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NGC 2392 (Eskimo nebula)



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Optically Thick Radio Cores of Narrow-Waist Bipolar Nebulae
We report our search for optically thick radio cores in 16 narrow-waistbipolar nebulae. Optically thick cores are a characteristic signature ofcollimated ionized winds. Eleven northern nebulae were observed with theVery Large Array (VLA) at 1.3 and 0.7 cm, and five southern nebulae wereobserved with the Australia Telescope Compact Array (ATCA) at 6 and 3.6cm. Two northern objects, 19W32 and M1-91, and three southern objects,He 2-25, He 2-84, and Mz 3, were found to exhibit a compact radio corewith a rising spectrum consistent with an ionized jet. Such jets havebeen seen in M2-9 and may be responsible for shaping bipolar structurein planetary nebulae.

X-ray emission from planetary nebulae calculated by 1D spherical numerical simulations
We calculate the X-ray emission from both constant and time-evolvingshocked fast winds blown by the central stars of planetary nebulae (PNe)and compare our calculations with observations. Using sphericallysymmetric numerical simulations with radiative cooling, we calculate theflow structure and the X-ray temperature and luminosity of the hotbubble formed by the shocked fast wind. We find that a constant fastwind gives results that are very close to those obtained from theself-similar solution. We show that in order for a fast shocked wind toexplain the observed X-ray properties of PNe, rapid evolution of thewind is essential. More specifically, the mass-loss rate of the fastwind should be high early on when the speed is ~300-700 kms-1, and then it needs to drop drastically by the time the PNage reaches ~1000 yr. This implies that the central star has a veryshort pre-PN (post-asymptotic giant branch) phase.

XMM-Newton Observations of the Bipolar Planetary Nebulae NGC 2346 and NGC 7026
We have obtained X-ray observations of the bipolar planetary nebulae(PNe) NGC 2346 and NGC 7026 with XMM-Newton. These observations detecteddiffuse X-ray emission from NGC 7026 but not from NGC 2346. The X-rayemission from NGC 7026 appears to be confined within the bipolar lobesof the PN and has spectral properties suggesting a thermal plasmaemitting at a temperature of1.1+0.5-0.2×106 K. The X-rayspectrum of NGC 7026 is modeled using nebular and stellar abundances toassess whether a significant amount of nebular material has been mixedinto the shocked wind, but the results of this comparison are notconclusive owing to the small number of counts detected. Observations ofbipolar PNe indicate that diffuse X-ray emission is much less likelydetected in open-lobed nebulae than closed-lobed nebulae, possiblybecause open-lobed nebulae do not have strong fast winds or are unableto retain hot gas.

Planetary Nebula Abundances and Morphology: Probing the Chemical Evolution of the Milky Way
This paper presents a homogeneous study of abundances in a sample of 79northern Galactic planetary nebulae (PNe) whose morphological classeshave been uniformly determined. Ionic abundances and plasma diagnosticswere derived from selected optical line strengths in the literature, andelemental abundances were estimated with the ionization correctionfactor developed by Kingsbourgh & Barlow in 1994. We compare theelemental abundances to the final yields obtained from stellar evolutionmodels of low- and intermediate-mass stars, and we confirm that mostbipolar PNe have high nitrogen and helium abundance and are the likelyprogeny of stars with main-sequence mass greater than 3Msolar. We derive =0.27 and discuss the implication of such ahigh ratio in connection with the solar neon abundance. We determine theGalactic gradients of oxygen and neon and foundΔlog(O/H)/ΔR=-0.01 dex kpc-1 andΔlog(Ne/H)/ΔR=-0.01 dex kpc-1. These flat PNgradients are irreconcilable with Galactic metallicity gradientsflattening with time.

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.

