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White dwarf masses derived from planetary nebula modelling
Aims.We compare the mass distribution of central stars of planetarynebulae (CSPNe) with those of their progeny, white dwarfs (WD). Methods: We use a dynamical method to measure masses with an uncertaintyof 0.02 M_ȯ. Results: The CSPN mass distribution is sharplypeaked at 0.61~M_ȯ. The WD distribution peaks at lower masses(0.58~M_ȯ) and shows a much broader range of masses. Some of thedifference can be explained if the early post-AGB evolution is fasterthan predicted by the Blöcker tracks. Between 30 and 50 per cent ofWD may avoid the PN phase because they have too low a mass. However, thediscrepancy cannot be fully resolved and WD mass distributions may havebeen broadened by observational or model uncertainties.Data is only available in electronic form at http://www.aanda.org
| HST and VLT observations of the symbiotic star Hen 2-147. Its nebular dynamics, its Mira variable and its distance
Aims.We investigate the dynamics of the nebula around the symbiotic starHen 2-147, determine its expansion parallax, and compare it with thedistance obtained via the period-luminosity relation for its Miravariable. Methods: A combination of multi-epoch HST images and VLTintegral field high-resolution spectroscopy is used to study the nebulardynamics both along the line of sight and in the plane of the sky. Theseobservations allow us to build a 3D spatio-kinematical model of thenebula, which, together with the measurement of its apparent expansionin the plane of the sky over a period of 3 years, provides the expansionparallax for the nebula. Additionally, SAAO near-infrared photometryobtained over 25 years is used to determine the Mira pulsation periodand derive an independent distance estimation via the period-luminosityrelationship for Mira variables. Results: The geometry of the nebula isfound to be that of a knotty annulus of ionized gas inclined to theplane of sky and expanding with a velocity of ~90 km s-1. Astraightforward application of the expansion parallax method provides adistance of 1.5 ± 0.4 kpc, which is a factor of two lower thanthe distance of 3.0 ± 0.4 kpc obtained from the period-luminosityrelationship for the Mira (which has a pulsation period of 373 days).The discrepancy is removed if, instead of expanding matter, we areobserving the expansion of a shock front in the plane of the sky. Thisshock interpretation is further supported by the broadening of thenebular emission lines.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 No. NAS5-26555; on observations obtained at the 8 m VLTtelescope of the European Southern Observatory in Chile; and onobservations made at the South African Astronomical Observatory. Themovie (Fig. 3) is only available in electronic form athttp://www.aanda.org Table 2 is only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/465/481
| 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.
| Evolution of elemental abundances in planetary nebulae
We study the evolution of elemental abundances in an ensemble ofGalactic planetary nebulae as a function of the masses of the centralstars (M cs) and their progenitors (M ini). We derive the dependences ofthe C, N, Ne, Cl, Ar, and S abundances on M cs and M ini for a largesample of nebulae. We calculate the theoretical elemental abundances innebulae under the assumption of complete mixing of theprogenitor’s matter ejected at different stages of its evolution.The theoretical dependences of the C and N abundances on M ini have beenfound to correspond to the observed ones. At the same time, the observedmean O abundance is approximately half its theoretical value. The Ne,Cl, Ar, and S abundances monotonically increase with increasing mass ofthe progenitor star, which reflects an increase in the mean abundancesof heavy elements during the chemical evolution of the Galaxy. We havederived the relation between the abundances of the elements underconsideration in planetary nebulae and the masses of their centralstars. This relation is used to construct the mass function for thenuclei of planetary nebulae.
| Observed Planetary Nebulae as Descendants of Interacting Binary Systems
We examine recent studies on the formation rate of planetary nebulae andfind this rate to be about one-third of the formation rate of whitedwarfs. This implies that only about one-third of all planetary nebulaethat evolve to form white dwarfs are actually bright enough to beobserved. This finding corresponds with the claim that it is necessaryfor a binary companion to interact with the asymptotic giant branchstellar progenitor for the descendant planetary nebulae to be brightenough to be detected. The finding about the formation rate alsostrengthens O. De Marco's conjecture that the majority of observedplanetary nebulae harbor binary systems. In other words, single starsalmost never form observed planetary nebulae.
