X.-W. Liu

Elemental abundances of Galactic bulge planetary nebulae from optical recombination lines

Deep long-slit optical spectrophotometric observations are presented for 25 Galactic bulge planetary nebulae (GBPNe) and six Galactic disc planetary nebulae (GDPNe). The spectra, combined with archival ultraviolet (UV) spectra obtained with the International Ultraviolet Explorer and infrared spectra obtained with the Infrared Space Observatory, have been used to carry out a detailed plasma diagnostic and element abundance analysis utilizing both collisional excited lines (CELs) and optical recombination lines (ORLs). Comparisons of plasma diagnostic and abundance analysis results obtained from CELs and ORLs reproduce many of the patterns previously found for GDPNe. In particular we show that the large discrepancies between electron temperatures (Te values) derived from CELs and ORLs appear to be mainly caused by abnormally low values yielded by recombination lines and/or continua. Similarly, the large discrepancies between heavy element abundances deduced from ORLs and CELs are largely caused by abnormally high values obtained from ORLs, up to tens of solar in extreme cases. It appears that whatever mechanisms are causing the ubiquitous dichotomy between CELs and ORLs, their main effects are to enhance the emission of ORLs, but hardly affect that of CELs. It seems that heavy element abundances deduced from ORLs may not reflect the bulk composition of the nebula. Rather, our analysis suggests that ORLs of heavy element ions mainly originate from a previously unseen component of plasma of Te values of just a few hundred kelvins, which is too cool to excite any optical and UV CELs. We find that GBPNe are on the average 0.1‐0.2 dex more metal-rich than GDPNe but have a mean C/O ratio that is approximately 0.2 dex lower. By comparing the observed relative abundances of heavy elements with recent theoretical predictions, we show that GBPNe probably evolved from a relatively metal-rich environment of initial Z ∼ 0.013, compared to an initial Z � 0.008 for GDPNe. In addition, we find that GBPNe tend to have more massive progenitor stars than GDPNe. GBPNe are found to have an average magnesium abundance about 0.13 dex higher than GDPNe. The latter have a mean magnesium abundance almost identical to the solar value. The enhancement of magnesium in GBPNe and the large [α/Fe] ratios of bulge giants suggest that the primary enrichment process in the bulge was Type II supernovae. PN observations yield a Ne/O abundance ratio much higher than the solar value, suggesting that the solar neon abundance may have been underestimated by 0.2 dex.

Integrated spectrum of the planetary nebula NGC 7027

We present deep optical spectra of the archetypal young planetary nebula (PN) NGC 7027, covering a wavelength range from 3310 to 9160 A. The observations were carried out by uniformly scanning a long slit across the entire nebular surface, thus yielding average optical spectra for the whole nebula. A total of 937 emission features are detected. The extensive line list presented here should prove valuable for future spectroscopic analyses of emission line nebulae. The optical data, together with the archival IUE and ISO spectra, are used to probe the temperature and density structures and to determine the elemental abundances from lines produced by different excitation mechanisms. Electron temperatures have been derived from the hydrogen recombination Balmer jump (BJ), from ratios of He  optical recombination lines (ORLs) and from a variety of diagnostic ratios of collisionally excited lines (CELs). Electron densities have been determined from the intensities of high-order H  Balmer lines and of He  Pfund lines, as well as from a host of CEL diagnostic ratios. CEL and ORL diagnostics are found to yield compatible results. Adopting respectively electron temperatures of Te = 12 600 and 15 500 K for ions with ionization potentials lower or higher than 50 eV and a constant density of Ne = 47 000 cm −3 , elemental abundances have been determined from a large number of CELs and ORLs. The C 2+ /H + , N 2+ /H + ,O 2+ /H + and Ne 2+ /H + ionic abundance ratios derived from ORLs are found to be only slightly higher than the corresponding CEL values. We conclude that whatever mechanism is causing the BJ/CEL temperature discrepanies and the ORL/CEL abundance discrepancies that have been observed in many PNe, it has an insignificant effect on this bright young compact PN. The properties of the central star are also discussed. Based on the integrated spectrum and using the energy-balance method, we have derived an effective temperature of 219 000 K for the ionizing star. Finally, we report the first detection in the spectrum of this bright young PN of Raman-scattered O  features at 6830 and 7088 A, pointing to the existence of abundant neutral hydrogen around the ionized regions.

