Magellanic Cloud Planetary Nebulae Research Articles

Binary central stars of planetary nebulae in the Large Magellanic Cloud

Context. Close binary central stars of planetary nebulae (PNe) must have formed through a common envelope evolution during the giant phase experienced by one of the stars. Transfer of the angular momentum from the binary system to the envelope leads to the shortening of the binary separations from the radius of red giant to the radius of the order of few tenths of AU. Thus, close binary central stars of planetary nebulae are laboratories to study the common envelope phase of evolution. The close binary fraction in the Galaxy has been measured in various sky surveys, but the close binary fraction is not yet well constrained for the Magellanic Clouds, and our results may help the study of common envelope evolution in low-metallicity environments. Aims. This paper presents a continuation of our study of variability in the Magellanic Cloud planetary nebulae on the basis of data from the OGLE survey. Previously, we had analysed the OGLE data in the Small Magellanic Cloud. Here, the study is extended to the Large Magellanic Cloud (LMC). In this paper we search for close binary central stars with the aim to constrain the binary fraction and period distribution in the LMC. Methods. We identified 290 counterparts of PNe in the LMC in the I-band images from the OGLE-III and OGLE-IV surveys. However, the light curves of ten objects were not accessible in the OGLE database, and thus we analysed the time series photometry of 280 PNe. Results. In total, 32 variables were found, but 5 of them turned out to be foreground objects. Another 18 objects show irregular or regular variability that is not attributable to the binarity of their central stars. Their status and the nature of their variability will be verified in the follow-up paper. Nine binary central stars of PNe with periods between 0.24 and 23.6 days were discovered. The obtained fraction for the LMC PNe is 3.3-1.6+2.6% without correcting for incompleteness. This number is significantly lower than the 12–21% derived in the analogous search in the Galactic bulge. We discuss this difference, taking into account observational biases. The lower binary fraction suggests a lower efficiency of the common envelope phase in producing close binaries in the LMC compared to the Galaxy.

Carbon Abundances in Compact Galactic Planetary Nebulae: An Ultraviolet Spectroscopic Study with the Space Telescope Imaging Spectrograph (STIS)

We surveyed a sample of compact Galactic planetary nebulae (PNe) with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope (HST) to determine their gas-phase carbon abundances. Carbon abundances in PNe constrain the nature of their asymptotic giant branch (AGB) progenitors, as well as cosmic recycling. We measured the carbon abundances, or the limits thereof, of 11 compact Galactic PNe, notably increasing the sample of Galactic PNe whose carbon abundance based on HST ultraviolet spectra is available. The dust content of most targets has been studied elsewhere from Spitzer spectroscopy; given the compact nature of the nebulae, both UV and IR spectra can be directly compared to study gas- and dust-phase carbon. We found that carbon-poor (C/O < 1) compact Galactic PNe have an oxygen-rich dust type, while their carbon-enhanced counterparts (C/O > 1) have carbon-rich dust, confirming the correlation between gas- and dust-phase carbon content that was known for Magellanic Cloud PNe. Based on models of expected final yields from AGB evolution, we interpret the majority of the carbon-poor PNe in this study as the progeny of ∼1.1–1.2 M ⊙ stars that experienced some extra mixing on the red giant branch. They went through the AGB but did not go through the carbon star phase. Most PNe in this group have a bipolar morphology, possibly due to the presence of a subsolar companion. The carbon-enhanced PNe in our sample could be the progeny of stars in the ∼1.5–2.5 M ⊙ range, depending on their original metallicity.

Rapid evolution of [WC] stars in the Magellanic Clouds

We obtained new spectra of fourteen Magellanic Cloud planetary nebulae with the South African Large Telescope to determine heating rates of their central stars and to verify evolutionary models of post-asymptotic giant branch stars. We compared new spectra with observations made in previous years. Five planetary nebulae showed an increase in excitation over time. Four of their central stars exhibit [WC] features in their spectra, including three new detections. This raises the total number of [WC] central stars of PNe in the Magellanic Clouds to ten. We compared determined heating rates of the four [WC] central stars with the He-burning post-asymptotic giant branch evolutionary tracks and the remaining star with the H-burning tracks. Determined heating rates are consistent with the evolutionary models for both H and He-burning post-asymptotic giant branch stars. The central stars of the PNe that show the fastest increase of excitation are also the most luminous in the sample. This indicates that [WC] central stars in the Magellanic Clouds evolve faster than H-burning central stars, and they originate from more massive progenitors.