| Working with VIMOS-IFU data: Searching and characterizing the faint haloes of planetary nebulae
In this contribution, we present the VIMOS-IFU data analysis of twoplanetary nebulae: NGC 3242 and NGC 4361. Due to the technical nature ofthe workshop, we emphasize some specific problems of data reduction andanalysis: accuracy in flux calibration, importance of scattered lightand improvement of the S/N ratio. We estimate that accuracy of our fluxcalibration is ˜15% in the spectral range 4200 6100 Å. At˜10″ from the center of a point source, the intensity is≲0.02% of the peak value, indicating that scattered light isnegligible. Some examples of science that we are doing are shown. Inparticular, we have established an apparent temperature gradient acrossthe halo of NGC NGC3242.
| Chemical abundances in seven galactic planetary nebulae
An observational study of chemical abundances in the galactic planetarynebulae NGC 1535, NGC 2438,NGC 2440, NGC 3132, NGC3242, NGC 6302, and NGC7009 based on long-slit spectra of high signal-to-noise ratioin the 3100 to 6900 Å range is presented. We determined the N, O,Ne, S, and Cl abundances from collisionally excited lines and the He andO++ abundances from recombination lines. TheO++/H+ estimates derived from recombination linesare about a factor of four and two higher than those derived fromforbidden lines for NGC 7009 and NGC3242, respectively. Spatial profiles ofO++/H+ abundance from O II permitted lines andfrom [O III] forbidden lines were obtained for the planetary nebulaNGC 7009. The differences betweenO++/H+ derived from recombination and fromforbidden lines present smooth variations along the nebular surface ofNGC 7009, with the differences decreasing from thecenter to the edges of the nebula. If these abundance differences areexplained by the presence of electron temperature fluctuations,quantified by the parameter t2, a value of aboutt2=0.09 is required for NGC 3242 andNGC 7009.
| Photospheric opacity and over-expanded envelopes of asymptotic giant branch stars
I suggest that the behavior of the photospheric opacity in oxygen-rich(similar to solar abundance) upper asymptotic giant branch stars maycause these stars to substantially expand for a few thousand years. Iterm this process over-expansion. This may occur when the photospheric(effective) temperature drops to Tp 3000 K, and becausethe opacity sharply increases as temperature further decreases down toTp 2000 K. The much higher opacity implies a much lowerphotospheric density, which stabilizes the envelope structure. As massloss proceeds, the star eventually contracts to become a post-asymptoticgiant branch star. Some possible outcomes of the over-expanded phase arediscussed: (1) The over-expanded phase may be connected to the formationof semi-periodic concentric arcs (rings; shells); (2) the over-expandedphase may be related to the positive correlation between the mass lossrate and the transition to axisymmetric mass loss geometry; and (3) anover-expanded asymptotic giant branch star, which doubles its radius, issomewhat more likely to swallow a low mass companion.
| The Detection of 3He+ in a Planetary Nebula Using the VLA
We used the VLA to search for 3He+ emission fromtwo Galactic planetary nebulae (PNe): NGC 6572 and J320. Standardstellar models predict that the 3He/H abundance ratios forPNe should be 1-2 orders of magnitude higher than the primordial value(3He/H~10-5 by number) determined from Galactic HII region abundances and confirmed by WMAP cosmic microwave backgroundresults. Chemical evolution models suggest that fewer than 5% of all PNeenrich the interstellar medium (ISM) with 3He at the level ofstandard stellar models. Our target PNe are therefore anomalous in thatthey were selected from a sample deliberately biased to contain objectswith properties that maximized the likelihood of a 3Hedetection by the VLA. We have detected the 8.665 GHz hyperfine3He+ transition in J320 at the 4 σ level.The 3He/H abundance ratio is 1.9×10-3 withroughly a factor of 2 uncertainty. For NGC 6572 we find an upper limitof 3He/H<~10-3. This detection of3He in J320 makes it the second PN known to have ananomalously high 3He abundance, confirming that at least somelow-mass stars produce significant amounts of 3He thatsurvives to the PN stage and enriches the ISM.
