Optical Recombination Lines Research Articles

Exploring the high abundance discrepancy in the planetary nebula IC 4663

This article presents an in-depth analysis of integral field unit spectroscopy performed on the Galactic planetary nebula (PN) IC 4663, with a primary focus on investigating its physical characteristics, chemical composition, and morphological properties. The examination involved the utilization of a series of emission-line maps representing various elements and ionization zones, allowing for a comprehensive assessment of morphological and ionization structure of IC 4663. The majority of these maps depict an overall elliptical shape, featuring a relatively faint core at the center. Emanating from this core are two distinct and illuminated lobes extending in opposite directions, forming a conspicuous double-lobed configuration. The chemical analysis conducted on IC 4663 has revealed that the PN is rich in both helium and nitrogen, characterized by an N/O ratio exceeding 0.5, classifying it as a Peimbert type I nebula. Furthermore, the presence of a notably intense He II emission line at λ4686, in conjunction with the existence of high-excitation lines within the nebular spectrum, indicate its classification as a high-excitation class nebula. The computation of O2+, N2+, and N3+ through optical recombination lines has unveiled extreme abundance discrepancy factors, with values of 39.0 ± 6 for O2+ and 37.0 ± 9 for N2+ and N3+. These findings suggest the possibility that the central star of IC 4663 may be part of a binary system with a sub-stellar companion, potentially engaged in a common envelope interaction. Recent astrometric data derived from the Gaia mission further indicate a significant likelihood of interaction between the asymptotic giant branch halo of the PN and its surrounding interstellar medium, particularly in the southeast direction.

Two-photon Production in Low-velocity Shocks

The Galactic interstellar medium abounds in shocks with low velocities v s ≲ 70 km s−1. Some are descendants of higher velocity shocks, while others start off at low velocity (e.g., stellar bow shocks, intermediate velocity clouds, spiral density waves). Low-velocity shocks cool primarily via Lyα and two-photon continuum, augmented by optical recombination lines (e.g., Hα), forbidden lines of metals and free-bound emission, free–free emission. The dark far-ultraviolet (FUV) sky, aided by the fact that the two-photon continuum peaks at 1400 Å, makes the FUV band an ideal tracer of low-velocity shocks. GALEX FUV images reaffirm this expectation, discovering faint and large interstellar structure in old supernova remnants and thin arcs stretching across the sky. Interstellar bow shocks are expected from fast stars from the Galactic disk passing through the numerous gas clouds in the local interstellar medium within 15 pc of the Sun. Using the bests atomic data available to date, we present convenient fitting formulae for yields of Lyα, two-photon continuum, and Hα for pure hydrogen plasma in the temperature range of 104–105 K. The formulae presented here can be readily incorporated into time-dependent cooling models as well as collisional ionization equilibrium models.

Spectral study of the planetary nebula NGC 2452

We provide a detailed study for the Galactic planetary nebula (PN) NGC 2452, which has long been suspected to be a member of the Galactic cluster NGC2453. The present study uses integral field unit (IFU) spectroscopic observations of the PN to explore its physical conditions, chemical abundances, kinematical characteristics, and morphological features. The strength of the singly ionized helium (HeII λ4686) line reveals that the PN is of high excitation class. The presence of the neutral lines of [OI] and [NI] a long with the value of the flux ratio F([Nii]λ6583)/F(Hα) indicates that the object is slightly optically thick. We compute the nebular density and temperature from a large number of diagnostic line ratios. The ionic and total abundances of helium (He) and carbon (C) are derived from their optical recombination lines (ORLs), while the abundances of nitrogen (N), oxygen (O), neon (Ne), argon (Ar), sulphur (S), and chlorine (Cl) are derived from their collisionlly-excitation lines (CELs). Faint lines like Krypton (Kr), Manganese (Mn), and Chromium (Cr), which are known to be products of the slow-process (s-process), are also seen in the PN spectrum. To illustrate the morphology and excitation structures of the PN, we generate a gallery of emission-line maps using the nebular data cubes. We find that the PN has an overall elliptical shape with a few interior clumps. In terms of physical and chemical properties, we find no significant differences between the entire PN and its accompanying clumps, considering the uncertainties associated with these parameters. Both the astronomical data from Gaia and the inferred radial velocity suggest that the PN belongs to the Galactic thin-disk.

