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The Use of K_S Band Photometric Excesses to Investigate H(2) Emission in Planetary Nebulae
We have determined the distribution of H(2) emission in 14 planetarynebulae (PNe), using imaging and photometry published by the 2MASSinfrared survey. This technique is only applicable under certainstringent conditions, and requires precise broad band photometry, andaccurate spatial registration between the K_S and H band images. It is,in addition, only applicable to certain sources, and excludes outflowsin which central star and grain thermal excesses are appreciable. Ourresults for NGC 3132, NGC 6720, IC 4406 and M 2-9 are closely similar tothose of previous narrow band imaging, and confirm that H(2) emissionis confined to narrow, highly fragmented shells. Similar results areobtained for M 1-7, M 1-8, and M 3-5. Our spatial profiles also confirmthat the emission extends outside of the primary ionised shells. Whereenvelopes are large, and the PNe are more evolved, then the fractionalextensions Deltatheta/$theta appear to be at their smallest. They arealso similar to the radial widths predicted for H(2) abundanceprofiles, and to the values DeltaR/R determined throughmagnetohydrodynamic modelling of shocks. There appears, finally, to beevidence for an evolution in this parameter, such that Deltatheta/$thetavaries with increasing envelope size d(H) as Deltatheta/$theta ~d(H)(-2.2) .

Rings around Lyra.
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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.

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.

C [III] imagery of planetary nebulae and H II regions.
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Filaments as Possible Signatures of Magnetic Field Structure in Planetary Nebulae
We draw attention to the extreme filamentary structures seen inhigh-resolution optical images of certain planetary nebulae. Wedetermine the physical properties of the filaments in the nebulae IC418, NGC 3132, and NGC 6537, and based on their large length-to-widthratios, longitudinal coherence, and morphology, we suggest that they maybe signatures of the underlying magnetic field. The fields needed forthe coherence of the filaments are probably consistent with thosemeasured in the precursor circumstellar envelopes. The filaments suggestthat magnetic fields in planetary nebulae may have a localized andthreadlike geometry.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the data archive at the Space Telescope Institute. STScIis operated by the association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555.

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.

Hot dust haloes in planetary nebulae
We point out that many planetary nebulae (PNe) have large infraredindices (H-KS), and that this is likely to result from thepresence of hot grains, and/or H2 S(1) line emission atλ= 2.122μm. We are able to identify two groups of sourcesassociated with each of these mechanisms, both of which appear topossess distinct physical characteristics. One difference between thesegroups concerns the near-infrared dimensions of the sources. It appearsthat hot dust outflows frequently have sizes θ(KS)> θ(H) > θ(J). Four of the sources are particularlyextreme in this regard, and show emission extending well outside of theprimary shells. We propose that this is likely to arise fromhigh-temperature grains located in low-density haloes. The location ofsuch grains at large distances from the central stars represents achallenge for any mechanism purporting to explain this phenomenon. Themost likely explanation appears to be in terms of photon heating of verysmall grains.

Polycyclic aromatic hydrocarbon emission bands in selected planetary nebulae: a study of the behaviour with gas phase C/O ratio
Airborne and space-based low-resolution spectroscopy in the 1980sdiscovered tantalizing quantitative relationships between the gas phaseC/O abundance ratio in planetary nebulae (PNe) and the fractions oftotal far-infrared (FIR) luminosity radiated by the 7.7- and 11.3-μmbands (the C = C stretch and C-H bend, respectively), of polycyclicaromatic hydrocarbons (PAHs). Only a very small sample of nebulae wasstudied in this context, limited by airborne observations of the7.7-μm band, or the existence of adequate IRAS Low ResolutionSpectrometer data for the 11.3-μm band. To investigate these trendsfurther, we have expanded the sample of planetaries available for thisstudy using Infrared Space Observatory (ISO) low-resolution spectrasecured with the Short Wavelength Spectrometer and the Long WavelengthSpectrometer. The new sample of 43 PNe, of which 17 are detected in PAHemission, addresses the range from C/O = 0.2-13 with the objective oftrying to delineate the pathways by which carbon dust grains might haveformed in planetaries. For the 7.7-μm and 11.3-μm bands, weconfirm that the ratio of band strength to total infrared (IR)luminosity is correlated with the nebular C/O ratio. Expressed inequivalent width terms, the cut-on C/O ratio for the 7.7-μm band isfound to be 0.6+0.2-0.4, in good accord with thatfound from sensitive ground-based measurements of the 3.3-μ band.