NEUTRON-CAPTURE ELEMENT ABUNDANCES IN MAGELLANIC CLOUD PLANETARY NEBULAE*

ABSTRACT We present near-infrared spectra of 10 planetary nebulae (PNe) in the Large and Small Magellanic Clouds (LMC and SMC), acquired with the FIRE and GNIRS spectrometers on the 6.5 m Baade and 8.1 m Gemini South Telescopes, respectively. We detect Se and/or Kr emission lines in eight of these objects, the first detections of n-capture elements in Magellanic Cloud PNe. Our abundance analysis shows large s-process enrichments of Kr (0.6–1.3 dex) in the six PNe in which it was detected, and Se is enriched by 0.5–0.9 dex in five objects. We also estimate upper limits to Rb and Cd abundances in these objects. Our abundance results for the LMC are consistent with the hypothesis that PNe with 2–3 M ⊙ progenitors dominate the bright end of the PN luminosity function in young gas-rich galaxies. We find no significant correlations between s-process enrichments and other elemental abundances, central star temperature, or progenitor mass, though this is likely due to our small sample size. We determine S abundances from our spectra and find that [S/H] agrees with [Ar/H] to within 0.2 dex for most objects, but is lower than [O/H] by 0.2–0.4 dex in some PNe, possibly due to O enrichment via third dredge-up. Our results demonstrate that n-capture elements can be detected in PNe belonging to nearby galaxies with ground-based telescopes, allowing s-process enrichments to be studied in PN populations with well-determined distances.

A test for asymptotic giant branch evolution theories: planetary nebulae in the Large Magellanic Cloud

We used a new generation of asymptotic giant branch (AGB) stellar models that include dust formation in the stellar winds to find the links between evolutionary models and the observed properties of a homogeneous sample of Large Magellanic Cloud (LMC) planetary nebulae (PNe). Comparison between the evolutionary yields of elements such as CNO and the corresponding observed chemical abundances is a powerful tool to shed light on evolutionary processes such as hot bottom burning (HBB) and third dredge-up (TDU). We found that the occurrence of HBB is needed to interpret the nitrogen-enriched (log(N/H)+12>8) PNe. In particular, N-rich PNe with the lowest carbon content are nicely reproduced by AGB models of mass M >=6 Mo, whose surface chemistry reflects the pure effects of HBB. PNe with log(N/H)+12<7.5 correspond to ejecta of stars that have not experienced HBB, with initial mass below about 3 Mo. Some of these stars show very large carbon abundances, owing to the many TDU episodes experienced. We found from our LMC PN sample that there is a threshold to the amount of carbon accumulated at AGB surfaces, log(C/H)+12<9. Confirmation of this constraint would indicate that, after the C-star stage is reached,AGBs experience only a few thermal pulses, which suggests a rapid loss of the external mantle, probably owing to the effects of radiation pressure on carbonaceous dust particles present in the circumstellar envelope. The implications of these findings for AGB evolution theories and the need to extend the PN sample currently available are discussed.

THE FORMATION OF FULLERENES: CLUES FROM NEW C 60 , C 70 , AND (POSSIBLE) PLANAR C 24 DETECTIONS IN MAGELLANIC CLOUD PLANETARY NEBULAE

We present ten new Spitzer detections of fullerenes in Magellanic Cloud Planetary Nebulae, including the first extragalactic detections of the C70 molecule. These new fullerene detections together with the most recent laboratory data permit us to report an accurate determination of the C60 and C70 abundances in space. Also, we report evidence for the possible detection of planar C24 in some of our fullerene sources, as indicated by the detection of very unusual emission features coincident with the strongest transitions of this molecule at ~6.6, 9.8, and 20 um. The infrared spectra display a complex mix of aliphatic and aromatic species such as hydrogenated amorphous carbon grains (HACs), PAH clusters, fullerenes, and small dehydrogenated carbon clusters (possible planar C24). The coexistence of such a variety of molecular species supports the idea that fullerenes are formed from the decomposition of HACs. We propose that fullerenes are formed from the destruction of HACs, possibly as a consequence of shocks driven by the fast stellar winds, which can sometimes be very strong in transition sources and young PNe. This is supported by the fact that many of our fullerene-detected PNe show altered [NeIII]/[NeII] ratios suggestive of shocks as well as P-Cygni profiles in their UV lines indicative of recently enhanced mass loss.

