SDSS-V LVM: A Spatially Resolved Study of the Physical Conditions and the Chemical Abundance Discrepancy in the Lagoon Nebula (M 8)

We present deep optical spectra of 23 galactic planetary nebulae, which are analysed in conjunction with archival infrared and ultraviolet spectra. We derive nebular electron temperatures based on standard collisionally excited line (CEL) diagnostics as well as the hydrogen Balmer jump and find that, as expected, the Balmer jump almost always yields a lower temperature than the [O III] nebular-to-auroral line ratio. We also make use of the weak temperature dependence of helium and O II recombination line ratios to further investigate the temperature structure of the sample nebulae. We find that, in almost every case, the derived temperatures follow the relation T e (CEL) ≥ T e (BJ) ≥ T e (He I) ≥ T e (O II), which is the relation predicted by two-component nebular models in which one component is cold and hydrogen-deficient. T e (O II) may be as low as a few hundred Kelvin, in line with the low temperatures found for the hydrogen-deficient knots of Abell 30 by Wesson, Liu and Barlow. Elemental abundances are derived for the sample nebulae from both CELs and optical recombination lines (ORLs). ORL abundances are higher than CEL abundances in every case, by factors ranging from 1.5 to 12. Five objects with O 2+ abundance discrepancy factors greater than 5 are found. DdDm 1 and Vy 2-2 are both found to have a very large abundance discrepancy factor of 11.8. We consider the possible explanations for the observed discrepancies. From the observed differences between T e (O III) and T e (BJ), we find that temperature fluctuations cannot resolve the abundance discrepancies in 22 of the 23 sample nebulae, implying some additional mechanism for enhancing ORL emission. In the one ambiguous case, the good agreement between abundances derived from temperature-insensitive infrared lines and temperature-sensitive optical lines also points away from temperature fluctuations being present. The observed recombination line temperatures, the large abundance discrepancies and the generally good agreement between infrared and optical CEL abundances all suggest instead the existence of a cold hydrogen-deficient component within the 'normal' nebular gas. The origin of this component is as yet unknown.

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.

We have obtained deep optical, long-slit spectrophotometry of the Galactic H ii regions M 17, NGC 3576 and of the Magellanic Cloud H ii regions 30 Doradus, LMC N11B and SMC N66, recording the optical recombination lines (ORLs) of C ii, N ii and O ii. A spatial analysis of 30 Doradus is performed, revealing that the forbidden-line [O iii] electron temperature is remarkably constant across the nebula. The forbidden-line O2+/H+ abundance mapped by the [O iii]λ4959 collisionally excited line (CEL) is shown to be consistently lower than the recombination-line abundance mapped by the O ii V1 multiplet at 4650 Å. In addition, the spatial profile of the C2+/O2+ ratio derived purely from recombination lines is presented for the first time for an extragalactic nebula. Temperature-insensitive ORL C2+/O2+ and N2+/O2+ ratios are obtained for all nebulae except SMC N66. The ORL C2+/O2+ ratios show remarkable agreement within each galactic system, while also being in agreement with the corresponding CEL ratios. The disagreement found between the ORL and CEL N2+/O2+ ratios for M 17 and NGC 3576 can be attributed to the N ii V3 and V5 ORLs that were used being affected by fluorescent excitation effects. For all five nebulae, the O2+/H+ abundance derived from multiple O ii ORLs is found to be higher than the corresponding value derived from the strong [O iii]λλ4959, 5007 CELs, by factors of 1.8 to 2.7 for four of the nebulae. The LMC N11B nebula exhibits a more extreme discrepancy factor for the O2+ ion, ∼5. Thus, these H ii regions exhibit ORL/CEL abundance discrepancy factors that are similar to those previously encountered amongst planetary nebulae. Our optical CEL O2+/H+ abundances agree to within 20–30 per cent with published O2+/H+ abundances that have been obtained from observations of infrared fine-structure lines. Since the low excitation energies of the latter make them insensitive to variations about typical nebular temperatures, fluctuations in temperature are ruled out as the cause of the observed ORL/CEL O2+ abundance discrepancies. We present evidence that the observed O ii ORLs from these H ii regions originate from gas of very similar density (<3500 cm−3) to that emitting the observed heavy-element optical and infrared CELs, ruling out models that employ high-density ionized inclusions in order to explain the abundance discrepancy. We consider a scenario whereby much of the heavy-element ORL emission originates from cold (≤500 K) metal-rich ionized regions. These might constitute haloes that are being evaporated from much denser neutral cores. The origin of these metal-rich inclusions is not clear — they may have been ejected into the nebula by evolved, massive Of and Wolf—Rayet stars, although the agreement found between heavy-element ion ratios derived from ORLs with the ratios derived from CELs provides no evidence for nuclear-processed material in the ORL-emitting regions.