Bubbles in planetary nebulae and clusters of galaxies: jet bending
We study the bending of jets in binary stellar systems. A compactcompanion accretes mass from the slow wind of the mass-losing primarystar, forms an accretion disc and blows two opposite jets. These fastjets are bent by the slow wind. Disregarding the orbital motion, we findthe dependence of the bending angle on the properties of the slow windand the jets. Bending of jets is observed in planetary nebulae which arethought to be the descendants of interacting binary stars. For example,in some of these planetary nebulae, the two bubbles (lobes) which areinflated by the two opposite jets are displaced to the same side of thesymmetry axis of the nebula. Similar displacements are observed inbubble pairs in the centre of some clusters and groups of galaxies. Wecompare the bending of jets in binary stellar systems with that inclusters of galaxies.

X-ray emission by a shocked fast wind from the central stars of planetary nebulae
We calculate the X-ray emission from the shocked fast wind blown by thecentral stars of planetary nebulae (PNe) and compare with observations.Using spherically symmetric self-similar solutions, we calculate theflow structure and X-ray temperature for a fast wind slamming into apreviously ejected slow wind. We find that the observed X-ray emissionof six PNe can be accounted for by shocked wind segments that wereexpelled during the early-PN phase, if the fast wind speed is moderate,v2~ 400-600kms-1, and the mass-loss rate is a fewtimes 10-7Msolaryr-1. We find, asproposed previously, that the morphology of the X-ray emission is in theform of a narrow ring inner to the optical bright part of the nebula.The bipolar X-ray morphology of several observed PNe, which indicates animportant role of jets, rather than a spherical fast wind, cannot beexplained by the flow studied here.

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.

Iron Depletion in the Hot Bubbles in Planetary Nebulae
We have searched for the emission from [Fe X] λ6374 and [Fe XIV]λ5303 that is expected from the gas emitting in diffuse X-rays inBD +30°3639, NGC 6543, NGC 7009, and NGC 7027. Neither line wasdetected in any object. Models that fit the X-ray spectra of theseobjects indicate that the [Fe X] λ6374 emission should be belowour detection thresholds, but the predicted [Fe XIV] λ5303emission exceeds our observed upper limits (1 σ) by factors of atleast 3.5-12. The best explanation for the absence of [Fe XIV]λ5303 is that the X-ray plasma is depleted in iron. In principle,this result provides a clear chemical signature that may be used todetermine the origin of the X-ray gas in either the nebular gas or thestellar wind. At present, although various lines of evidence appear tofavor a nebular origin, the lack of atmospheric and nebular ironabundances in the objects studied here precludes a definitiveconclusion.Based on observations obtained at the Observatorio AstronómicoNacional in San Pedro Mártir, Baja California, Mexico.Based on observations obtained with XMM-Newton, an ESA science missionwith instruments and contributions directly funded by ESA Member Statesand NASA.

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.

The distances of less-evolved planetary nebulae: a further test of statistical distance scales
It has recently been pointed out that a number of the methods used todetermine planetary nebulae (PNe) distances may be appreciably in error.Whilst the scales of Zhang (1995), Bensby & Lundstrom (2001) andothers are appropriate for higher radio brightness temperaturesTB, those of Phillips and Daub are more relevant whereTB is small.We note, in the following, that the absolute bolometric magnitudes ofless-evolved PNe are likely to be similar. The mean value of can therefore be used to constrain PNe distancesD, and confirm the distance scales for higher TB outflows. Wehave used this procedure to evaluate distances to a further 47 PNe, andwe find that the mean values of are consistent with those ofCahn, Kaler & Stanghellini (1992), Zhang (1995), Phillips et al.(2004) and van de Steene & Zijlstra (1995). They are, as expected,inconsistent with the lower TB scale of Phillips (2002a).

The mean properties of planetary nebulae as a function of Peimbert class
Planetary nebulae are known to possess a broad range of abundances, andthese (with other characteristics) have been used to define five classesof outflow. Peimbert Type I sources, for instance, possess high N and Heabundances, filamentary structures, and low mean scaleheights above theGalactic plane, whilst those of Type III have much lower abundances,high peculiar velocities, and belong to the Galactic thick disc. Apartfrom some rather ill-defined indications, however, very little is knownconcerning their mean physical, spatial, structural, kinematic andthermal characteristics.We have performed a comprehensive study of all of these properties, andfind evidence for strong variations between the various Peimbertclasses. Certain of these differences are consistent with Type I sourceshaving the highest progenitor masses, although it seems that thesenebulae also possess the lowest rms densities and 5-GHz brightnesstemperatures. The latter results are in conflict with a range of recentmodelling.