A detailed look at chemical abundances in the Magellanic Clouds

AbstractWe determine elemental abundances of He, N, O, Ne, S, and Ar in Magellanic Cloud planetary nebulae (PNe) using direct methods and a large set of observed ions, minimizing the need for ionization correction factors. In contrast to prior studies, we find a clear separation between Type I and non-Type I PNe in these low-metallicity environments, and no evidence that the O-N nucleosynthesis cycle is active in low-mass progenitors. We find that the S/O abundance ratio is anomalously low compared to H ii regions, confirming the “sulfur anomaly” found for Galactic PNe. We also found that Ne/O is elevated in some cases, raising the possibility that Ne yields in low-mass AGB stars may be enhanced at low metallicity.

Improved prospects for the detection of new Large Magellanic Cloud planetary nebulae

The Large Magellanic Cloud (LMC) contains the nearest large extragalactic population of planetary nebulae (PNe). A shallow viewing angle and low interstellar reddening towards the LMC potentially means a larger, more complete flux-limited population can be assembled than for any other galaxy. These advantages appear to be reflected by the small gap between the catalogued (~700 PNe) and estimated (1000 ± 250 PNe) population size. With more detailed multi-wavelength studies the catalogued number of LMC PNe may fall, potentially widening this gap. We demonstrate here that the gap can be further bridged with improved optical and near-infrared imaging surveys. We present three [O III]-selected PNe discovered from ESO WFI observations of the 30 Doradus region and one serendipitous discovery from near-infrared Vista Magellanic Cloud (VMC) survey observations. The WFI PNe have resolved [O III] and Hα nebulae that verify their PN nature and their [O III] fluxes place them 6–7 mag (m5007 = 20–21 mag) fainter than the bright-end of the planetary nebula luminosity function (PNLF). Their faintness, small angular size and surrounding complex emission-line background explains why previous surveys of the region did not select them. We estimate there may be as many as 50–75 similar PNe awaiting discovery in the central 5 × 5 degrees of the LMC. The VMC survey routinely detects PNe as resolved nebulae that may allow some of this expected population to be recovered without traditional narrow-band imaging surveys. We demonstrate this potential with the first new VMC-selected PN which has a rare Wolf-Rayet [WC9]–[WC11] central star.

A DETAILED LOOK AT CHEMICAL ABUNDANCES IN MAGELLANIC CLOUD PLANETARY NEBULAE. I. THE SMALL MAGELLANIC CLOUD

We present an analysis of elemental abundances of He, N, O, Ne, S, and Ar in Magellanic Cloud planetary nebulae (PNe), and focus initially on 14 PNe in the Small Magellanic Cloud (SMC). We derived the abundances from a combination of deep, high dispersion optical spectra, as well as mid-infrared (IR) spectra from the Spitzer Space Telescope. A detailed comparison with prior SMC PN studies shows that significant variations among authors of relative emission line flux determinations lead to systematic discrepancies in derived elemental abundances between studies that are >~0.15 dex, in spite of similar analysis methods. We used ionic abundances derived from IR emission lines, including those from ionization stages not observable in the optical, to examine the accuracy of some commonly used recipes for ionization correction factors (ICFs). These ICFs, which were developed for ions observed in the optical and ultraviolet, relate ionic abundances to total elemental abundances. We find that most of these ICFs work very well even in the limit of substantially sub-Solar metallicities, except for PNe with very high ionization. Our abundance analysis shows enhancements of He and N that are predicted from prior dredge-up processes of the progenitors on the AGB, as well as the well known correlations among O, Ne, S, and Ar that are little affected by nucleosynthesis in this mass range. We identified MG_8 as an interesting limiting case of a PN central star with a ~3.5 M_sun progenitor in which hot-bottom burning did not occur in its prior AGB evolution. We find no evidence for O depletion in the progenitor AGB stars via the O-N cycle, which is consistent with predictions for lower-mass stars. We also find low S/O ratios relative to SMC H_II regions, with a deficit comparable to what has been found for Galactic PNe.