| Blowing up warped disks in 3D. Three-dimensional AMR simulations of point-symmetric nebulae
The Generalized Interacting Stellar Winds model has been very successfulin explaining observed cylindrical and bipolar shapes of planetarynebulae. However, many nebulae have a multipolar or point-symmetricshape. Previous two-dimensional calculations showed that these seeminglyenigmatic forms can be reproduced by a two-wind model in which theconfining disk is warped, as is expected to occur in irradiated disks.In this paper we present the extension to fully three-dimensionalAdaptive Mesh Refinement simulations using the publicly availablehydrodynamics package Flash. We briefly describe the mechanism leadingto a radiation driven warped disk, and give an equation for its shape.We derive time scales related to the disk and compare them to theradiative cooling time scale of the gas, thereby determining therelevant part of parameter space. By comparing two-dimensionalcalculations including realistic radiative cooling through a coolingcurve, with ones employing a low value for the adiabatic index γ,we show that the latter, computationally less expensive approach, is avalid approximation for treating cooling in our nebulae. The results ofthe fully three-dimensional wind-disk simulations show our mechanism tobe capable of producing a plethora of multipolar (and quadrupolar)morphologies, which can explain the observed shape of a number of(proto-)planetary nebulae.
| Electron temperature fluctuations in planetary nebulae
An observational study of the spatial variation of the electrontemperature and density in 10 galactic planetary nebulae is presented.The data consist of long-slit spectra of high signal-to-noise ratio inthe 3100 to 6900 Å range. Electron temperatures were determinedfrom the [O III](λ 4959 + λ 5007)/λ 4363 and [NII](λ 6548 + λ 6583)/λ 5755 ratios and from theBalmer discontinuity. Electron densities were estimated from the [SII]λ 6716/λ 6731, [Cl III]λ 5517/λ 5537, and[Ar IV]λ 4711/λ 4740 ratios. Electron temperaturevariations of low amplitude were found across the nebular surface in theplanetary nebulae studied. The temperature distribution across eachnebula presents a variance relative to the mean corresponding to 0.0003≤ t2s(Bal) ≤ 0.0078, 0.0003 ≤t2s(N II) ≤ 0.0097, and 0.0011 ≤t2s(O III) ≤ 0.0050. A systematic spatialvariation of electron density has been detected in most of objects(NGC 1535, NGC 2438, NGC2440, NGC 3132, NGC3242, NGC 6302, NGC6563, and NGC 7009). The remaining objects(NGC 6781 and NGC 6853) have notshown any significant electron density dependence on position.NGC 2438, NGC 6563, NGC6781, and NGC 6853 are in general the mostdiffuse and probably evolved objects studied here, with low meandensities in the range Ne(S II) ≈ 95-158~cm-3.An anti-correlation between temperature and density was found forNGC 2438 and NGC 3132, with theelectron temperature increasing with the decrease of electron densityand a correlation between temperature and density was found forNGC 2440, NGC 3242, NGC6302, and NGC 7009, with the electrontemperature increasing with the increase of electron density. Theserelationships seem to be associated with the structure of the nebula.The nebulae in which the correlation between temperature and density ispresent are ring shaped. The anti-correlation between temperature anddensity is found in bipolar planetary nebulae that are denser in thecentre of the nebula.
| Oxygen Recombination Line Abundances in Gaseous Nebulae
The determination of the heavy element abundances from giantextragalactic H II regions has been generally based on collisionallyexcited lines. We will discuss the reasons to study the characteristicsof recombination lines, and then use these lines to determine chemicalabundances. Of these lines the oxygen (specifically the O II) lines arethe most important; and, of them, the lines of multiplet 1 of O II arethe most accessible. It has often been assumed that by measuring theintensity of a single line within a multiplet the intensities of all thelines in the multiplet can be determined; in recent studies we havefound that the intensity ratios of lines within a multiplet can dependon density; we will present empirical density-intensity relationshipsfor multiplet 1 based on recent observations of H II regions andplanetary nebulae. From observations of H II regions we find that thecritical density for collisional redistribution of the multiplet 1 O IIrecombination lines amounts to 2800+/-500 cm-3. We point out that theO/H recombination abundances of H II regions in the solar vicinity arein excellent agreement with the O/H solar value, while the abundancesderived from collisionally excited lines are not. We present acalibration of Pagel's method in the 8.2 < 12 + log O/H < 8.8range based on O recombination lines.