The dark lane of the planetary nebula NGC 6302*†
The butterfly-shaped planetary nebula, NGC 6302, shows a unique, denseequatorial dark lane, which is presumably a dusty disc, obscuring anunobserved, very hot central star. We trace the structure of this discusing Hubble Space Telescope Hα and [N II] images, Very LargeTelescope L-and M-band images at 0.4-arcsec resolution, includingBrα and polycyclic aromatic hydrocarbon (PAH) images, and a JamesClerk Maxwell Telescope 450-μm image. Extinction maps are derivedfrom these images. Within the disc, the extinction isAHα= 5-7 mag and ABrα= 1-2 mag. The450-μm map shows a north-south elongated central core, tracing themassive dust disc, and extended emission from dust in the bipolar flows.A fit to the spectral energy distribution yields the disc dust mass of0.03 Msolar. The innermost region shows an ionized shell. Theorientation of the polar axis shows a marked change between shell, discand inner and outer outflow. The structures are well described by thewarped-disc model of Icke (2003). PAH images are presented: PAH emissionis found in the shell but avoids the disc. An infrared source is foundclose to the expected location of the central star.

The Chemical Composition of Galactic Planetary Nebulae with Regard to Inhomogeneity in the Gas Density in Their Envelopes
The results of a study of the chemical compositions of Galacticplanetary nebulae taking into account two types of inhomogeneity in thenebular gas density in their envelopes are reported. New analyticalexpressions for the ionization correction factors have been derived andare used to determine the chemical compositions of the nebular gas inGalactic planetary nebulae. The abundances of He, N, O, Ne, S, and Arhave been found for 193 objects. The Y Z diagrams for various Heabundances are analyzed for type II planetary nebulae separately andjointly with HII regions. The primordial helium abundance Y p andenrichment ratio dY/dZ are determined, and the resulting values arecompared with the data of other authors. Radial abundance gradients inthe Galactic disk are studied using type II planetary nebulae.

Recombination Line versus Forbidden Line Abundances in Planetary Nebulae
Recombination lines (RLs) of C II, N II, and O II in planetary nebulae(PNs) have been found to give abundances that are much larger in somecases than abundances from collisionally excited forbidden lines (CELs).The origins of this abundance discrepancy are highly debated. We presentnew spectroscopic observations of O II and C II recombination lines forsix planetary nebulae. With these data we compare the abundances derivedfrom the optical recombination lines with those determined fromcollisionally excited lines. Combining our new data with publishedresults on RLs in other PNs, we examine the discrepancy in abundancesderived from RLs and CELs. We find that there is a wide range in themeasured abundance discrepancyΔ(O+2)=logO+2(RL)-logO+2(CEL),ranging from approximately 0.1 dex (within the 1 σ measurementerrors) up to 1.4 dex. This tends to rule out errors in therecombination coefficients as a source of the discrepancy. Most RLsyield similar abundances, with the notable exception of O II multipletV15, known to arise primarily from dielectronic recombination, whichgives abundances averaging 0.6 dex higher than other O II RLs. Wecompare Δ(O+2) against a variety of physical propertiesof the PNs to look for clues as to the mechanism responsible for theabundance discrepancy. The strongest correlations are found with thenebula diameter and the Balmer surface brightness; high surfacebrightness, compact PNs show small values of Δ(O+2),while large low surface brightness PNs show the largest discrepancies.An inverse correlation of Δ(O+2) with nebular densityis also seen. A marginal correlation of Δ(O+2) is foundwith expansion velocity. No correlations are seen with electrontemperature, He+2/He+, central star effectivetemperature and luminosity, stellar mass-loss rate, or nebularmorphology. Similar results are found for carbon in comparing C II RLabundances with ultraviolet measurements of C III].