THE GALACTIC STRUCTURE AND CHEMICAL EVOLUTION TRACED BY THE POPULATION OF PLANETARY NEBULAE

We use an extended and homogeneous data set of Galactic planetary nebulae (PNe) to study the metallicity gradients and the Galactic structure and evolution. The most up-to-date abundances, distances (calibrated with Magellanic Cloud PNe) have been employed, together with a novel homogeneous morphological classification, to characterize the different PN populations. We confirm that morphological classes have a strong correlation with PN Peimbert's Type, and also with their distribution on the Galactic landscape. We studied the alpha-element distribution within the Galactic disk, and found that the best selected disk population, together with the most reliable PN distance scale yields to a radial oxygen gradient of d[log(O/H)]/dR=-0.023 +- 0.006 dex/ kpc for the whole disk sample, and of d[log(O/H)]/dR= -0.035+-0.024, -0.023+-0.005, and -0.011+-0.013 dex/kpc respectively for Type I, II, and III PNe. Neon gradients for the same PN types confirm the trend. Accurate statistical analysis show moderately high uncertainties in the slopes, but also confirm the trend of steeper gradient for PNe with more massive progenitors, indicating a possible steepening with time of the Galactic disk metallicity gradient. The PN metallicity gradients presented here are consistent with the local metallicity distribution; furthermore, oxygen gradients determined with young and intermediate age PNe show good consistency with oxygen gradients derived respectively from other young (OB stars, HII regions) and intermediate (open cluster) Galactic populations. We also extend the Galactic metallicity gradient comparison by revisiting the open cluster [Fe/H] data from high resolution spectroscopy. The analysis suggests that they could be compliant with the same general picture of a steepening of gradient with time.

CARBON ABUNDANCE IN SMALL MAGELLANIC CLOUD PLANETARY NEBULAE THROUGH ADVANCED CAMERA FOR SURVEYS PRISM SPECTROSCOPY: CONSTRAINING STELLAR EVOLUTION AT LOW METALLICITY

We perform near ultraviolet ACS prism spectroscopy of 11 Small Magellanic Cloud (SMC) planetary nebulae (PNe) with the main aim of deriving the abundance of carbon. The analysis of the ACS spectra provides reliable atomic carbon abundances for all but a couple of our targets; ionic C{sup 2+} abundances are calculated for all target PNe. With the present paper we more than double the number of SMC PNe with known carbon abundances, providing a good database to study the elemental evolution in low- and intermediate-mass stars at low metallicity. We study carbon abundances of Magellanic Cloud PNe in the framework of stellar evolution models and the elemental yields. Constraining SMC and LMC stellar evolutionary models is now possible with the present data, through the comparison of the final yields calculated and the CNO abundances observed. We found that SMC PNe are almost exclusively carbon rich, and that for the most part they have not undergone the hot bottom burning phase, contrary to about half of the studied LMC PNe. The yields from stellar evolutionary models with LMC and SMC metallicities broadly agree with the observations. In particular, evolutionary yields for M {sub to} < 3.5 M {sub sun} well encompassmore » the abundances of round and elliptical PNe in the SMC. We found that the carbon emission lines are major coolants for SMC PNe, more so than in their LMC counterparts, indicating that metallicity has an effect on the physics of PNe, as predicted by Stanghellini et al.« less

The mid-infrared colours of Galactic bulge, disc and Magellanic planetary nebulae