Modelling variable linear polarization produced by Co-rotating Interaction Regions (CIRs) across optical recombination lines of Wolf–Rayet stars

ABSTRACT Massive star winds are structured both stochastically (‘clumps’) and often coherently (Co-rotation Interaction Regions, or CIRs). Evidence for CIRs threading the winds of Wolf–Rayet (WR) stars arises from multiple diagnostics including linear polarimetry. Some observations indicate changes in polarization position angle across optical recombination emission lines from a WR star wind but limited to blueshifted Doppler velocities. We explore a model involving a spherical wind with a single conical CIR stemming from a rotating star as qualitative proof-of-concept. To obtain a realistic distribution of limb polarization and limb darkening across the pseudo-photosphere formed in the optically thick wind of a WR star, we used Monte Carlo radiative transfer (MCRT). Results are shown for a parameter study. For line properties similar to WR 6 (EZ CMa; HD 50896), combining the MCRT results, a simple model for the CIR, and the Sobolev approximation for the line formation, we were able to reproduce variations in both polarization amplitude and position angle commensurate with observations. Characterizing CIRs in WR winds has added importance for providing stellar rotation periods since the vsin i values are unobtainable because the pseudo-photosphere forms in the wind itself.

Non-detection of broad hydrogen radio recombination lines in the Circinus galaxy

ABSTRACT The line widths of broad-line regions (BLRs) of active galactic nuclei (AGNs) are key parameters for understanding central supermassive black holes. However, owing to obscuration from dusty tori, optical recombination lines from BLRs in type II AGNs cannot be directly detected. Radio recombination lines (RRLs), with low extinction, could be ideal tracers to probe the emission from BLRs in type II AGNs. We performed RRL observations for H35α and H36α towards the centre of the Circinus galaxy with Atacama Large Millimeter/submillimeter Array (ALMA). The narrow components of H35α and H36α, which are thought to be mainly from star-forming regions around the nuclear region, are detected. However, only upper limits are obtained for broad H35α and H36α. Because Circinus is one of the nearest AGNs, the non-detection of broad RRLs in Circinus in this band tells us that it is hopeless to try to detect broad RRL emission in local AGNs with current facilities. Submillimetre RRLs, with flux densities that are dozens of times higher than those at the millimetre level, could be the tools to directly detect BLRs in type II AGNs with ALMA, once its backend frequency coverage has been upgraded to several times better than its current capabilities.

No correlation of the Lyman continuum escape fraction with spectral hardness

The properties that govern the production and escape of hydrogen-ionizing photons (Lyman continuum, LyC; with energies > 13.6 eV) in star-forming galaxies are still poorly understood, but they are key to identifying and characterizing the sources that reionized the Universe. Here we empirically explore the relation between the hardness of ionizing radiation and the LyC leakage in a large sample of low-z star-forming galaxies from the recent Hubble Space Telescope Low-z Lyman Continuum Survey. Using Sloan Digital Sky Survey stacks and deep X-shooter observations, we investigate the hardness of the ionizing spectra (QHe+/QH) between 54.4 eV (He+) and 13.6 eV (H) from the optical recombination lines He II 4686 Å and Hβ 4861 Å for galaxies with LyC escape fractions spanning a wide range, fesc(LyC) ≃ 0−90%. We find that the observed intensity of He II/Hβ is primarily driven by variations in the metallicity, but is not correlated with LyC leakage. Both very strong (<fesc(LyC)> ≃ 0.5) and nonleakers (<fesc(LyC)> ≃ 0) present similar observed intensities of He II and Hβ at comparable metallicity, between ≃0.01 and ≃0.02 for 12 + log(O/H)> 8.0 and < 8.0, respectively. Our results demonstrate that QHe+/QH does not correlate with fesc(LyC), which implies that strong LyC emitters do not show harder ionizing spectra than nonleakers at similar metallicity.

Physical conditions and chemical abundances of the variable planetary nebula IC 4997

ABSTRACT The planetary nebula (PN) IC 4997 is one of a few rapidly evolving objects with variable brightness and nebular emission around a hydrogen-deficient star. In this study, we have determined the physical conditions and chemical abundances of this object using the collisionally excited lines (CELs) and optical recombination lines (ORLs) measured from the medium-resolution spectra taken in 2014 July with the FIbre-fed Échelle Spectrograph on the Nordic Optical Telescope at La Palma Observatory. We derived electron densities of ≳ 3 × 104 cm−3 and electron temperatures of ≳ 14 000 K from CELs, whereas cooler temperatures of ∼11 000 and ∼7000 K were obtained from helium and heavy element ORLs, respectively. The elemental abundances deduced from CELs point to a metal-poor progenitor with [O/H] ≲ −0.75, whereas the ORL abundances are slightly above the solar metallicity, [O/H] ≈ 0.15. Our abundance analysis indicates that the abundance discrepancy factors (ADFs ≡ ORLs/CELs) of this PN are relatively large: ADF(O2+) ≳ 8 and ADF(N2+) ≳ 7. Further research is needed to find out how the ADFs and variable emissions are formed in this object and whether they are associated with a binary companion or a very late thermal pulse.