Dust cloud formation in stellar environments. II. Two-dimensional models for structure formation around AGB stars
This paper reports on computational evidence for the formation ofcloud-like dust structures around C-rich AGB stars. This spatio-temporalstructure formation process is caused by a radiative/thermal instabilityof dust-forming gases as identified by Woitke et al. (2000, A&A,358, 665). Our 2D (axisymmetric) models combine a time-dependentdescription of the dust formation process according to Gail &Sedlmayr (1988, A&A, 206, 153) with detailed, frequency-dependentcontinuum radiative transfer by means of a Monte Carlo method (Niccoliniet al. 2003, A&A, 399, 703) in an otherwise static medium (v = 0).These models show that the formation of dust behind already condensedregions, which shield the stellar radiation field, is strongly favoured.In the shadow of these clouds the temperature decreases by severalhundred Kelvin, which triggers the subsequent formation of dust andensures its thermal stability. Considering an initially dust-free gaswith small density inhomogeneities, we find that finger-like duststructures develop which are cooler than the surroundings and pointtowards the centre of the radiant emission, similar to the“cometary knots” observed in planetary nebulae and starformation regions. Compared to a spherical symmetric reference model,the clumpy dust distribution has little effect on the spectral energydistribution, but dominates the optical appearance in near IRmonochromatic images.

Some implications of the introduction of scattered starlight in the spectrum of reddened stars
This paper presents new investigations on coherent scattering in theforward direction (orders of magnitude; conservation of energy;dependence of scattered light on geometry and wavelength), and on howscattered light contamination in the spectrum of reddened stars ispossibly related to as yet unexplained observations (the diminution ofthe 2200 Å bump when the obscuring material is close to the star,the difference between Hipparcos and photometric distances). This paperthen goes on to discuss the fit of the extinction curve, a possible roleof extinction by the gas in the far-UV, and the reasons of theinadequacy of the Fitzpatrick and Massa [ApJSS, 72 (1990) 163] fit.

XMM-Newton detection of hot gas in the Eskimo Nebula: Shocked stellar wind or collimated outflows?
The Eskimo Nebula (NGC 2392) is a double-shell planetary nebula (PN)known for the exceptionally large expansion velocity of its inner shell,˜90 km s-1, and the existence of a fast bipolar outflowwith a line-of-sight expansion velocity approaching 200 kms-1. We have obtained XMM-Newton observations of the Eskimoand detected diffuse X-ray emission within its inner shell. The X-rayspectra suggest thin plasma emission with a temperature of ˜2× 106 K and an X-ray luminosity of LX =(2.6±1.0) × 1031 (d/1150 pc)2 ergs-1, where d is the distance in parsecs. The diffuse X-rayemission shows noticeably different spatial distributions between the0.2-0.65 keV and 0.65-2.0 keV bands. High-resolution X-ray images of theEskimo are needed to determine whether its diffuse X-ray emissionoriginates from shocked fast wind or bipolar outflows.Based on observations obtained with XMM-Newton, an ESA science missionwith instruments and contributions directly funded by ESA Member Statesand NASA.

Radial-Velocity Survey of Central Stars of Southern Planetary Nebulae .
We have monitored selected southern-hemisphere planetary-nebula nuclei(PNNi) in order to search for radial-velocity (RV) variations. Theobservations have been carried out regularly since early 2003 with theSMARTS Consortium 1.5-m telescope and Cassegrain spectrograph at CerroTololo Inter-American Observatory, Chile. This study is a followup to anearlier survey of northern PNNi made by \citet{demarco04}, whichsuggested that there is a high incidence of RV variability among PNNi.If the variations are due to motion in binary orbits, the fraction ofclose binaries among PNNi must be very high, suggesting that mostplanetary nebulae are ejected through binary-star processes, such ascommon-envelope interactions. We presente here the results of thesouthern portion of our RV survey. Preliminary results indicate that thefraction of variable RVs is also very high among southern PNNi.