Electron temperatures and densities of planetary nebulae determined from the nebular hydrogen recombination spectrum and temperature and density variations

A method is presented to derive electron temperatures and densities of planetary nebulae (PNe) simultaneously, using the observed hydrogen recombination spectrum, which includes continuum and line emission. By matching theoretical spectra to observed spectra around the Balmer jump at about 3646 A, we determine electron temperatures and densities for 48 Galactic PNe. The electron temperatures based on this method - hereafter T e (Bal)- are found to be systematically lower than those derived from [O III] λ4959/λ4363 and [O III] (88 μm + 52 μm)/λ4959 ratios- hereafter T e ([O III] na ) and T e ([O III] fn ). The electron densities based on this method are found to be systematically higher than those derived from [O II] λ3729/λ3726, [S II] λ6731/λ6716, [Cl III] λ5537/λ5517, [ArIV] λ4740/λ4711 and [O III] 88 μm/52 μm ratios. These results suggest that temperature and density fluctuations are generally present within nebulae. The comparison of T e ([O III] na ) and T e (Bal) suggests that the fractional mean-square temperature variation (t 2 ) has a representative value of 0.031. A majority of temperatures derived from the T e ([O III] fn ) ratio are found to be higher than those of T e ([O III] na ), which is attributed to the existence of dense clumps in nebulae - those [O III] infrared fine-structure lines are suppressed by collisional de-excitation in the clumps. By comparing T e ([O III] fn ), T e ([O III] na ) and T e (Bal) and assuming a simple two-density-component model, we find that the filling factor of dense clumps has a representative value of 7 x 10 -5 . The discrepancies between T e ([O III] na ) and T e (Bal) are found to be anticorrelated with electron densities derived from various density indicators; high-density nebulae have the smallest temperature discrepancies. This suggests that temperature discrepancy is related to nebular evolution. In addition, He/H abundances of PNe are found to be positively correlated with the difference between T e ([O III] na ) and T e (Bal), suggesting that He/H abundances might have been overestimated generally because of the possible existence of H-deficient knots. Electron temperatures and densities deduced from spectra around the Paschen jump regions at 8250 A are also obtained for four PNe: NGC 7027, NGC 6153, M 1-42 and NGC 7009. Electron densities derived from spectra around the Paschen jump regions are in good agreement with the corresponding values derived from spectra around the Balmer jump, whereas temperatures deduced from the spectra around the Paschen jump are found to be lower than the corresponding values derived from spectra around the Balmer jump for all the four cases. The reason remains unclear.

Integral field spectroscopy of planetary nebulae: mapping the line diagnostics and hydrogen‐poor zones with VLT FLAMES

Results from the first dedicated study of Galactic planetary nebulae (PNe) by means of optical integral field spectroscopy with the Very Large Telescope Fibre Large Array Multi Element Spectrograph Argus integral field unit are presented. Three typical Galactic disc PNe have been mapped with the 11.5 × 7.2-arcsec2 Argus array: 2D spectral maps of the main shell of NGC 5882 and of large areas of NGC 6153 and NGC 7009 with 297 spatial pixels per target were obtained at subarcsec resolutions. A corresponding number of 297 spectra per target were obtained in the 396.4-507.8 nm range. Spatially resolved maps of emission lines and of nebular physical properties such as electron temperatures, densities and ionic abundances were produced. The abundances of helium and of doubly ionized carbon and oxygen, relative to hydrogen, were derived from optical recombination lines (ORLs), while those of O2+ were also derived from the classic collisionally excited lines (CELs). The occurrence of the abundance discrepancy problem, pertaining to oxygen, was investigated by mapping the ratio of ORL/CEL abundances for O 2+ [the abundance discrepancy factor (ADF)] across the face of the PNe. The ADF varies between targets and also with position within the targets, attaining values of ∼40 in the case of NGC 6153 and ∼30 in the case of NGC 7009. Correlations of the ADF with geometric distance from the central star and plasma surface brightness (for NGC 6153), as well as with [O iii] electron temperature, plasma ionization state and other physical properties of the targets are established. Very small values of the temperature fluctuation parameter in the plane of the sky, t2A(O2+), are found in all cases. It is argued that these results provide further evidence for the existence in run-of-the-mill PNe of a distinct nebular component consisting of hydrogen-deficient, super-metal-rich plasma. The zones containing this posited component appear as undulations in the C ii and O ii ORL abundance diagnostics of about 2 spatial pixels across, and so any associated structures should have physical sizes of less than ∼1000 astronomical units. Regarding the origin of the inferred zones, we propose that circumstellar discs, Abell 30-type knots, or Helix-type cometary globules may be involved. Implications for emission-line studies of nebulae are discussed.