Abundance discrepancy factors in high-density planetary nebulae

ABSTRACT 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.

Physical and Chemical Properties of Wolf–Rayet Planetary Nebulae

Abstract 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.

Can nebular He II emission be explained by ultra-luminous X-ray sources?

Context. The shape of the ionising spectra of galaxies is a key ingredient to reveal their physical properties and for our understanding of the ionising background radiation. A long-standing unsolved problem is the presence of He II nebular emission in many low-metallicity star-forming galaxies. This emission requires ionising photons with an energy > 54 eV; it is important to note that these particular photons are not produced in sufficient amounts by normal stellar populations. Aims. To examine if high mass X-ray binaries and ultra-luminous X-ray sources (HMXBs and ULXs) can explain the observed He II nebular emission and how their presence alters other emission lines, we computed photoionisation models of galaxies including such sources. Methods. We combined spectral energy distributions (SEDs) of integrated stellar populations with constrained SEDs of ULXs to obtain composite spectra with varying amounts of X-ray luminosity, parameterised by LX/SFR. With these, we computed photoionisation models to predict the emission line fluxes of the optical recombination lines of H and He+, as well as the main metal lines of [O III], [O II], [O I], and [N II]. The predictions were then compared to a large sample of low-metallicity galaxies. Results. We find that it is possible to reproduce the nebular He II λ4686 and other line observations with our spectra and with amounts of LX/SFR compatible with the observations. Our work suggests that HMBXs and ULXs could be responsible for the observed nebular He II emission. However, the strengths of the high and low ionisation lines, such as He II and [O I] λ6300, strongly depend on the X-ray contribution and on the assumed SEDs of the high energy source(s), the latter being poorly known.

Empirical mass-loss rates and clumping properties of Galactic early-type O supergiants

Aims. We investigate the impact of optically thick clumping on spectroscopic stellar wind diagnostics in O supergiants and constrain wind parameters associated with porosity in velocity space. This is the first time the effects of optically thick clumping have been investigated for a sample of massive hot stars, using models which include a full optically thick clumping description. Methods. We re-analyse existing spectroscopic observations of a sample of eight O supergiants previously analysed with the non-local-thermodynamic-equilibrium (NLTE) atmosphere code CMFGEN. Using a genetic algorithm wrapper around the NLTE atmosphere code FASTWIND we obtain simultaneous fits to optical and ultraviolet spectra and determine photospheric properties, chemical surface abundances and wind properties. Results. We provide empirical constraints on a number of wind parameters including the clumping factors, mass-loss rates and terminal wind velocities. Additionally, we establish the first systematic empirical constraints on velocity filling factors and interclump densities. These are parameters that describe clump distribution in velocity-space and density of the interclump medium in physical-space, respectively. We observe a mass-loss rate reduction of a factor of 3.6 compared to theoretical predictions from Vink et al. (2020, A&A, 362, 295) and mass-loss rates within a factor 1.4 of theoretical predictions from Björklund et al. (2021, A&A, 648, A36). Conclusions. We confirm that including optically thick clumping allows simultaneous fitting of optical recombination lines and ultraviolet resonance lines, including the unsaturated ultraviolet phosphorus lines (P V λλ1118–1128), without reducing the phosphorus abundance. We find that, on average, half of the wind velocity field is covered by dense clumps. We also find that these clumps are 25 times denser than the average wind, and that the interclump medium is 3–10 times less dense than the mean wind. The former result agrees well with theoretical predictions, the latter suggests that lateral filling-in of radially compressed gas might be critical for setting the scale of the rarefied interclump matter.

Physical conditions and chemical abundances in PN M 2-36. Results from deep echelle observations

ABSTRACT 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 impact of strong recombination on temperature determination in planetary nebulae

ABSTRACT The long-standing difference in chemical abundances determined from optical recombination lines and collisionally excited lines raises questions about our understanding of atomic physics, as well as the assumptions made when determining physical conditions and chemical abundances in astrophysical nebulae. Here, we study the recombination contribution of [O iii] 4363 and the validity of the line ratio [O iii] 4363/4959 as a temperature diagnostic in planetary nebulae with a high abundance discrepancy. We derive a fit for the recombination coefficient of [O iii] 4363 that takes into account the radiative and dielectronic recombinations, for electron temperatures from 200 to 30 000 K. We estimate the recombination contribution of [O iii] 4363 for the planetary nebulae Abell 46 and NGC 6778 by subtracting the collisional contribution from the total observed flux. We find that the spatial distribution for the estimated recombination contribution in [O iii] 4363 follows that of the O ii 4649 recombination line, both peaking in the central regions of the nebula, especially in the case of Abell 46 that has a much higher abundance discrepancy. The estimated recombination contribution reaches up to 70 and 40 per cent of the total [O iii] 4363 observed flux, for Abell 46 and NGC 6778, respectively.