X-ray Observations of Hot Gas in Planetary Nebulae
The formation and shaping of planetary nebulae (PNe) is a complexprocess that involves the action of multiple agents, including faststellar winds and collimated outflows. Both fast stellar winds andcollimated outflows can produce shock-heated gas that emits diffuseX-rays. Hot gas in PN interiors was hinted by ROSAT observations,but unambiguous detections of diffuse X-ray emission were not made untilChandra and XMM-Newton became available. The unprecedentedangular resolution and sensitivity of these new X-ray observations allowus to investigate in detail the physical properties and origin of thehot gas content of PNe and to assess its dynamical effects on theshaping and expansion of PNe. This paper reviews the results from recentX-ray observations of PNe and discusses their implications to ourunderstanding of the formation and evolution of PNe.

The Rice University CCD Imager for the AEOS Telescope (poster)
Not Available

XMM-Newton Detection of Hot Gas in Two Evolved Elliptical Planetary Nebulae: the Eskimo Nebula and the Ghost of Jupiter
Planetary nebulae (PNe) consist of the stellar material ejected by low-and intermediate-mass stars (1-8 Mȯ) at the end of theasymptotic giant branch phase (AGB). As such a star evolves off the AGBphase, the copious mass-loss strips off the stellar envelope and exposesthe hot stellar core that ionizes the nebular material. The centralstars of PNe present fast stellar winds with terminal velocities1000-4000 km s-1, while fast collimated outflows withvelocities up to 1000 km s-1 are also observed in PNe. Theinteractions of the fast stellar wind and/or collimated outflows withnebular material can give rise to diffuse X-ray emission from PNe.Diffuse X-ray emission has been detected only in young PNe previously.To investigate the evolution of hot gas in PN interiors, we obtainedXMM-Newton observations of NGC 2392 (the Eskimo Nebula) and NGC 3242(the Ghost of Jupiter), two evolved elliptical PNe. Diffuse X-rayemission is detected in both nebulae. In both cases, the hot gas isconfined within the innermost shell, the X-ray spectrum can be describedby a thin plasma emission model with temperature˜2×106 K, and the X-ray luminosity is˜1×1031 ergs s-1. Furthermore, theX-ray spectrum of NGC 3242 shows evidence of enhanced nitrogenabundance, while the X-ray morphology of NGC 2392 hints a possibleassociation with its fast collimated outflows.

A deep survey of heavy element lines in planetary nebulae - II. Recombination-line abundances and evidence for cold plasma
In our Paper I, we presented deep optical observations of the spectra of12 Galactic planetary nebulae (PNe) and three Magellanic Cloud PNe,carrying out an abundance analysis using the collisionally excitedforbidden lines. Here, we analyse the relative intensities of faintoptical recombination lines (ORLs) from ions of carbon, nitrogen andoxygen in order to derive the abundances of these ions relative tohydrogen. The relative intensities of four high-l CII recombinationlines with respect to the well-known 3d-4f λ4267 line are foundto be in excellent agreement with the predictions of recombinationtheory, removing uncertainties about whether the high C2+abundances derived from the λ4267 line could be due tonon-recombination enhancements of its intensity.We define an abundance discrepancy factor (ADF) as the ratio of theabundance derived for a heavy element ion from its recombination linesto that derived for the same ion from its ultraviolet, optical orinfrared collisionally excited lines (CELs). All of the PNe in oursample are found to have ADFs that exceed unity. Two of the PNe, NGC2022 and LMC N66, have O2+ ADFs of 16 and 11, respectively,while the remaining 13 PNe have a mean O2+ ADF of 2.6, withthe smallest value being 1.8.Garnett and Dinerstein found that for a sample of about 12 PNe themagnitude of the O2+ ADF was inversely correlated with thenebular Balmer line surface brightness. We have investigated this for alarger sample of 20 PNe, finding weak correlations with decreasingsurface brightness for the ADFs of O2+ and C2+.The C2+ ADFs are well correlated with the absolute radii ofthe nebulae, although no correlation is present for the O2+ADFs. We also find both the C2+ and O2+ ADFs to bestrongly correlated with the magnitude of the difference between thenebular [OIII] and Balmer jump electron temperatures (ΔT),corroborating a result of Liu et al. for the O2+ ADF.ΔT is found to be weakly correlated with decreasing nebularsurface brightness and increasing absolute nebular radius.There is no dependence of the magnitude of the ADF upon the excitationenergy of the ultraviolet, optical or infrared CEL transition used,indicating that classical nebular temperature fluctuations - i.e. in achemically homogeneous medium - are not the cause of the observedabundance discrepancies. Instead, we conclude that the main cause of thediscrepancy is enhanced ORL emission from cold ionized gas located inhydrogen-deficient clumps inside the main body of the nebulae, as firstpostulated by Liu et al. for the high-ADF PN, NGC 6153. We havedeveloped a new electron temperature diagnostic, based upon the relativeintensities of the OII 4f-3d λ4089 and 3p-3s λ4649recombination transitions. For six out of eight PNe for which bothtransitions are detected, we derive O2+ ORL electrontemperatures of <=300 K, very much less than the O2+forbidden-line and H+ Balmer jump temperatures derived forthe same nebulae. These results provide direct observational evidencefor the presence of cold plasma regions within the nebulae, consistentwith gas cooled largely by infrared fine-structure transitions; at suchlow temperatures, recombination transition intensities will besignificantly enhanced due to their inverse power-law temperaturedependence, while ultraviolet and optical CELs will be significantlysuppressed.