We present C and O abundances in the Magellanic Clouds derived from deep spectra of HII regions. The data have been taken with the Ultraviolet-Visual Echelle Spectrograph at the 8.2-m VLT. The sample comprises 5 HII regions in the Large Magellanic Cloud (LMC) and 4 in the Small Magellanic Cloud (SMC). We measure pure recombination lines (RLs) of CII and OII in all the objects, permitting to derive the abundance discrepancy factors (ADFs) for O^2+, as well as their O/H, C/H and C/O ratios. We compare the ADFs with those of other HII regions in different galaxies. The results suggest a possible metallicity dependence of the ADF for the low-metallicity objects, but more uncertain for high-metallicity objects. We compare nebular and B-type stellar abundances and we find that the stellar abundances agree better with the nebular ones derived from collisionally excited lines (CELs). Comparing these results with other galaxies we observe that stellar abundances seem to agree better with the nebular ones derived from CELs in low-metallicity environments and from RLs in high-metallicity environments. The C/H, O/H and C/O ratios show almost flat radial gradients, in contrast with the spiral galaxies where such gradients are negative. We explore the chemical evolution analysing C/O vs. O/H and comparing with the results of HII regions in other galaxies. The LMC seems to show a similar chemical evolution to the external zones of small spiral galaxies and the SMC behaves as a typical star-forming dwarf galaxy.

We re-examine the well-known discrepancy between ionic abundances determined via the analysis of recombination lines (RLs) and collisionally excited lines (CELs). We show that abundance variations can be mimicked in a chemically homogeneous medium by the presence of dense (nH &amp;gt; rsim 104 cm−3) X-ray irradiated regions which present different ionization and temperature structures from those of the more diffuse medium they are embedded in, which is predominantly ionized by extreme-ultraviolet radiation. The presence of X-ray ionized dense clumps or filaments also naturally explains the lower temperatures often measured from O ii RLs and from the Balmer jump when compared to temperatures determined by CELs. We discuss the implications for abundances determined via the analysis of CELs and RLs and provide a simple analytical procedure to obtain upward corrections for CEL-determined abundance. While we show that the abundance discrepancy factor and the Balmer Jump temperature determined from observations of the Orion Nebula can simultaneously be reproduced by this model (implying upward corrections for CELs by a factor of 1.15), we find that the required X-ray fluxes exceed the known Orion's stellar and diffuse X-ray budget, if we assume that the clumps are located at the edge of the blister. We propose, however, that spatially resolved observations may be used to empirically test the model, and outline how the framework developed in this Letter may be applied in the future to objects with better constrained geometries (e.g. planetary nebulae).

In Planetary Nebulae (PNe) and HII regions ionic abundances can be derived by using collisionally excited lines (CELs) or recombination lines (ORLs). Such abundances do not coincide for the same ion and usually abundances from ORLs are larger than those from CELs by factors of 2 or larger. The origin of the discrepancy, known as the Abundance Discrepancy Factor is an open problem in astrophysics of gaseous nebulae. It has been attributed to temperature fluctuations in the plasma, tiny metal-rich inclusions embedded in the H-rich plasma, gas inhomogeneities or other processes. In this work we analyze the kinematical behavior of CELs and ORLs in two PNe ionized by [WC] stars, finding that kinematics of ORLS is incompatible with the kinematics of CELs. In particular the expansion velocities from CELs and ORLs for the same ion are different, indicating that ORLs seem to be produced in zones nearer the central star than CELs. This is in agreement with results found by other authors for individual PNe.