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.

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.

Antoine Darquier et la decouverte de la nebuleuse de la Lyre.
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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.

A CO Survey of Young Planetary Nebulae
We report the results of a sensitive survey of young planetary nebulaein the CO J=2-1 line that significantly increases the available data onwarm, dense, molecular gas in the early phases of planetary nebulaformation. The observations were made using the IRAM 30 m telescope withthe 3×3 pixel Heterodyne Receiver Array (HERA). The array providesan effective means of discriminating the CO emission of planetarynebulae in the Galactic plane from contaminating emission ofinterstellar clouds along the line of sight. A total of 110 planetarynebulae were observed in the survey, and 40 were detected. The resultsincrease the number of young planetary nebulae with known CO emission byapproximately a factor of 2. The CO spectra yield radial velocities forthe detected nebulae, about half of which have uncertain or no velocitymeasurements at optical wavelengths. The CO profiles range fromparabolic to double-peaked, tracing the evolution of structure in themolecular gas. The line widths are significantly larger than on theasymptotic giant branch, and many of the lines show extended wings,which probably result from the effects on the envelopes of high-velocityjets.Based on observations carried out with the IRAM 30 m telescope. IRAM issupported by INSU/CNRS (France), MPG (Germany), and IGN (Spain).

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].

A Map of the Universe
We have produced a new conformal map of the universe illustrating recentdiscoveries, ranging from Kuiper Belt objects in the solar system to thegalaxies and quasars from the Sloan Digital Sky Survey. This mapprojection, based on the logarithm map of the complex plane, preservesshapes locally and yet is able to display the entire range ofastronomical scales from the Earth's neighborhood to the cosmicmicrowave background. The conformal nature of the projection, preservingshapes locally, may be of particular use for analyzing large-scalestructure. Prominent in the map is a Sloan Great Wall of galaxies 1.37billion light-years long, 80% longer than the Great Wall discovered byGeller and Huchra and therefore the largest observed structure in theuniverse.

The Multitude of Molecular Hydrogen Knots in the Helix Nebula
We present Hubble Space Telescope NICMOS imaging of the H22.12 μm emission in five fields in the Helix Nebula ranging in radialdistance from 250" to 450" from the central star. The images revealarcuate structures with their apexes pointing toward the central star.These molecular hydrogen knots are most highly structured in the fieldsclosest to the central star and become increasingly less structured withincreasing radius. Comparison of these images with ground-based imagesof comparable resolution reveals that the molecular gas is more highlyclumped than the ionized gas line tracers. From our images, we determinean average number density of knots in the molecular gas ranging from 162knots arcmin-2 in the denser regions to 18 knotsarcmin-2 in the lower density outer regions. The decreasingnumber density of H2 knots in the outer regions creates alower filling factor of neutral and molecular gas emission in the radioobservations of CO and H I and may explain why these outer regions,where we clearly detect H2 2.12 μm, fall below thedetection limit of the radio observations. Using this new numberdensity, we estimate the total number of knots in the Helix to be~23,000, which is a factor of 6.5 larger than previous estimates. Thetotal neutral gas mass in the Helix is 0.35 Msolar assuming amass of ~1.5×10-5 Msolar for the individualknots. The H2 emission structure of the entire Helix Nebulasupports the recent interpretation of the Helix as a nearly pole-onpolypolar planetary nebula (PN). The H2 intensity,(5-9)×10-5 ergs s-1 cm-2sr-1, remains relatively constant with projected distancefrom the central star, suggesting a heating mechanism for the moleculargas that is distributed almost uniformly in the knots throughout thenebula. The temperature and H2 2.12 μm intensity of theknots can be approximately explained by photodissociation regions (PDRs)in the individual knots; however, theoretical PDR models of PNsunderpredict the intensities of some knots by a factor of 10. Thebrightest H2 emission (~3×10-4 ergss-1 cm-2 sr-1) may be enhanced by alarger than unity area filling factor of H2 knots or may bean individual H2 knot exposed to direct starlight, causingrapid photoevaporation compared with the more embedded knots of thedisk.Based in part on observations with the NASA/ESA Hubble Space Telescopeobtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS5-26555.