| The evolution of planetary nebulae. III. Internal kinematics and expansion parallaxes
A detailed theoretical study of the basic internal kinematics ofplanetary nebulae is presented, based on 1D radiation-hydrodynamicssimulations of circumstellar envelopes around central stars of 0.595 and0.696 Mȯ. By means of observable quantities like radialsurface-brightness distributions and emission-line profiles computedfrom the models, a comparison with real objects was performed andrevealed a reasonable agreement. This allowed to draw importantconclusions by investigating the kinematics of these models in detail.Firstly, it is shown that the determination of kinematical ages,normally considered to be simple if size and expansion rate of an objectare given, can seriously be flawed. Secondly, the expansion law of aplanetary nebula is different from what is assumed for derivingspatio-kinematical models. Thirdly and most importantly, ourhydrodynamical models help to correctly use existing angular expansionmeasurements for distance determinations. The mere combination of theangular expansion rates with the spectroscopic expansion velocitiesleads always to a serious underestimate of the distance, the degree ofwhich depends on the evolutionary state of the object. The necessarycorrection factor varies between 3 and 1.3. Individual correctionfactors can be estimated with an accuracy of about 10% by matching ourhydrodynamical models to real objects. As a result, revised distancesfor a few objects with reliable angular expansion rates are presented.But even these corrected distances are not always satisfying: they stillappear to be inconsistent with other distance determinations and, evenmore disturbing, with the accepted theory of post-asymptotic giantbranch evolution. As a byproduct of the angular expansion measurements,the transition times from the vicinity of the asymptotic giant branch tothe planetary-nebula regime could be estimated. They appear to beshorter than assumed in the present evolutionary calculations.
| Fluorine Abundances in Planetary Nebulae
We have determined fluorine abundances from the [F II] λ4789 and[F IV] λ4060 nebular emission lines for a sample of planetarynebulae (PNe). Our results show that fluorine is generally overabundantin PNe, thus providing new evidence for the synthesis of fluorine inasymptotic giant branch (AGB) stars. [F/O] is found to be positivelycorrelated with the C/O abundance ratio, in agreement with thepredictions of theoretical models of fluorine production in thermallypulsing AGB stars. A large enhancement of fluorine is observed in theWolf-Rayet PN NGC 40, suggesting that high mass-loss rates probablyfavor the survival of fluorine.
| Integral Field Spectroscopy of Faint Halos of Planetary Nebulae
We present the first integral field spectroscopy observations of the twoplanetary nebulae NGC 3242 and NGC 4361 with the VIMOS instrumentattached to VLT-UT3. By co-adding a large number of spaxels, we reach anemission-line detection limit of 5×10-18 ergscm-2 s-1 arcsec-2. In the case of NGC3242, we succeed in determining some properties of the halo. The radialsurface brightness profile in [O III] implies increasing mass lossbefore the formation of the planetary nebula. Traces of the mysterious``rings'' are clearly visible. We find for the first time an apparenttemperature gradient across a halo: from about 16,000 K close to theshell/halo transition to 20,000 K at the halo's outer edge. No lineemission is seen in the suspected halo region of NGC 4361 down to thesensitivity limit.
| 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.
| Modelling of aspherical nebulae - I. A quick pseudo-3D photoionization code
We describe a pseudo-3D photoionization code, NEBU&LOWBAR;3D, and itsassociated visualization tool, VISNEB&LOWBAR;3D, which are able to treata wide variety of nebular geometries easily and rapidly, by combiningmodels obtained with a 1D photoionization code. The only requirement forthe code to work is that the ionization source is unique and notextended. It is applicable as long as the diffuse ionizing radiationfield is not dominant and strongly inhomogeneous. As examples of thecapabilities of these new tools, we consider two very differenttheoretical cases. One is that of a high-excitation planetary nebulathat has an ellipsoidal shape with two polar density knots. The other isthat of a blister HII region, for which we have also constructed aspherical model (the spherical impostor), which has exactly the sameHβ surface brightness distribution as the blister model and thesame ionizing star.We present and comment upon line intensity maps corresponding todifferent viewing angles. We also use the computed line intensities toderive physical properties of the model in the same way as an observerwould do for a real object. For example, we derive the `apparent' valueof N/O for the entire nebula and along spectral slits of differentorientations. For this, we take the electron temperature and densityderived from the [NII]5755Å/[NII]6583Åand[OII]3726Å/[OII]3729Åratios, respectively, and we adopt thecommon recipe: N/O = N+/O+. Interestingly, we findthat, in the case of our high-excitation nebula, the derived N/O iswithin 10-20 per cent of the real value, even when the slit crosses thehigh-density knots. On the other hand, for the blister HII region andits spherical impostor, we find that the apparent N/O is much smallerthan the true one (about 0.68 and 0.5 of it, respectively).These two examples warn against preconceived ideas when interpretingspectroscopic and imaging data of HII regions and planetary nebulae. Thetools NEBU&LOWBAR;3D and VISNEB&LOWBAR;3D, which will be made publiclyavailable in the future, should facilitate the performance of numericalexperiments, to yield a better understanding of the physics ofaspherical ionized nebulae.