Instability of a Thin Photoevaporable Circumstellar Envelope
We investigate the stability of a dense neutral shell that isaccelerated outward by the hot-gas pressure and that loses its massthrough photoionization by radiation from the central star. We assumethe H I shell to be thin and use the Lagrangian coordinates to describeits motion. We show that a flow accompanied by cumulative effectsemerges during the nonlinear development of the instability. We estimatethe influence of the radiative cooling rate on the motion and determineparameters of the gas in the cumulative region. The results obtained arecompared with the observations of the nebulae NGC 7293 and NGC 2392.

Radiation-driven winds of hot luminous stars. XV. Constraints on the mass-luminosity relation of central stars of planetary nebulae
We present a new model atmosphere analysis of nine central stars ofplanetary nebulae. This study is based on a new generation of realisticstellar model atmospheres for hot stars; state-of-the-art,hydrodynamically consistent, spherically symmetric model atmospheresthat have been shown to correctly reproduce the observed UV spectra ofmassive Population I O-type stars. The information provided by the windfeatures (terminal velocity, mass loss rate) permits to derive thephysical size of each central star, from which we can derive the stellarluminosity, mass, and distance, without having to assume a relationbetween stellar mass and luminosity taken from the theory of stellarstructure and AGB and post-AGB evolution. The results of our analysisare quite surprising: we find severe departures from the generallyaccepted relation between post-AGB central star mass and luminosity.

A reanalysis of chemical abundances in galactic PNe and comparison with theoretical predictions
New determinations of chemical abundances for He, N, O, Ne, Ar and Sare derived for all galactic planetary nebulae (PNe) so far observedwith a relatively high accuracy, in an effort to overcome differences inthese quantities obtained over the years by different authors usingdifferent procedures. These include: ways to correct for interstellarextinction, the atomic data used to interpret the observed line fluxes,the model nebula adopted to represent real objects and the ionizationcorrections for unseen ions. A unique `good quality' classical-typeprocedure, i.e. making use of collisionally excited forbidden lines toderive ionic abundances of heavy ions, has been applied to allindividual sets of observed line fluxes in each specific position withineach PN. Only observational data obtained with linear detectors, andsatisfying some `quality' criteria, have been considered. Suchobservations go from the mid-1970s up to the end of 2001. Theobservational errors associated with individual line fluxes have beenpropagated through the whole procedure to obtain an estimate of theaccuracy of final abundances independent of an author's `prejudices'.Comparison of the final abundances with those obtained in relevantmulti-object studies on the one hand allowed us to assess the accuracyof the new abundances, and on the other hand proved the usefulness ofthe present work, the basic purpose of which was to take full advantageof the vast amount of observations done so far of galactic PNe, handlingthem in a proper homogeneous way. The number of resulting PNe that havedata of an adequate quality to pass the present selection amounts to131. We believe that the new derived abundances constitute a highlyhomogeneous chemical data set on galactic PNe, with realisticuncertainties, and form a good observational basis for comparison withthe growing number of predictions from stellar evolution theory. Owingto the known discrepancies between the ionic abundances of heavyelements derived from the strong collisonally excited forbidden linesand those derived from the weak, temperature-insensitive recombinationlines, it is recognized that only abundance ratios between heavyelements can be considered as satisfactorily accurate. A comparison withtheoretical predictions allowed us to assess the state of the art inthis topic in any case, providing some findings and suggestions forfurther theoretical and observational work to advance our understandingof the evolution of low- and intermediate-mass stars.