Star Formation Traced by Optical and Millimeter Hydrogen Recombination Lines and Free–Free Emissions in the Dusty Merging Galaxy NGC 3256—MUSE/VLT and ALMA Synergy

Abstract A galaxy–galaxy merger and the subsequent triggering of starburst activity are fundamental processes linked to the morphological transformation of galaxies and the evolution of star formation across the history of the universe. Both nuclear and disk-wide starbursts are assumed to occur during the merger process. However, quantifying both nuclear and disk-wide star formation activity is nontrivial because the nuclear starburst is dusty in the most active merging starburst galaxies. This paper presents a new approach to this problem: combining hydrogen recombination lines in optical, millimeter, and free–free emission. Using NGC 3256 as a case study, Hβ, H40α, and free–free emissions are investigated using the Multi Unit Spectroscopic Explorer at the Very Large Telescope of the European Southern Observatory (MUSE) and the Atacama Large Millimeter/submillimeter Array (ALMA). The Hβ image obtained by MUSE identifies star-forming regions outside the nuclear regions, suggesting a disk-wide starburst. In contrast, the H40α image obtained by ALMA identifies a nuclear starburst where optical lines are undetected due to dust extinction (A V ∼ 25). Combining both MUSE and ALMA observations, we conclude that the total star formation rate (SFR) is 49 ± 2 M ⊙ yr−1 and the contributions from nuclear and disk-wide starbursts are ∼34% and ∼66%, respectively. This suggests the dominance of disk-wide star formation in NGC 3256. In addition, pixel-by-pixel analyses for disk-wide star-forming regions suggest that shock gas tracers (e.g., CH3OH) are enhanced where gas depletion time (τ gas = M gas/SFR) is long. This possibly means that merger-induced shocks regulate disk-wide star formation activities.

Realistic models for filling and abundance discrepancy factors in photoionized nebulae

ABSTRACT 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 < 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.

H-, He-like recombination spectra – III. n-changing collisions in highly excited Rydberg states and their impact on the radio, IR, and optical recombination lines

ABSTRACT At intermediate to high densities, electron (de-)excitation collisions are the dominant process for populating or depopulating high Rydberg states. In particular, the accurate knowledge of the energy changing (n-changing) collisional rates is determinant for predicting the radio recombination spectra of gaseous nebula. The different data sets present in the literature come either from impact parameter calculations or semi-empirical fits and the rate coefficients agree within a factor of 2. We show in this paper that these uncertainties cause errors lower than 5 per cent in the emission of radio recombination lines of most ionized plasmas of typical nebulae. However, in special circumstances where the transitions between Rydberg levels are amplified by maser effects, the errors can increase up to 20 per cent. We present simulations of the optical depth and Hnα line emission of active galactic nuclei broad-line regions and the Orion Nebula Blister to showcase our findings.

An imaging spectroscopic survey of the planetary nebula NGC 7009 with MUSE

Aims. The spatial structure of the emission lines and continuum over the 50″ extent of the nearby, O-rich, PN NGC 7009 (Saturn Nebula) have been observed with the MUSE integral field spectrograph on the ESO Very Large Telescope. This study concentrates on maps of line emission and their interpretation in terms of physical conditions. Methods. MUSE Science Verification data, in <0.6″ seeing, have been reduced and analysed as maps of emission lines and continuum over the wavelength range 4750–9350 Å. The dust extinction, the electron densities and temperatures of various phases of the ionized gas, abundances of species from low to high ionization and some total abundances are determined using standard techniques. Results. Emission line maps over the bright shells are presented, from neutral to the highest ionization available (He II and [Mn V]). For collisionally excited lines (CELs), maps of electron temperature (Te from [N II] and [S III]) and density (Ne from [S II] and [Cl III]) are available and for optical recombination lines (ORLs) temperature (from the Paschen jump and ratio of He I lines) and density (from high Paschen lines). These estimates are compared: for the first time, maps of the differences in CEL and ORL Te’s have been derived, and correspondingly a map of t2 between a CEL and ORL temperature, showing considerable detail. Total abundances of only He and O were formed, the latter using three ionization correction factors. However, the map of He/H is not flat, departing by ~2% from a constant value, with remnants corresponding to ionization structures. An integrated spectrum over an area of 2340 arcsec2 was also formed and compared to 1D photoionization models. Conclusions. The spatial variation of a range of nebular parameters illustrates the complexity of the ionized media in NGC 7009. These MUSE data are very rich with detections of hundreds of lines over areas of hundreds of arcsec2 and follow-on studies are outlined.