A Multi-Instrument Study of the Helix Nebula Knots with the Hubble Space Telescope
We have conducted a combined observational and theoretical investigationof the ubiquitous knots in the Helix Nebula (NGC 7293). We haveconstructed a combined hydrodynamic + radiation model for the ionizedportion of these knots and have accurately calculated a static model fortheir molecular regions. Imaging observations in optical emission lineswere made with the Hubble Space Telescope's STIS, operating in a``slitless'' mode, complemented by WFPC2 images in several of the samelines. The NICMOS camera was used to image the knots in H2.These observations, when combined with other studies of H2and CO, provide a complete characterization of the knots. They possessdense molecular cores of densities about 106 cm-3surrounded on the central star side by a zone of hot H2. Thetemperature of the H2-emitting layer defies explanationeither through detailed calculations for radiative equilibrium orthrough simplistic calculations for shock excitation. Farther away fromthe core is the ionized zone, whose peculiar distribution of emissionlines is explained by the expansion effects of material flowing throughthis region. The shadowed region behind the core is the source of mostof the CO emission from the knot and is of the low temperature expectedfor a radiatively heated molecular region.Based in part on observations with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS 5-26555.Based in part on observations obtained at the Cerro TololoInter-American Observatory, which is operated by AURA, Inc., under acooperative agreement with the National Science Foundation.

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.

Molecular hydrogen kinematics in the ring-like planetary nebula NGC 6781
This paper presents the radial velocity mapping of the molecularhydrogen in the ring-like planetary nebula NGC 6781. Observations of theH2 v=1{-}0 S(1) emission line (2.122 μm, Δ λ=0.02 μm)were obtained by Fabry-Pérot spectroscopy. These observationsprovide a very detailed map of the kinematic structure of the molecularhydrogen envelope in NGC 6781. The kinematic structure is explained witha model consisting of a thin hollow cylinder whose axis is tilted withrespect to the line of sight, and gas expanding radially outward with avelocity proportional to the distance to the central star. Molecularhydrogen emission was detected at LSR velocities ranging from -6.2 to+57.1 km s-1, with peak intensities between +15.7 and +25.5km s-1. There is evidence that the molecular hydrogenenvelope is excited by shocks from the ionization front. The mass ofmolecular gas is estimated to be of the order of 0.2 M_ȯ. Theemergent picture from the H2 kinematic structure supports the scenariowhere the structure of NGC 6781 was formed from the ionization anddestruction of an ellipsoidal molecular envelope that began with thedestruction and ionization of the least dense polar caps and willcontinue until the densest molecular material at nebular waist is fullyionized.