Observations and three‐dimensional photoionization modelling of the Wolf–Rayet planetary nebula NGC 1501

Recent observations reveal that the central star of the planetary nebula Abell 48 exhibits spectral features similar to massive nitrogen-sequence Wolf–Rayet stars. This raises a pertinent question, whether it is still a planetary nebula or rather a ring nebula of a massive star. In this study, we have constructed a three-dimensional photoionization model of Abell 48, constrained by our new optical integral field spectroscopy. An analysis of the spatially resolved velocity distributions allowed us to constrain the geometry of Abell 48. We used the collisionally excited lines to obtain the nebular physical conditions and ionic abundances of nitrogen, oxygen, neon, sulphur and argon, relative to hydrogen. We also determined helium temperatures and ionic abundances of helium and carbon from the optical recombination lines. We obtained a good fit to the observations for most of the emission-line fluxes in our photoionization model. The ionic abundances deduced from our model are in decent agreement with those derived by the empirical analysis. However, we notice obvious discrepancies between helium temperatures derived from the model and the empirical analysis, as overestimated by our model. This could be due to the presence of a small fraction of cold metal-rich structures, which were not included in our model. It is found that the observed nebular line fluxes were best reproduced by using a hydrogen-deficient expanding model atmosphere as the ionizing source with an effective temperature of Teff = 70 kK and a stellar luminosity of L� = 5500 L� , which corresponds to a relatively low-mass progenitor star (∼ 3M � ) rather than a massive Pop I star.

Very deep spectroscopy of the bright Saturn nebula NGC 7009 - I. Observations and plasma diagnostics

We present very deep CCD spectrum of the bright, medium-excitation planetary nebula NGC 7009, with a wavelength coverage from 3040 to 11000 A. Traditional emission line identification is carried out to identify all the emission features in the spectra, based on the available laboratory atomic transition data. Since the spectra are of medium resolution, we use multi-Gaussian line profile fitting to deblend faint blended lines, most of which are optical recombination lines (ORLs) emitted by singly ionized ions of abundant second-row elements such as C, N, O and Ne. Computer-aided emission-line identification, using the code EMILI developed by Sharpee et al., is then employed to further identify all the emission lines thus obtained. In total about 1200 emission features are identified, with the faintest ones down to fluxes 10- 4 of Hβ. The flux errors for all emission lines, estimated from multi-Gaussian fitting, are presented. Plots of the whole optical spectrum, with identified emission lines labelled, are presented along with the results of multi-Gaussian fits. Of all the properly identified emission lines, permitted lines contribute 81 per cent to the total line number. More than 200 O II permitted lines are presented, as well as many others from N II and Ne II. Due to its relatively simple atomic structure, C II presents few lines. Within the flux range 10 ―2 ―10 ―4 Hβ where most permitted lines of C II, N II, O II and Ne II fall, the average flux measurement uncertainties are about 10-20 per cent. Comparison is also made of the number of emission lines identified in the current work of NGC 7009 and those of several other planetary nebulae (PNe) that have been extensively studied in the recent literature, and it shows that our line-deblending procedure increases the total line number significantly, especially for emission lines with fluxes lower than 10- 3 of Hβ. Higher resolution is still needed to obtain more reliable fluxes for those extremely faint emission lines, lines of fluxes of the order of 10 ―5 ―10 ―6 of Hβ. Plasma diagnostics using optical forbidden line ratios give an average electron temperature of 10020 K, which agrees well with previous results of the same object. The average electron density of NGC 7009 derived from optical forbidden line ratios is 4290 cm ―3 . The [O III] λ4959/λ4363 nebular-to-auroral line ratio yields an electron temperature of 9800 K. The ratio of the nebular continuum Balmer discontinuity at 3646 A to H II reveals an electron temperature of 6500 K, about 600 K lower than the measurements published in the literature. The Balmer decrement reveals a density of about 3000 cm ―3 . Also derived are electron temperatures from the He I line ratios, and a value of 5100 K from the λ7281/λ6678 ratio is adopted. Utilizing the effective recombination coefficients newly available, we find an electron temperature around 1000 K from the O II ORL spectrum. Thus the general pattern of electron temperatures, T e ([O III]) ≳ T e (H I BJ) ≳ T e (He I) ≳ T e (O II), which is seen in many PNe, is repeated in NGC 7009. Far-infrared fine-structure lines, with observed fluxes adopted from the literature, are also used to derive T e and N e . The [O III] (52 + 88 μm)/λ4959 line ratio gives an electron temperature of 9260 K, and the 52 μm/88 μm ratio yields an electron density of 1260 cm ―3 .

Fluorine abundances in planetary nebulae

We have determined fluorine abundances from the [F II] λ4789 and [F IV] λ4060 nebular emission lines for a sample of planetary nebulae (PNe). Our results show that fluorine is generally overabundant in PNe, thus providing new evidence for the synthesis of fluorine in asymptotic giant branch (AGB) stars. [F/O] is found to be positively correlated with the C/O abundance ratio, in agreement with the predictions of theoretical models of fluorine production in thermally pulsing AGB stars. A large enhancement of fluorine is observed in the Wolf-Rayet PN NGC 40, suggesting that high mass-loss rates probably favor the survival of fluorine.