| 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.
| Helium recombination spectra as temperature diagnostics for planetary nebulae
Electron temperatures derived from the HeI recombination line ratios,designated Te(HeI), are presented for 48 planetary nebulae(PNe). We study the effect that temperature fluctuations inside nebulaehave on the Te(HeI) value. We show that a comparison betweenTe(HeI) and the electron temperature derived from the Balmerjump of the HI recombination spectrum, designated Te(HI),provides an opportunity to discriminate between the paradigms of achemically homogeneous plasma with temperature and density variations,and a two-abundance nebular model with hydrogen-deficient materialembedded in diffuse gas of a `normal' chemical composition (i.e.~solar), as the possible causes of the dichotomy between the abundancesthat are deduced from collisionally excited lines and those deduced fromrecombination lines. We find that Te(HeI) values aresignificantly lower than Te(HI) values, with an averagedifference of = 4000 K. Theresult is consistent with the expectation of the two-abundance nebularmodel but is opposite to the prediction of the scenarios of temperaturefluctuations and/or density inhomogeneities. From the observeddifference between Te(HeI) and Te(HI), we estimatethat the filling factor of hydrogen-deficient components has a typicalvalue of 10-4. In spite of its small mass, the existence ofhydrogen-deficient inclusions may potentially have a profound effect inenhancing the intensities of HeI recombination lines and thereby lead toapparently overestimated helium abundances for PNe.
| 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].
| The evolution of planetary nebulae. II. Circumstellar environment and expansion properties
We investigate and discuss the expansion properties of planetary nebulaeby means of 1D radiation-hydrodynamics models computed for differentinitial envelope configurations and central star evolutionary tracks. Inparticular, we study how the expansion depends on the initial densitygradient of the circumstellar envelope and show that it is possible toderive information on the very last mass-loss episodes during the star'sfinal evolution along and off the asymptotic giant branch. To facilitatethe comparison of the models with real objects, we have also computedobservable quantities like surface brightness and emission-lineprofiles. With the help of newly acquired high-resolution emission-lineprofiles for a sample of planetary nebulae we show that models withinitial envelopes based on the assumption of a stationary wind outflowfail to explain the observed expansion speeds of virtually all of theobserved planetary nebulae. Instead it must be assumed that during thevery last phase of evolution along the final asymptotic giant branchevolution the mass-loss rate increases in strength, resulting in a muchsteeper slope of the circumstellar radial density distribution. Underthese conditions, the expansion properties of the nebular gas differconsiderably from the self-similar solutions found for isothermalconditions. Furthermore, the mass loss must remain at a rather highlevel until the stellar remnant begins to evolve quickly towards thecentral star regime. Current theoretical computations of dust-drivenmass-loss which are restricted to rather low temperatures cannot beapplied during the star's departure from the asymptotic giant branch.Based on observations obtained at the 3.5 μm NTT and the 1.2 μmCAT telescope of the European Southern Observatory, La Silla, and at the2.6 μm NOT telescope operated on the island of La Palma by NOTSA, inthe Spanish Observatorio del Roque de Los Muchachos of the InstitutodeAstrofísica de Canarias.Dedicated to Prof. V. Weidemann on the occasion of his 80th birthday,October 3, 2004.
| Unresolved Hα Enhancements at High Galactic Latitude in the WHAM Sky Survey Maps
We have identified 85 regions of enhanced Hα emission at|b|>10deg subtending approximately 1° or less on theWisconsin Hα Mapper (WHAM) sky survey. These high-latitude ``WHAMpoint sources'' have Hα fluxes of 10-11-10-9ergs cm-2 s-1, radial velocities within about 70km s-1 of the LSR, and line widths that range from less than20 to about 80 km s-1 (FWHM). Twenty-nine of theseenhancements are not identified with either cataloged nebulae or hotstars and appear to have kinematic properties that differ from thoseobserved for planetary nebulae. Another 14 enhancements are near hotevolved low-mass stars that had no previously reported detections ofassociated nebulosity. The remainder of the enhancements are catalogedplanetary nebulae and small, high-latitude H II regions surroundingmassive O and early B stars.