Wolf–Rayet ([WR]) and weak-emission-line (wels) central stars of planetary nebulae (PNs) have hydrogen-deficient atmospheres, whose origins are not well understood. In the present study, we have conducted plasma diagnostics and abundance analyses of 18 Galactic PNs surrounding [WR] and wels nuclei, using collisionally excited lines (CELs) and optical recombination lines (ORLs) measured with the Wide Field Spectrograph on the Australian National University 2.3 m telescope at the Siding Spring Observatory complemented with optical archival data. Our plasma diagnostics imply that the electron densities and temperatures derived from CELs are correlated with the intrinsic nebular Hβ surface brightness and excitation class, respectively. Self-consistent plasma diagnostics of heavy-element ORLs of N2+ and O2+ suggest that a small fraction of cool (≲7000 K), dense (∼104–105 cm−3) materials may be present in some objects, though with large uncertainties. Our abundance analyses indicate that the abundance discrepancy factors (ADFs ≡ ORLs/CELs) of O2+ are correlated with the dichotomies between forbidden-line and He i temperatures. Our results likely point to the presence of a tiny fraction of cool, oxygen-rich dense clumps within diffuse warm ionized nebulae. Moreover, our elemental abundances derived from CELs are mostly consistent with asymptotic giant branch models in the range of initial masses from 1.5 to 5 M ⊙. Further studies are necessary to understand better the origins of abundance discrepancies in PNs around [WR] and wels stars.

From high-resolution spectra, chemical abundances from collisionally excited lines (CELs) and optical recombination lines (ORLs) have been determined for planetary nebulae (PNe) Cn 3-1, Vy 2-2, Hu 2-1, Vy 1-2 and IC 4997, which are young and dense objects. The main aim of this work is to derive their O+2/H+ abundance discrepancy factors (ADFs) between CELs and ORLs. He, O, N, Ne, Ar, S, and Cl abundances were obtained and our values are in agreement with those previously reported. We found that Cn 3-1, Hu 2-1, and Vy 1-2 have O abundances typical of disc PNe, while Vy 2-2 and IC 4997 are low O abundance objects ($\rm {12+log(O/H) \sim 8.2}$), which can be attributed to possible O depletion into dust grains. ADFs(O+2) of $4.30^{+1.00}_{-1.16}$, 1.85 ± 1.05, $5.34^{+1.27}_{-1.08}$ and $4.87^{+4.34}_{-2.71}$ were determined for Vy 2-2, Hu 2-1, Vy 1-2, and IC 4997, respectively. The kinematics of CELs and ORLs was analysed for each case to study the possibility that different coexisting plasmas in the nebula emit them. Expansion velocities of [O iii] and O ii are equal within uncertainties in three PNe, providing no evidence for these lines being emitted in different zones. Exceptions are Hu 2-1 and Vy 2-2, where ORLs might be emitted in different zones than CELs. For Vy 2-2 and IC 4997, we found that nebular and auroral lines of the same ion (S+, N+, Ar+2, Ar+3, O+2) might present different expansion velocities. Auroral lines show lower $\rm {V_{exp}}$, which might indicate that they are emitted in a denser and inner zone than the nebular ones.

We have acquired high spectral resolution observations (R=150,000) of the planetary nebulae NGC 7009 and NGC 6153, using bHROS on Gemini South. Observations of this type may provide a key to understanding why optical recombination lines (ORLs) yield systematically higher heavy element abundances for photoionized nebulae than do the classical forbidden collisionally excited lines (CELs) emitted by the same ions; NGC 7009 and NGC 6153 have notably high ORL/CEL abundance discrepancy factors (ADFs) of 5 and 10, respectively. Due to the opposite temperature dependences of ORLs and CELs, ORLs should be preferentially emitted by colder plasma. Our bHROS observations of NGC 7009 reveal that the [O III] 4363 Å CEL has a FWHM linewidth that is 1.5 times larger than that shown by O II ORLs in the same spectrum, despite the fact that all of these lines are emitted by the O$^{2+}$ ion. The bHROS spectra of NGC 6153 also show that its O II ORLs have significantly narrower linewidths than do the [O III] 4363 Å and 5007 Å lines but, in addition, the [O III] 4363 Å and 5007 Å lines show very different velocity profiles, implying the presence of large temperature variations in the nebula.