We present mid-infrared (MIR) photometry for 367 Galactic disk, bulge and Large Magellanic Cloud (LMC) planetary nebulae, determined using GLIMPSE II and SAGE data acquired using the Spitzer Space Telescope. This has permitted us to make a comparison between the luminosity functions of bulge and LMC planetary nebulae, and between the MIR colours of all three categories of source. It is determined that whilst the 3.6 microns luminosity function of the LMC and bulge sources are likely to be closely similar, the [3.6]-[5.8] and [5.8]-[8-0] indices of LMC nebulae are different from those of their disk and bulge counterparts. This may arise because of enhanced 6.2 microns PAH emission within the LMC sources, and/or as a result of differences between the spectra of LMC PNe and those of their Galactic counterparts. We also determine that the more evolved disk sources listed in the MASH catalogues of Parker et al. and Miszalski et al. (2008) have similar colours to those of the less evolved (and higher surface brightness) sources in the catalogue of Acker et al. (1992); a result which appears at variance with previous studies of these sources.

The population of Magellanic Cloud planetary nebulae

AbstractIn this review we address the progress that has been made toward the understanding of Magellanic Cloud planetary nebulae (PNe) and their evolution since the last Magellanic Cloud Symposium. Planetary nebulae in the Magellanic Clouds are the key probes of stellar and circumstellar evolution, both for their known distances and relative vicinity, and for their broad metallicity range A selection of recent results is presented, including the HST study of PNe and their central stars, the study of the population of Magellanic Cloud PNe based on abundance analysis, the recent Spitzer analysis of their dust contents, and the use of Magellanic Cloud PNe to constrain the distance scale of Galactic PNe.

The IRS Spitzer Spectra of the Magellanic Cloud Planetary Nebulae: Revealing the Dust and Gas Chemistry

Planetary nebulae (PNe) in the Magellanic Clouds (LMC, SMC) offer a unique opportunity to study both the population and evolution of low- and intermediate-mass stars in an environment which is free of the distance scale bias that hinders Galactic PN studies. The emission shown by PNe in the 5–40 m range is characterized by the presence of a combination of solid state features (from the dust grains) and nebular emission lines superimposed on a strong dust continuum. We acquired low resolution IRS spectroscopy of a selected sample of LMC and SMC PNe whose morphology, size, central star brightness, and chemical composition are known. The data have been acquired and reduced, and the IRS spectra show outstanding quality as well as very interesting features. The preliminary analysis presented here allows to determine strong correlations between gas and dust composition, and nebular morphology. More detailed analysis in the future will deepen our knowledge of the mass-loss mechanism, its efficiency, and its relation to PN morphology.

Carbon Abundances in the Small Magellanic Cloud Planetary Nebulae

As an ongoing study of Magellanic Cloud PNe we have obtained UV spectra of 9 PNe in the SMC to measure their carbon abundances. The spectra were acquired with ACS HRC/PR200L and SBC/PR130L. The ACS prisms give a reasonable resolution in the range of 1200–2500 Å to detect C IV, C III], and C II] nebular emission, essential for chemical studies of PNe. The carbon abundances of SMC PNe, together with those of the LMC previously determined with STIS spectroscopy, will allow a comparative study of nebular enrichment and provide a basis for comparison with stellar evolution models at various metallicities.

Space Telescope Imaging Spectrograph Ultraviolet Spectra of Large Magellanic Cloud Planetary Nebulae: A Study of Carbon Abundances and Stellar Evolution

We acquired spectra of 24 LMC planetary nebulae (PNs) in the 1150-3000 A range in order to determine carbon and other ionic abundances. The sample more than doubles the number of LMC PNs with high-quality UV spectra in this wavelength range and whose optical images are available in the Hubble Space Telescope archive. The Space Telescope Imaging Spectrograph was used with a very large aperture to obtain virtually slitless spectra; thus, the monochromatic images in the major nebula emission lines are also available. The analysis of the data shows extremely high quality spectra. This paper presents the emission lines identified and measured and the calculation of the ionic abundances of the emitting carbon and other ions, as well as total carbon abundance. P Cygni profiles have been found in a fraction of the nebulae, and the limiting velocities of the stellar winds estimated. The total carbon abundance can be inferred reliably in most nebulae. We found that the average carbon abundance in round and elliptical PNs is one order of magnitude larger than that of the bipolar PNs, while elliptical and round PNs with a bipolar core have a bimodal behavior. This results confirm that bipolarity in LMC PNs is tightly correlated with high-mass progenitors. When compared with predicted yields, we found that the observed abundance ratio shows a shift toward higher carbon abundances, which may be due to initial conditions assumed in the models not appropriate for LMC PNs.