Physical conditions in Photo-Dissociation Regions around Planetary Nebulae
We present observations of the infrared fine-structure lines of [Si II](34.8 μm), [O I] (63.2 and 145.5 μm) and [C II] (157.7 μm)obtained with the ISO SWS and LWS spectrographs of nine PlanetaryNebulae (PNe). These lines originate in the Photo-Dissociation Regions(PDRs) associated with the nebulae and provide useful information on theevolution and excitation conditions of the ejected material in theseregions. In order to interpret the observations, the measured lineintensities have been compared with those predicted byphoto-dissociation models. This comparison has been done taking intoaccount the C/O content in the nebulae. The densities derived with thiscomparison show a large scatter for some nebulae, probably because thedensity is higher than the critical density. Therefore, they are nolonger sensitive to this parameter implying that transitions from otherspecies with higher critical density should be used. The possiblecontribution of shocks to the observed emission characteristics of thesePNe is briefly discussed and it is shown that the radiation field is themain driving force responsible for the atomic lines in the PNe that havebeen studied. In addition, data on the pure rotational lines ofH2 in three nebulae (NGC 7027, NGC 6302 and Hb 5) are alsopresented. Assuming local thermal equilibrium the rotational temperatureand densities have been derived. We have derived the mass of atomic gasin the PDR associated with these PNe and compared those to ionic massesderived from Hβ and molecular masses derived from low J COobservations. This comparison shows that for these nebulae, the PDR isthe main reservoir of gas surrounding these objects. A comparison of theresults of these evolved PNe with very young PNe from the literaturesuggests that as the nebula ages the relative amount of ionic gasincreases at the expense of the atomic and molecular mass.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the UK) and with the participation of ISAS and NASA.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr ( or via http: / /cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/523

Molecular Hydrogen Kinematics and Structure in the Ring Nebula
This paper presents the radial velocity structure of the molecularhydrogen distribution in the Ring Nebula (NGC 6720). The structure wasderived from the Doppler shift of the H2 v=1-0 S(1) lineemission (2.122 μm, Δλ=0.02 μm), obtained usingFabry-Pérot spectroscopy. The kinematic results point to the factthat the molecular hydrogen emission is concentrated in an expandinghollow cylinder containing H2 clumps. In spite of the mediumresolution of the observations, the kinematics of the H2clumps in NGC 6720 are revealed, showing that they are distributed allover the ellipsoidal shell, with a wide range of values for their radialvelocities and sizes.Based on observations made at the Observatorio AstronómicoNacional at San Pedro Mártir, Baja California, México.

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.

Chemical abundances of planetary nebulae from optical recombination lines - I. Observations and plasma diagnostics
We have obtained deep optical spectra of medium resolution for a sampleof 12 Galactic planetary nebulae (PNe). Optical recombination lines(ORLs) from carbon, nitrogen and oxygen have been detected in 11 of themand neon ORLs in nine of them. All spectra were obtained by scanning along slit across the nebular surface, yielding relative line intensitiesfor the entire nebula that are suitable for comparison with integratedline fluxes measured in other wavelength regions using space-bornefacilities, such as the Infrared Space Observatory (ISO) and theInternational Ultraviolet Explorer (IUE). For 11 PNe, ISO infraredspectra between 2.4 and 197 μm are available, most of them taken byourselves, plus a Kuiper Airborne Observatory (KAO) infrared spectrum ofNGC 6210. IUE ultraviolet (UV) spectra are available for all nebulaeexcept one in our sample. The UV, optical and infrared spectra have beencombined to study nebular thermal and density structures and todetermine elemental abundances.We have determined UV to optical extinction curves towards these PNe byexamining observed fluxes of HI and HeII recombination lines, radiofree-free continuum flux density, and UV to optical nebular continua.For 11 PNe in our sample, the derived optical reddening curves are foundto be consistent with the standard Galactic extinction law for atotal-to-selective extinction ratio, R≡A(V)/EB-V= 3.1.However, the optical extinction curve towards Hu 1-2 yields R= 2.0. TheUV extinction towards Hu 1-2 and NGC 6572 is also found to be muchsteeper than the standard Galactic reddening law. In contrast, the UVextinction curve along the sight lines towards NGC 6210 is found to bemuch shallower, although in the latter case the uncertainties involvedare quite large.Electron temperatures and densities have been derived using a variety ofdiagnostic ratios of collisionally excited lines (CELs) in the UV,optical and infrared. The results show clear stratifications, both intemperature and density. Lines emitted by ions formed in regions ofhigher ionization degree yield higher temperatures than lines arisingfrom regions of lower ionization degree, while densities deduced fromratios of infrared diagnostic CELs of low critical densities, such asthe [OIII] 88-μm/52-μm ratio, are systematically lower than thosederived from UV and optical diagnostic lines, which in general have muchhigher critical densities than the infrared fine-structure lines.Electron temperatures have also been derived from the ratio of thenebular continuum Balmer discontinuity to H 11 for 11 PNe. For four ofthese, the Balmer jump temperatures are more than 1000 K lower thanvalues derived from the [OIII] optical collisionally excited diagnosticline ratio. With a difference of 3580 K, NGC 40 has the lowest Balmerjump temperature relative to the [OIII] optical forbidden-linetemperature. High-order Balmer line decrements have been used todetermine electron densities. The results are consistent with valuesderived from forbidden-line density-diagnostics.