We present results from integral field spectroscopy of a field located near the Trapezium Cluster using the Potsdam Multi-Aperture Spectrophotometer (PMAS). The observed field contains a variety of morphological structures: five externally ionized protoplanetary discs (also known as proplyds), the high-velocity jet HH 514 and a bowshock. Spatial distribution maps are obtained for different emission line fluxes, the c(H) extinction coefficient, electron densities and temperatures, ionic abundances of different ions from collisionally excited lines (CELs), C 2+ and O 2+ abundances from recombination lines (RLs) and the abundance discrepancy factor of O 2+ , ADF(O 2+ ). We distinguish the three most prominent proplyds (177-341, 170-337 and 170-334) and analyse their impact on the spatial distributions of the above mentioned quantities. We find that collisional de-excitation has a major influence on the line fluxes in the proplyds. If this is not properly accounted for then physical conditions deduced from commonly used line ratios will be in error, leading to unreliable chemical abundances for these objects. We obtain the intrinsic emission of the proplyds 177-341, 170-337 and 170-334 by a direct subtraction of the background emission, though the last two present some background contamination due to their small sizes. A detailed analysis of 177-341 spectra making use of suitable density diagnostics reveals the presence of high-density gas (3.8 10 5 cm -3 ) in contrast to the typical values observed in the background gas of the nebula (3800 cm -3 ). We also explore how the background subtraction could be affected by the possible opacity of the proplyd and its effect on the derivation of physical conditions and chemical abundances of the proplyd 177-341. We construct a physical model for the proplyd 177-341 finding a good agreement between the predicted and observed line ratios. Finally, we find that the use of reliable physical conditions returns an ADF(O 2+ ) about zero for the intrinsic spectra of 177-341, while the background emission presents the typical ADF(O 2+ ) observed in the Orion nebula (0.16 0.11 dex). We conclude that the presence of high-density ionized gas is severely affecting the abundances determined from CELs and, therefore, those from RLs should be considered as a better approximation to the true abundances.

We present deep spectroscopy of three Galactic planetary nebulae (PNe) with large abundance discrepancy factors (ADFs): NGC6153, M1-42 and Hf2-2. The spectra were obtained with VLT/UVES and cover the whole optical range (3040-11,000 A) with a spectral resolution of ~20,000. For all three PNe, several hundred emission lines were detected and identified, with more than 70 per cent of them as permitted lines. Most of these permitted lines are excited by recombination. Numerous weak optical recombination lines (ORLs) of O II, C II, N II and Ne II were detected in the spectra and accurate fluxes measured. Line flux tables were compiled and ready for use by the community of nebular astrophysics. These ORLs were critically analyzed using the effective recombination coefficients recently calculated for the optical recombination spectrum of N II and O II under the physical conditions of photoionized gaseous nebulae. Plasma diagnostics based on the heavy element ORLs were carried out using the new atomic data. Elemental abundances derived from the ORLs were systematically higher than those derived from the collisionally excited lines (CELs) by a factor of ~10, 22 and 80 for NGC6153, M1-42 and Hf2-2, respectively. The electron temperatures derived from the heavy element ORLs are systematically lower than those derived from the CELs. These ORL versus CEL abundance and temperature discrepancies, previously observed in the three PNe through deep spectroscopy with medium to low spectral resolution, are thus confirmed by our analysis of the deep echelle spectra using the new atomic data.

It has recently been noted that there seems to be a strong correlation between planetary nebulae with close binary central stars, and highly enhanced recombination line abundances. We present new deep spectra of seven objects known to have close binary central stars, and find that the heavy element abundances derived from recombination lines exceed those from collisionally excited lines by factors of 5-95, placing several of these nebulae among the most extreme known abundance discrepancies. This study nearly doubles the number of nebulae known to have a binary central star and an extreme abundance discrepancy. A statistical analysis of all nebulae with measured recombination line abundances reveals no link between central star surface chemistry and nebular abundance discrepancy, but a clear link between binarity and the abundance discrepancy, as well as an anticorrelation between abundance discrepancies and nebular electron densities: all nebulae with a binary central star with a period of less than 1.15 days have an abundance discrepancy factor exceeding 10, and an electron density less than $\sim$1000 cm$^{-3}$; those with longer period binaries have abundance discrepancy factors less than 10 and much higher electron densities. We find that [O~{\sc ii}] density diagnostic lines can be strongly enhanced by recombination excitation, while [S~{\sc ii}] lines are not. These findings give weight to the idea that extreme abundance discrepancies are caused by a nova-like eruption from the central star system, occurring soon after the common-envelope phase, which ejects material depleted in hydrogen, and enhanced in CNONe but not in third-row elements.