Sulfur, Chlorine, and Argon Abundances in Planetary Nebulae. IV. Synthesis and the Sulfur Anomaly
We have compiled a large sample of O, Ne, S, Cl, and Ar abundances thathave been determined for 85 Galactic planetary nebulae in a consistentand homogeneous manner using spectra extending from 3600 to 9600Å. Sulfur abundances have been computed using the near-IR lines of[S III] λλ9069, 9532 along with [S III] temperatures. Wefind average values, expressed logarithmically with a standarddeviation, of log(S/O)=-1.91+/-0.24, log(Cl/O)=-3.52+/-0.16, andlog(Ar/O)=-2.29+/-0.18, numbers consistent with previous studies of bothplanetary nebulae and H II regions. We also find a strong correlationbetween [O III] and [S III] temperatures among planetary nebulae. Inanalyzing abundances of Ne, S, Cl, and Ar with respect to O, we find atight correlation for Ne-O, and loose correlations for Cl-O and Ar-O.All three trends appear to be colinear with observed correlations for HII regions. S and O also show a correlation, but there is a definiteoffset from the behavior exhibited by H II regions and stars. We suggestthat this S anomaly is most easily explained by the existence ofS+3, whose abundance must be inferred indirectly when onlyoptical spectra are available, in amounts in excess of what is predictedby model-derived ionization correction factors in PNe. Finally for thedisk PNe, abundances of O, Ne, S, Cl, and Ar all show gradients whenplotted against Galactocentric distance. The slopes are statisticallyindistinguishable from one another, a result which is consistent withthe notion that the cosmic abundances of these elements evolve inlockstep.

The 3-D ionization structure and evolution of NGC 7009 (Saturn Nebula)
Tomographic and 3-D analyses for extended, emission-line objects areapplied to long-slit ESO NTT + EMMI high-resolution spectra of theintriguing planetary nebula NGC 7009, covered at twelve position angles.We derive the gas expansion law, the diagnostics and ionic radialprofiles, the distance and the central star parameters, the nebularphoto-ionization model and the spatial recovery of the plasma structureand evolution. The Saturn Nebula (distance≃1.4 kpc, age≃6000yr, ionized mass≃0.18 Mȯ) consists of severalinterconnected components, characterized by different morphology,physical conditions, excitation and kinematics. We identify four``large-scale'', mean-to-high excitation sub-systems (the internalshell, the main shell, the outer shell and the halo), and as many``small-scale'' ones: the caps (strings of low-excitation knots withinthe outer shell), the ansae (polar, low-excitation, likely shockedlayers), the streams (high-excitation polar regions connecting the mainshell with the ansae), and an equatorial, medium-to-low excitationpseudo-ring within the outer shell. The internal shell, the main shell,the streams and the ansae expand at Vexp≃4.0 × Rarcsec km s-1, the outer shell, the caps and the equatorialpseudo-ring at Vexp≃3.15 × R arcsec kms-1, and the halo at Vexp≃10 kms-1. We compare the radial distribution of the physicalconditions and the line fluxes observed in the eight sub-systems withthe theoretical profiles coming from the photo-ionization code CLOUDY,inferring that all the spectral characteristics of NGC 7009 areexplainable in terms of photo-ionization by the central star, a hot (log T* ≃4.95) and luminous ( logL*/Lȯ≃3.70) 0.60-0.61Mȯ post-AGB star in the hydrogen-shell nuclear burningphase. The 3-D shaping of the Saturn Nebula is discussed within anevolutionary scenario dominated by photo-ionization and supported by thefast stellar wind: it begins with the superwind ejection (firstisotropic, then polar deficient), passes through the neutral, transitionphase ({lasting} ≃3000 yr), the ionization start (occurred≃2000 yr ago), and the full ionization of the main shell(≃1000 yr ago), at last reaching the present days: the wholenebula is optically thin to the UV stellar flux, except the caps (meanlatitude condensations in the outer shell, shadowed by the main shell)and the ansae (supersonic ionization fronts along the major axis).Based on observations made with: ESO Telescopes at the La SillaObservatories (program ID 65.I-0524), and the NASA/ESA Hubble SpaceTelescope, obtained from the data archive at the Space TelescopeInstitute. Observing programs: GO 6117 (P.I. Bruce Balick), GO 6119(P.I. Howard Bond) and GO 8390 (P.I. Arsen Hajian). STScI is operated bythe association of Universities for Research in Astronomy, Inc. underthe NASA contract NAS 5-26555. We extensively apply the photo-ionizationcode CLOUDY, developed at the Institute of Astronomy of the CambridgeUniversity (Ferland et al. 1998).