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 of 12 Galactic planetary nebulae (PNe) and three Magellanic Cloud PNe, carrying out an abundance analysis using the collisionally excited forbidden lines. Here, we analyse the relative intensities of faint optical recombination lines (ORLs) from ions of carbon, nitrogen and oxygen in order to derive the abundances of these ions relative to hydrogen. The relative intensities of four high-l C ii recombination lines with respect to the well-known 3d–4f λ4267 line are found to be in excellent agreement with the predictions of recombination theory, removing uncertainties about whether the high C2+ abundances derived from the λ4267 line could be due to non-recombination enhancements of its intensity. We define an abundance discrepancy factor (ADF) as the ratio of the abundance derived for a heavy element ion from its recombination lines to that derived for the same ion from its ultraviolet, optical or infrared collisionally excited lines (CELs). All of the PNe in our sample are found to have ADFs that exceed unity. Two of the PNe, NGC 2022 and LMC N66, have O2+ ADFs of 16 and 11, respectively, while the remaining 13 PNe have a mean O2+ ADF of 2.6, with the smallest value being 1.8. Garnett and Dinerstein found that for a sample of about 12 PNe the magnitude of the O2+ ADF was inversely correlated with the nebular Balmer line surface brightness. We have investigated this for a larger sample of 20 PNe, finding weak correlations with decreasing surface brightness for the ADFs of O2+ and C2+. The C2+ ADFs are well correlated with the absolute radii of the nebulae, although no correlation is present for the O2+ ADFs. We also find both the C2+ and O2+ ADFs to be strongly correlated with the magnitude of the difference between the nebular [O iii] 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 nebular surface brightness and increasing absolute nebular radius. There is no dependence of the magnitude of the ADF upon the excitation energy of the ultraviolet, optical or infrared CEL transition used, indicating that classical nebular temperature fluctuations – i.e. in a chemically homogeneous medium – are not the cause of the observed abundance discrepancies. Instead, we conclude that the main cause of the discrepancy is enhanced ORL emission from cold ionized gas located in hydrogen-deficient clumps inside the main body of the nebulae, as first postulated by Liu et al. for the high-ADF PN, NGC 6153. We have developed a new electron temperature diagnostic, based upon the relative intensities of the O ii 4f–3d λ4089 and 3p–3s λ4649 recombination transitions. For six out of eight PNe for which both transitions are detected, we derive O2+ ORL electron temperatures of ≤300 K, very much less than the O2+ forbidden-line and H+ Balmer jump temperatures derived for the same nebulae. These results provide direct observational evidence for the presence of cold plasma regions within the nebulae, consistent with gas cooled largely by infrared fine-structure transitions; at such low temperatures, recombination transition intensities will be significantly enhanced due to their inverse power-law temperature dependence, while ultraviolet and optical CELs will be significantly suppressed.

Comprehensive modelling of the planetary nebula LMC-SMP 61 and its [WC]-type central star

We present a comprehensive study of the Magellanic Cloud planetary nebula SMP 61 and of its nucleus, a Wolf-Rayet type star classified [WC 5-6]. We have performed a detailed spectral analysis of the central star, using the Potsdam code for expanding atmospheres in non-LTE. The fluxes from the model stellar atmosphere were used to compute photoionization models of the nebula. All the available observations, within their error bars, were used to constrain these models. We find that the ionizing fluxes predicted by the stellar model are basically consistent with the fluxes needed by the photoionization model to reproduce the nebular emission, within the error margins. However, there are indications that the stellar model overestimates the number and hardness of Lyman continuum photons. The photoionization models imply a clumped density structure of the nebular material. The observed CIII] 1909/CII 4267 line ratio implies the existence of carbon-rich clumps in the nebula. Such clumps are likely produced by stellar wind ejecta, possibly mixed with the nebular material. We discuss our results with regard to the stellar and nebular post-AGB evolution.