Electron temperatures and densities of planetary nebulae determined from the nebular hydrogen recombination spectrum and temperature and density variations
A method is presented to derive electron temperatures and densities ofplanetary nebulae (PNe) simultaneously, using the observed hydrogenrecombination spectrum, which includes continuum and line emission. Bymatching theoretical spectra to observed spectra around the Balmer jumpat about 3646 Å, we determine electron temperatures and densitiesfor 48 Galactic PNe. The electron temperatures based on this method -hereafter Te(Bal) - are found to be systematically lower thanthose derived from [OIII] λ4959/λ4363 and [OIII] (88 μm+ 52 μm)/λ4959 ratios - hereafterTe([OIII]na) andTe([OIII]fn). The electron densities based on thismethod are found to be systematically higher than those derived from[OII] λ3729/λ3726, [SII] λ6731/λ6716,[ClIII] λ5537/λ5517, [ArIV] λ4740/λ4711 and[OIII] 88 μm/52 μm ratios. These results suggest that temperatureand density fluctuations are generally present within nebulae. Thecomparison of Te([OIII]na) and Te(Bal)suggests that the fractional mean-square temperature variation(t2) has a representative value of 0.031. A majority oftemperatures derived from the Te([OIII]fn) ratioare found to be higher than those of Te([OIII]na),which is attributed to the existence of dense clumps in nebulae - those[OIII] infrared fine-structure lines are suppressed by collisionalde-excitation in the clumps. By comparingTe([OIII]fn), Te([OIII]na)and Te(Bal) and assuming a simple two-density-componentmodel, we find that the filling factor of dense clumps has arepresentative value of 7 × 10-5. The discrepanciesbetween Te([OIII]na) and Te(Bal) arefound to be anticorrelated with electron densities derived from variousdensity indicators; high-density nebulae have the smallest temperaturediscrepancies. This suggests that temperature discrepancy is related tonebular evolution. In addition, He/H abundances of PNe are found to bepositively correlated with the difference betweenTe([OIII]na) and Te(Bal), suggestingthat He/H abundances might have been overestimated generally because ofthe possible existence of H-deficient knots. Electron temperatures anddensities deduced from spectra around the Paschen jump regions at 8250Åare also obtained for four PNe: NGC 7027, NGC 6153, M 1-42 andNGC 7009. Electron densities derived from spectra around the Paschenjump regions are in good agreement with the corresponding values derivedfrom spectra around the Balmer jump, whereas temperatures deduced fromthe spectra around the Paschen jump are found to be lower than thecorresponding values derived from spectra around the Balmer jump for allthe four cases. The reason remains unclear.

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.

Flux Ratio [Nev] 14.3/24.3 as a Test of Collision Strengths
From ISO [Nev] 14.3/24.3 μm line flux ratios, we find that 10 out of20 planetary nebulae (PNs) have measured ratios below the low-electrondensity (Ne) theoretical predicted limit. Such astronomicaldata serve to provide important tests of atomic data, collisionstrengths in this case. In principle, well-calibrated measurements ofthe [Nev] 14.3/24.3 flux ratio could improve upon the existing atomicdata.