| 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.
| 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 reexamination of electron density diagnostics for ionized gaseous nebulae
We present a comparison of electron densities derived from opticalforbidden line diagnostic ratios for a sample of over a hundred nebulae.We consider four density indicators, the [O II]λ3729/λ3726, [S II] λ6716/λ6731, [Cl III]λ5517/λ5537 and [Ar IV] λ4711/λ4740 doubletratios. Except for a few H II regions for which data from the literaturewere used, diagnostic line ratios were derived from our own high qualityspectra. For the [O II] λ3729/λ3726 doublet ratio, we findthat our default atomic data set, consisting of transition probabilitiesfrom Zeippen (\cite{zeippen1982}) and collision strengths from Pradhan(\cite{pradhan}), fit the observations well, although at high electrondensities, the [O II] doublet ratio yields densities systematicallylower than those given by the [S II] λ6716/λ6731 doubletratio, suggesting that the ratio of transition probabilities of the [OII] doublet, A(λ3729)/A(λ3726), given by Zeippen(\cite{zeippen1982}) may need to be revised upwards by approximately 6per cent. Our analysis also shows that the more recent calculations of[O II] transition probabilities by Zeippen (\cite{zeippen1987a}) andcollision strengths by McLaughlin & Bell (\cite{mclaughlin}) areinconsistent with the observations at the high and low density limits,respectively, and can therefore be ruled out. We confirm the earlierresult of Copetti & Writzl (\cite{copetti2002}) that the [O II]transition probabilities calculated by Wiese et al. (\cite{wiese}) yieldelectron densities systematically lower than those deduced from the [SII] λ6716/λ6731 doublet ratio and that the discrepancy ismost likely caused by errors in the transition probabilities calculatedby Wiese et al. (\cite{wiese}). Using our default atomic data set for [OII], we find that Ne([O II]) Ne([S II]) ≈Ne([Cl III])< Ne([Ar IV]).
| Identification and Characterization of Faint Emission Lines in the Spectrum of the Planetary Nebula IC 418
We present high signal-to-noise ratio echelle spectra of the compacthigh surface brightness, low-ionization planetary nebula (PN) IC 418.These reveal 807 emission lines down to intensities less than10-5 that of Hβ for which we determine widths andrelative intensities. We show that line profiles are a valuableparameter for making line identifications and in constraining theexcitation mechanism of the lines. We present evidence that indicatesthat many supposed high-level recombination lines may in fact be excitedby a process other than recombination. We contend from the detection ofdielectronic recombination lines that their relatively low intensitiesargue against their making a significant contribution to levelpopulations of the heavy ions in this object. Following similar analysesof other PNe we find that IC 418 shows a small discrepancy in ionabundances derived from forbidden versus recombination lines of theheavy elements.
| Observations and three-dimensional photoionization modelling of the Wolf-Rayet planetary nebula NGC 1501
Deep optical spectra of the high-excitation planetary nebula NGC 1501and its W04 central star are presented. A recombination line abundanceanalysis of the emission-line spectrum of the central star yields He:C:Omass fractions of 0.36:0.48:0.16, similar to those of PG 1159 stars. Adetailed empirical analysis of the nebular collisionally excited line(CEL) and optical recombination line (ORL) spectrums are presented,together with fully three-dimensional photoionization modelling of thenebula. We found very large ORL-CEL abundance discrepancy factors (ADFs)for O2+ (32) and Ne2+ (33). The mean value of~5100 K for the Te derived from HeI recombination linesratios is 6000 K, lower than the value of 11100 K implied by the [OIII]line ratio. This result indicates the existence of a second,low-temperature nebular component, which could account for the observedORL emission. Electron temperature fluctuations (t2) cannotaccount for the high ADFs found from our optical spectra of this nebula.A three-dimensional photoionization model of NGC 1501 was constructedusing the photoionization code MOCASSIN, based on our new spectroscopicdata and using the three-dimensional electron density distributiondetermined from long-slit echellograms of the nebula by Ragazzoni et al.The central star ionizing radiation field is approximated by a modelatmosphere, calculated using the Tübingen non-local thermodynamicequilibrium model atmosphere package, for abundances typical of the W04nucleus of NGC 1501 and PG 1159 stars. The nebular emission-linespectrum was best reproduced using a central star model with aneffective temperature of Teff= 110 kK and a luminosity ofL*= 5000Lsolar. The initial models showed higherdegrees of ionization of heavy elements than indicated by observations.We investigated the importance of the missing low-temperaturedielectronic recombination rates for third-row elements and haveestimated upper limits to their rate coefficients.Our single-phase, three-dimensional photoionization model heavilyunderpredicts the optical recombination line emission. We conclude thatthe presence of a hydrogen-deficient, metal-rich component is necessaryto explain the observed ORL spectrum of this object. The existence ofsuch knots could also provide a softening of the radiation field, viathe removal of ionizing photons by absorption in the knots, therebyhelping to alleviate the overionization of the heavy elements in ourmodels.
