of the nebula is a toroidal disk filled with doublyionized helium and is ionization bounded, with an extended verticalcomponent resembling the structure of numerous bipolar nebulae. Theouter structure is less well defined, with one construction being thatthere is an open-center outer-disk that is nearly perpendicular to theinner-disk and an alternate construction that explains the features seenjust outside the inner-disk as being the result of the observer's lineof sight passing through extended lobes perpendicular to the inner-disk.A definitive model awaits thorough velocity mapping in the majordiagnostic emission lines. However, even our well-defined knowledge ofthe inner-disk defies explanation by the simplest application of thebroadly accepted two-wind model for the formation of PN.

The kinematics of the large western knot in the halo of the young planetary nebula NGC 6543
A detailed analysis of the dominant ionized knot in the halo of theplanetary nebula NGC 6543 is presented. Observations were made at highspectral and spatial resolution of the [OIII]λ5007-Ålineusing the Manchester echelle spectrometer combined with the 2.1-m SanPedro Martir Telescope. A 20-element multislit was stepped across thefield to give almost complete spatial coverage of the large western knotand surrounding halo.The spectra reveal, for the first time, gas flows around thekinematically inert knot. The gas flows are found to have velocitiescomparable to the sound speed as gas is photoevaporated off an ionizedsurface. No evidence is found of fast wind interaction with the knot,and we find it likely that the fast wind is still contained in apressure-driven bubble in the core of the nebula. This rules out thepossibility of the knot having its origin in instabilities at theinterface of the fast and asymptotic giant branch (AGB) stellar winds.We suggest that the knot is embedded in the slowly expanding red giantwind, and that its surfaces are being continually photoionized by thecentral star.

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Observation and Astrometry data

Right ascension:22h29m38.55s
Apparent magnitude:99.9

Catalogs and designations:
Proper NamesHelix Nebula
Ojo de Dios   (Edit)
NGC 2000.0NGC 7293

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