We present a spectrum of the planetary nebula (PN) M 2-36 obtained using the Ultraviolet and Visual Echelle Spectrograph (UVES) at the Very Large Telescope. 446 emission lines are detected. We perform an analysis of the chemical composition using multiple electron temperature (Te) and density (ne) diagnostics. Te and ne are computed using a variety of methods, including collisionally excited line (CEL) ratios, O++ optical recombination lines (ORLs), and measuring the intensity of the Balmer jump. Besides the classical CEL abundances, we also present robust ionic abundances from ORLs of heavy elements. From CELs and ORLs of O++, we obtain a new value for the Abundance Discrepancy Factor (ADF) of this nebula, being ADF(O++) = 6.76 ± 0.50. From all the different line ratios that we study, we find that the object cannot be chemically homogeneous; moreover, we find that two-phased photoionization models are unable to simultaneously reproduce critical ${\rm O\, \small {II}}$ and [${\rm O\, \small {III}}$] line ratios. However, we find a three-phased model able to adequately reproduce such ratios. While we consider this to be a toy model, it is able to reproduce the observed temperature and density line diagnostics. Our analysis shows that it is important to study high ADF PNe with high spectral resolution, since its physical and chemical structure may be more complicated than previously thought.

The origin of the abundance discrepancy, i.e., the fact that abundances derived from recombination lines are larger than those from collisionally excited lines, is one of the key problems in the physics of photoionized nebulae. In this work, we analyze and discuss data for a sample of Galactic and extragalactic H II regions where this abundance discrepancy has been determined. We find that the abundance discrepancy factor (ADF) is fairly constant and of order 2 in the available sample of H II regions. This is a rather different behavior than that observed in planetary nebulae, where the ADF shows a much wider range of values. We do not find correlations between the ADF and the O/H, O++/H+ ratios, the ionization degree, Te(High), Te(Low)/Te(High), FWHM, and the effective temperature of the main ionizing stars within the observational uncertainties. These results indicate that whatever mechanism is producing the abundance discrepancy in H II regions it does not substantially depend on those nebular parameters. On the contrary, the ADF seems to be slightly dependent on the excitation energy, a fact that is consistent with the predictions of the classical temperature fluctuations paradigm. Finally, we find that Te-values obtained from O II recombination lines in H II regions are in agreement with those obtained from collisionally excited line ratios, a behavior that is again different from that observed in planetary nebulae. These similar temperature determinations are in contradiction with the predictions of the model based on the presence of chemically inhomogeneous clumps but are consistent with the temperature fluctuations paradigm. We conclude that all the indications suggest that the physical mechanism responsible for the abundance discrepancy in H II regions and planetary nebulae are different.

When comparing nebular electron densities derived from collisionally excited lines (CELs) to those estimated using the emission measure, significant discrepancies are common. The standard solution is to view nebulae as aggregates of dense regions of constant density in an otherwise empty void. This porosity is parametrized by a filling factor f &amp;lt; 1. Similarly, abundance and temperature discrepancies between optical recombination lines (ORLs) and CELs are often explained by invoking a dual delta distribution of a dense, cool, metal-rich component immersed in a diffuse, warm, metal-poor plasma. In this paper, we examine the possibility that the observational diagnostics that lead to such discrepancies can be produced by a realistic distribution of density and temperature fluctuations, such as might arise in plasma turbulence. We produce simulated nebulae with density and temperature fluctuations described by various probability distribution functions (pdfs). Standard astronomical diagnostics are applied to these simulated observations to derive estimates of nebular densities, temperatures, and abundances. Our results show that for plausible density pdfs, the simulated observations lead to filling factors in the observed range. None of our simulations satisfactorily reproduce the abundance discrepancy factors (ADFs) in planetary nebulae, although there is possible consistency with H ii regions. Compared to the case of density-only and temperature-only fluctuations, a positive correlation between density and temperature reduces the filling factor and ADF (from optical CELs), whereas a negative correlation increases both, eventually causing the filling factor to exceed unity. This result suggests that real observations can provide constraints on the thermodynamics of small-scale fluctuations.