V605 Aquilae: a born-again star, a nova or both?

V605 Aquilae is today widely assumed to have been the result of a final helium shell flash occurring on a single-post-asymptotic giant branch star. The fact that the outbursting star is in the middle of an old planetary nebula and that the ejecta associated with the outburst is hydrogen deficient supports this diagnosis. However, the material ejected during that outburst is also extremely neon rich, suggesting that it derives from an oxygen–neon–magnesium star, as is the case in the so-called neon novae. We have therefore attempted to construct a scenario that explains all the observations of the nebula and its central star, including the ejecta abundances. We find two scenarios that have the potential to explain the observations, although neither is a perfect match. The first scenario invokes the merger of a main-sequence star and a massive oxygen–neon–magnesium white dwarf. The second invokes an oxygen– neon–magnesium classical nova that takes place shortly after a final helium shell flash. The main drawback of the first scenario is the inability to determine whether the ejecta would have the observed composition and whether a merger could result in the observed hydrogendeficient stellar abundances observed in the star today. The second scenario is based on better-understood physics, but, through a population synthesis technique, we determine that its frequency of occurrence should be very low and possibly lower than what is implied by the number of observed systems. While we could not envisage a scenario that naturally explains this object, this is the second final flash star which, upon closer scrutiny, is found to have hydrogen-deficient ejecta with abnormally high neon abundances. These findings are in stark contrast with the predictions of the final helium shell flash and beg for an alternative

New effective recombination coefficients for nebular N ii lines

Aims. In nebular astrophysics, there has been a long-standing dichotomy in plasma diagnostics between abundance determinations using the traditionalmethod based on collisionally excited lines(CELs),on the one hand, and (optical)recombination lines/continuum, on the other. A number of mechanisms have been proposed to explain the dichotomy. Deep spectroscopy and recombination line analysis of emission line nebulae (planetary nebulae and H ii regions) in the past decade have pointed to the existence of another previously unknown component of cold, H-deficient material as the culprit. Better constraints are needed on the physical conditions (electron temperature and density), chemical composition, mass, and spatial distribution of the postulated H-deficient inclusions in order to unravel their astrophysical origins. This requires knowledge of the relevant atomic parameters, most importantly the effective recombination coefficients of abundant heavy element ions such as C ii ,O ii ,N ii ,a nd Neii, appropriate for the physical conditions prevailing in those cold inclusions (e.g. Te ≤ 1000 K). Methods. Here we report new ab initio calculations of the effective recombination coefficients for the N ii recombination spectrum. We have taken into account the density dependence of the coefficients arising from the relative populations of the fine-structure levels of the ground term of the recombining ion ( 2 P ◦ 1/2 and 2 P ◦ 3/2 in the case of N iii), an elaboration that has not been attempted before for this ion, and it opens up the possibility of electron density determination via recombination line analysis. Photoionization cross-sections, bound state energies, and the oscillator strengths of N ii with n ≤ 11 and l ≤ 4 have been obtained using the closecoupling R-matrix method in the intermediate coupling scheme. Photoionization data were computed that accurately map out the near-threshold resonances and were used to derive recombination coefficients, including radiative and dielectronic recombination. Also new is including the effects of dielectronic recombination via high-n resonances lying between the 2 P ◦ 1/2 and 2 P ◦ 3/2 levels. The new calculations are valid for temperatures down to an unprecedentedly low level (approximately 100 K). The newly calculated effective recombination coefficients allow us to construct plasma diagnostics based on the measured strengths of the N ii optical recombination lines (ORLs). Results. The derived effective recombination coefficients are fitted with analytic formulae as a function of electron temperature for different electron densities. The dependence of the emissivities of the strongest transitions of N ii on electron density and temperature is illustrated. Potential applications of the current data to electron density and temperature diagnostics for photoionized gaseous nebulae are discussed. We also present a method of determining electron temperature and density simultaneously.

Me 1-1: A PN containing a cool star

We report the detection of a cool stellar component at the center of the planetary nebula (PN) Me 1-1 and present optical spectra of the system. From measurements of nebular emission lines, we have derived electron temperature, density and chemical composition. Heavy elemental abundances deduced from collisionally excited lines (CELs) are compared with those derived from optical recombination lines (ORLs). The electron temperature and density deduced from the nebular analysis were used to calculate the nebular continuum emission, which was then subtracted from the observed spectrum in order to obtain the spectrum of the cool stellar component apparent in the observed spectrum. We calculate B and V magnitudes of the cool companion and obtain a color index of B − V = 1.20. By comparing the spectrum of the cool star with standard spectra in Pickless 1998 Stellar Flux Library, we find that the cool component has the spectral type of a K3-4 bright giant. Our analysis suggests that Me 1-1 is probably a yellow symbiotic system.