| Chemical abundances of planetary nebulae from optical recombination lines - II. Abundances derived from collisionally excited lines and optical recombination lines
In Paper I, we presented spectrophotometric measurements of emissionlines from the ultraviolet (UV) to the far-infrared for 12 Galacticplanetary nebulae (PNe) and derived nebular thermal and densitystructures using a variety of plasma diagnostics. The measurements andplasma diagnostic results are used in the current paper to determineelemental abundances in these nebulae. Abundance analyses are carriedout using both strong collisionally excited lines (CELs) and weakoptical recombination lines (ORLs) from heavy element ions.Assuming electron temperatures and densities derived from HIrecombination spectra (line and continuum), we are able to determine theORL C abundance relative to hydrogen for all the PNe in our sample, Nand O abundances for 11 of them and Ne abundances for nine of them. Inall cases, ORL abundances are found to be systematically higher than thecorresponding values deduced from CELs. In NGC 40, the discrepancybetween the abundances derived from the two types of emission linereaches a factor of 17 for oxygen. For the other 10 PNe, thediscrepancies for oxygen vary from 1.6 to 3.1. In general, collisionallyexcited infrared fine-structure lines, which have excitation energiesless than 103 K and consequently emissivities that areinsensitive to electron temperature and temperature fluctuations, yieldionic abundances comparable to those derived from optical/UV CELs. For agiven nebula, the discrepancies between the ORL and CEL abundances areof similar magnitude for different elements. In other words, relativeabundance ratios such as C/O, N/O and Ne/O deduced from the traditionalmethod based on strong CELs are comparable to those yielded by ORLs, fora wide range of ORL to CEL oxygen abundance ratios, varying from nearunity to over a factor of 20.We have also determined ORL abundances relative to hydrogen for thethird-row element magnesium for 11 nebulae in our sample. In strongcontrast to the cases for second-row elements, Mg abundances derivedfrom the MgII 3d-4f λ4481 ORL are nearly constant for all the PNeanalysed so far and agree within the uncertainties with the solarphotospheric value.In accordance with results from previous studies, the ORL to CELabundance ratio is correlated with the difference between the electrontemperatures derived from the [OIII] forbidden-line ratio, on the onehand, and from the hydrogen recombination Balmer discontinuity, on theother. We find that the discrepancy between the ORL and CEL abundancesis correlated with nebular absolute diameter, surface brightness, theelectron density derived from [SII] CELs, and excitation class. Theresults confirm that the dichotomy of temperatures and heavy elementalabundances determined from the two types of emission line, which hasbeen widely observed in PNe, is a strong function of nebular evolution,as first pointed out by Garnett and Dinerstein.Our analyses show that temperature fluctuations and/or densityinhomogeneities are incapable of explaining the large discrepanciesbetween the heavy elemental abundances and electron temperaturesdetermined from the two types of emission line. Our analyses support thebi-abundance model of Liu et al., who have proposed that PNe containanother previously unseen component of ionized gas which, highlyenriched in heavy elements, has an electron temperature of<~103 K and emits strongly in recombination lines but notin CELs. Our determinations of low average emission temperatures fromthe observed line intensity ratios of HeI and OII ORLs lend furthersupport to this scenario.
| 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.
| 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.
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