A unique Galactic planetary nebula with a [WN] central star
We report the discovery of the first probable Galactic [WN] central starof a planetary nebula (CSPN). The planetary nebula candidate was foundduring our systematic scans of the AAO/UKST Hα Survey of the MilkyWay. Subsequent confirmatory spectroscopy of the nebula and central starreveals the remarkable nature of this object. The nebular spectrum showsemission lines with large expansion velocities exceeding 150 kms-1, suggesting that perhaps the object is not a conventionalplanetary nebula. The central star itself is very red and is identifiedas being of the [WN] class, which makes it unique in the Galaxy. A largebody of supplementary observational data supports the hypothesis thatthis object is indeed a planetary nebula and not a Population IWolf-Rayet star with a ring nebula.

The relation between Zanstra temperature and morphology in planetary nebulae
We have created a master list of Zanstra temperatures for 373 galacticplanetary nebulae based upon a compilation of 1575 values taken from thepublished literature. These are used to evaluate mean trends intemperature for differing nebular morphologies. Among the most prominentresults of this analysis is the tendency forη=TZ(HeII)/TZ(HeI) to increase with nebularradius, a trend which is taken to arise from the evolution of shelloptical depths. We find that as many as 87 per cent of nebulae may beoptically thin to H ionizing radiation where radii exceed ~0.16 pc. Wealso note that the distributions of values η and TZ(HeII)are quite different for circular, elliptical and bipolar nebulae. Acomparison of observed temperatures with theoretical H-burning trackssuggests that elliptical and circular sources arise from progenitorswith mean mass ≅ 1 Msolar(although the elliptical progenitors are probably more massive).Higher-temperature elliptical sources are likely to derive fromprogenitors with mass ≅2 Msolar, however, implying thatthese nebulae (at least) are associated with a broad swathe ofprogenitor masses. Such a conclusion is also supported by trends in meangalactic latitude. It is found that higher-temperature ellipticalsources have much lower mean latitudes than those with smallerTZ(HeII), a trend which is explicable where there is anincrease in with increasing TZ(HeII).This latitude-temperature variation also applies for most other sources.Bipolar nebulae appear to have mean progenitor masses ≅2.5Msolar, whilst jets, Brets and other highly collimatedoutflows are associated with progenitors at the other end of the massrange (~ 1 Msolar). Indeed it ispossible, given their large mean latitudes and low peak temperatures,that the latter nebulae are associated with the lowest-mass progenitorsof all.The present results appear fully consistent with earlier analyses basedupon nebular scale heights, shell abundances and the relativeproportions of differing morphologies, and offer further evidence for alink between progenitor mass and morphology.

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Observation and Astrometry data

Right ascension:07h29m10.77s
Apparent magnitude:10

Catalogs and designations:
Proper NamesEskimo nebula
Eskimo   (Edit)
NGC 2000.0NGC 2392

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