Space Telescope Imaging Spectrograph Slitless Observations of Small Magellanic Cloud Planetary Nebulae: A Study on Morphology, Emission‐Line Intensity, and Evolution

A sample of 27 planetary nebulae (PNs) in the Small Magellanic Cloud (SMC) have been observed with the Hubble Space Telescope Imaging Spectrograph (HST/STIS) to determine their morphology, size, and the spatial variation of the ratios of bright emission lines. The morphologies of SMC PNs are similar to those of LMC and Galactic PNs. However, only a third of the resolved SMC PNs are asymmetric, compared to half of those in the LMC. The low-metallicity environment of the SMC seems to discourage the onset of bipolarity in PNs. We measured the line intensity, average surface brightness, and photometric radius of each nebula in Hα, Hβ, [O III] λλ4959 and 5007, [N II] λλ6548 and 6584, [S II] λλ6716 and 6731, He I λ6678, and [O I] λλ6300 and 6363. We show that the surface brightness-to-radius relationship is the same as in LMC PNs, indicating its possible use as a distance scale indicator for Galactic PNs. We determine the electron densities and the ionized masses of the nebulae where the [S II] lines were measured accurately, and we find that the SMC PNs are denser than the LMC PNs by a factor of 1.5. The average ionized mass of the SMC PNs is 0.3 M☉. We also found that the median [O III]/Hβ intensity ratio in the SMC is about half that of the corresponding LMC median. We use CLOUDY to model the dependence of the [O III]/Hβ ratio on the oxygen abundance. Our models encompass very well the average observed physical quantities. We suggest that the SMC PNs are principally cooled by the carbon lines, making it hard to study their excitation based on the optical lines at our disposal.

A deep survey of heavy element lines in planetary nebulae - I. Observations and forbidden-line densities, temperatures and abundances

We present deep optical spectrophotometry of 12 Galactic planetary nebulae (PNe) and three Magellanic Cloud PNe. Nine of the Galactic PNe were observed by scanning the slit of the spectrograph across the nebula, yielding relative line intensities for the entire nebula that are suitable for comparison with integrated nebular fluxes measured in other wavelength regions. In this paper we use the fluxes of collisionally excited lines (CELs) from the nebulae to derive electron densities and temperatures, and ionic abundances. We find that the nebular electron densities derived from optical CEL ratios are systematically higher than those derived from the ratios of the infrared (IR) fine-structure (FS) lines of [O III]. The latter have lower critical densities than the typical nebular electron densities derived from optical CELs, indicating the presence of significant density variations within the nebulae, with the IR CELs being biased towards lower density regions. We find that for several nebulae the electron temperatures obtained from [O II] and [N II] optical CELs are significantly affected by recombination excitation of one or more of the CELs. When allowance is made for recombination excitation, much better agreement is obtained with the electron temperatures obtained from optical [O III] lines. We also compare electron temperatures obtained from the ratio of optical nebular to auroral [O III] lines with temperatures obtained from the ratio of [O III] optical lines to [O III] IR FS lines. We find that when the latter are derived using electron densities based on the [O III]52 μm/88 μm line ratio, they yield values that are significantly higher than the optical [O III] electron temperatures. In contrast to this, [O III] optical/IR temperatures derived using the higher electron densities obtained from optical [Cl III]λ5517/λ5537 ratios show much closer agreement with optical [O III] electron temperatures, implying that the observed [O III] optical/IR ratios are significantly weighted by densities in excess of the critical densities of both [O III] FS lines. Consistent with this, ionic abundances derived from [O III] and [N III] FS lines using electron densities from optical CELs show much better agreement with abundances derived for the same ions from optical and ultraviolet CELs than do abundances derived from the FS lines using the lower electron densities obtained from the observed [O III]52 μm/88 μm ratios. The behaviour of these electron temperatures, obtained making use of the temperature-insensitive [O III] IR FS lines, provides no support for significant temperature fluctuations within the nebulae being responsible for derived Balmer jump electron temperatures that are lower than temperatures obtained from the much more temperature sensitive [O III] optical lines.