Robotic Observatory Tools for Wide-Field Observations
The Burst Observer and Optical Transient Exploring System (BOOTES) is aproject mainly devoted to gamma-ray burst (GRB) research. It has beenoperating and producing important scientific results since 1998. Here wepresent two of the technical improvements that will allow BOOTES to keepon the leading edge of GRB investigation: the automatic image analysissoftware and the wide-field spectrograph.

A Uniform Database of 2.2-16.5 μm Spectra from the ISOCAM CVF Spectrometer
We present all ISOCAM circular variable filter (CVF) spectra that covermore than one-third of the 2.2-16.5 μm spectral range of theinstrument. The 364 spectra have been classified according to theclassification system of Kraemer et al., as modified by Hodge et al. toaccount for the shorter wavelength range. Prior to classification, thespectra were processed and recalibrated to create a uniform database.Aperture photometry was performed at each wavelength centered on thebrightest position in each image field and the various spectral segmentsmerged into a single spectrum. The aperture was the same for all scalesizes of the images. Since this procedure differs fundamentally fromthat used in the initial ISOCAM calibration, a recalibration of thespectral response of the instrument was required for the aperturephotometry. The recalibrated spectra and the software used to createthem are available to the community on-line via the ISO Data Archive.Several new groups were added to the KSPW system to describe spectrawith no counterparts in either the SWS or PHT-S databases: CA, E/SA,UE/SA, and SSA. The zodiacal dust cloud provides the most commonbackground continuum to the spectral features, visible in almost 40% ofthe processed sources. The most characteristic and ubiquitous spectralfeatures observed in the CVF spectral atlas are those of theunidentified infrared bands (UIR), which are typically attributed toultraviolet-excited fluorescence of large molecules containing aromatichydrocarbons. The UIR features commonly occur superimposed on thezodiacal background (18%) but can also appear in conjunction with otherspectral features, such as fine-structure emission lines or silicateabsorption. In at least 13 of the galaxies observed, the pattern of UIRemission features has been noticeably shifted to longer wavelengths.Based on observations with the Infrared Space Observatory, a EuropeanSpace Agency (ESA) project with instruments funded by ESA Member States(especially the Principal Investigator countries: France, Germany, theNetherlands, and the United Kingdom) and with the participation of theInstitute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).

Molecular Hydrogen in the Ionized Region of Planetary Nebulae
This paper presents an analysis of the concentration of hydrogenmolecules inside the ionized region of planetary nebulae (PNs). Theequations corresponding to the ionization and chemical equilibria of H,H+, H-, H2, H+2,and H+3 are coupled with the equations ofionization and thermal balance for a photoionized atomic gas. A total of40 different reactions related to the formation or destruction of thesespecies are included. The presence of dust is taken into account, sincegrains act as catalysts for the production of H2 as well asshield the molecules against the stellar ionizing radiation. We analyzethe effect of the stellar ionizing continuum as well as of the gas andgrain properties on the calculated H2 mass. It is shown thata significant concentration of H2 can survive inside theionized region of planetary nebulae, mostly in the inner region of therecombination zone. The total H2 to total hydrogen mass ratioinside the ionized region increases with the central star temperature,and, depending on the PN physical conditions, it may be of the order of~10-6 or even higher. The increase of the recombination zonewith stellar temperature can account for such correlation. This mayexplain why H2 emission is more frequently observed inbipolar PNs (Gatley's rule), since this kind of object typically hashotter stars. Applying our results to the planetary nebula NGC 6720, weobtain an H2 to hydrogen mass ratio similar to the valueobtained from the observed H2 line emission.

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Observation and Astrometry data

Right ascension:18h53m36.00s
Apparent magnitude:9.3

Catalogs and designations:
Proper NamesRing Nebula
MessierM 57
NGC 2000.0NGC 6720

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