Identification of the central compact object in the young supernova remnant 1E 0102.2–7219

Oxygen-rich young supernova remnants 1 are valuable objects for probing the outcome of nucleosynthetic processes in massive stars, as well as the physics of supernova explosions. Observed within a few thousand years after the supernova explosion 2 , these systems contain fast-moving oxygen-rich and hydrogen-poor filaments visible at optical wavelengths: fragments of the progenitor’s interior expelled at a few thousand kilometres per second during the supernova explosion. Here we report the identification of the compact object in the supernova remnant 1E 0102.2–7219 in reprocessed Chandra X-ray Observatory data, enabled by the discovery of a ring-shaped structure visible primarily in optical recombination lines of Ne i and O i. The optical ring has a radius of (2.10 ± 0.35)″ ≡ (0.63 ± 0.11) pc, and is expanding at a velocity of $$90.{5}_{-30}^{+40}$$ 90 . 5 - 30 + 40 km s−1. It surrounds an X-ray point source with an intrinsic X-ray luminosity L i (1.2–2.0 keV) = (1.4 ± 0.2) × 1033 erg s−1. The energy distribution of the source indicates that this object is an isolated neutron star: a central compact object akin to those present in the Cas A3–5 and Pup A 6 supernova remnants, and the first of its kind to be identified outside of our Galaxy.

Bi-Abundance Ionisation Structure of the Wolf–Rayet Planetary Nebula PB 8

AbstractThe planetary nebula PB 8 around a [WN/WC]-hybrid central star is one of planetary nebulae with moderate abundance discrepancy factors (ADFs ~ 2–3), which could be an indication of a tiny fraction of metal-rich inclusions embedded in the nebula (bi-abundance). In this work, we have constructed photoionisation models to reproduce the optical and infrared observations of the planetary nebula PB 8 using a non-LTE stellar model atmosphere ionising source. A chemically homogeneous model initially used cannot predict the optical recombination lines. However, a bi-abundance model provides a better fit to most of the observed optical recombination lines from N and O ions. The metal-rich inclusions in the bi-abundance model occupy 5.6% of the total volume of the nebula, and are roughly 1.7 times cooler and denser than the mean values of the surrounding nebula. The N/H and O/H abundance ratios in the metal-rich inclusions are ~1.0 and 1.7 dex larger than the diffuse warm nebula, respectively. To reproduce the Spitzer spectral energy distribution of PB 8, dust grains with a dust-to-gas ratio of 0.01 (by mass) were also included. It is found that the presence of metal-rich inclusions can explain the heavy element optical recombination lines, while a dual-dust chemistry with different grain species and discrete grain sizes likely produces the infrared continuum of this planetary nebula. This study demonstrates that the bi-abundance hypothesis, which was examined in a few planetary nebulae with large abundance discrepancies (ADFs > 10), could also be applied to those typical planetary nebulae with moderate abundance discrepancies.

C/O ratios in planetary nebulae with dual-dust chemistry from faint optical recombination lines

We present deep high-resolution (R~15,000) and high-quality UVES optical spectrophotometry of nine planetary nebulae with dual-dust chemistry. We compute physical conditions from several diagnostics. Ionic abundances for a large number of ions of N, O, Ne, S, Cl, Ar, K, Fe and Kr are derived from collisionally excited lines. Elemental abundances are computed using state-of-the-art ionization correction factors. We derive accurate C/O ratios from optical recombination lines. We have re-analyzed additional high-quality spectra of 14 PNe from the literature following the same methodology. Comparison with asymptotic giant branch models reveals that about half of the total sample objects are consistent with being descendants of low-mass progenitor stars (M < 1.5 Msun). Given the observed N/O, C/O, and He/H ratios, we cannot discard that some of the objects come from more massive progenitor stars (M > 3--4 Msun) that have suffered a mild HBB. None of the objects seem to be a descendant of very massive progenitors. We propose that in most of the planetary nebulae studied here, the PAHs have been formed through the dissociation of the CO molecule. The hypothesis of a last thermal pulse that turns O-rich PNe into C-rich PNe is discarded, except in three objects, that show C/O > 1. We also discuss the possibility of a He pre-enrichment to explain the most He-enriched objects. We cannot discard other scenarios like extra mixing, stellar rotation or binary interactions to explain the chemical abundances behaviour observed in our sample.