Endogenous oxygen in the extremely metal-poor planetary nebula PN G135.9+55.9

It is shown that, in contrast to recent claims, oxygen (and helium) may not be extraordinarily underabundant in the new galactic halo planetary nebula (GHPN) PN G135.9+55.9 (hereafter PN G135). Determining elemental abundances in hot, highly ionized objects such as PN G135 depends critically on a proper description of the collisional excitation of the hydrogen Balmer lines, the departure from Case B recombination of hydrogen, the underlying stellar absorption lines, the shape of the primary continuum and the ionization equilibrium of highly ionized species of both oxygen and neon. Conversely, PN G135 provides unique checks of atomic data in unusual conditions: the H I collision strengths obtained by Aggarwal et al. (1991) for 1s - n transitions (3 ≤ n ≤ 5) are too large, while those obtained by Anderson et al. (2002) are acceptable. Empirical collision strengths are presented for n > 5. Photoionization models of PN G135 that fit all available optical data can be demonstrated only for oxygen abundances 12 + log (O/H) > 7.2 (>1/30 solar) and values 0.6 dex larger are possible, depending on the assumed C/O abundance ratio. Plausible variations in the geometry of the nebula, the primary stellar continuum and the atomic data do not alter this conclusion. The C/O ratio is less than 10 by number and Ne/O is at most solar. A satisfactory model for PN G135 can be obtained in which elemental abundances are nearly the same as those of a new detailed model for K 648, the prototypical GHPN in the old globular cluster M 15 (with 12 + log (O/H) = 7.58 ∼ 1/13 solar), although C/O may be smaller. Nonetheless, given the paucity of argon and iron in the nebula, PN G135 is likely to be a more extreme Population II object than K 648, reinforcing the idea of an endogenous origin for part of the oxygen in very metal-poor PNe. Assuming a standard H-burning post-Asymptotic Giant Branch evolution, timescale and spectroscopic considerations lead to an optimal solution, in which the distance to PN G135 is 8 kpc, the effective temperature of the nucleus slightly less than 1.3 × 105 K, its luminosity 1.4 × 1037 erg s-1, its mass 0.59 M⊙, the age of the ionized shell 104 yrs, the ionized mass 0.05 M⊙ and the abundances by number (H:He:C:O:Ne) = (106:81 500:90:30:4.5), with C/H being rather an upper limit and O/H and Ne/H uncertain by ±0.3 and ±0.1 dex respectively. Line intensities that could be used as diagnostics of the nebular elemental abundances are provided. Detailed imaging together with ultraviolet and very deep far-red spectra of PN G135 will be essential to definitely narrow the range of acceptable parameters and help us decide whether this exceptional PN is so oxygen-poor as to possibly influence current views on stellar evolution

Chemical abundances in the protoplanetary disc LV 2 (Orion) - II. High-dispersion VLT observations and microjet properties

Integral field spectroscopy of the LV 2 proplyd is presented taken with the Very Large Telescope (VLT)/FLAMES Argus array at an angular resolution of 0.31 × 0.31 arcsec2 and velocity resolutions down to 2 km s-1 pixel-1. Following subtraction of the local M42 emission, the spectrum of LV 2 is isolated from the surrounding nebula. We measured the heliocentric velocities and widths of a number of lines detected in the intrinsic spectrum of the proplyd, as well as in the adjacent Orion nebula falling within a 6.6 × 4.2 arcsec2 field of view. It is found that far-ultraviolet to optical collisional lines with critical densities, Ncr, ranging from 103 to 109 cm-3 suffer collisional de-excitation near the rest velocity of the proplyd correlating tightly with their critical densities. Lines of low Ncr are suppressed the most. The bipolar jet arising from LV 2 is spectrally and spatially well detected in several emission lines. We compute the [O III] electron temperature profile across LV 2 in velocity space and measure steep temperature variations associated with the red-shifted lobe of the jet, possibly being due to a shock discontinuity. From the velocity-resolved analysis the ionized gas near the rest frame of LV 2 has Te= 9200 ± 800 K and Ne∼ 106 cm-3, while the red-shifted jet lobe has Te≈ 9000–104 K and Ne∼ 106–107 cm-3. The jet flow is highly ionized but contains dense semineutral clumps emitting neutral oxygen lines. The abundances of N+, O2+, Ne2+, Fe2+, S+and S2+ are measured for the strong red-shifted jet lobe. Iron in the core of LV 2 is depleted by 2.54 dex with respect to solar as a result of sedimentation on dust, whereas the efficient destruction of dust grains in the fast microjet raises its Fe abundance to at least 30 per cent solar. Sulphur does not show evidence of significant depletion on dust, but its abundance both in the core and the jet is only about half solar

Unravelling the chemical inhomogeneity of PNe with VLT FLAMES integral-field unit spectroscopy

Recent weak emission-line long-slit surveys and modelling studies of PNe have convincingly argued in favour of the existence of an unknown component in the planetary nebula plasma consisting of cold, hydrogen-deficient gas, as an explanation for the long-standing recombination-line versus forbidden-line temperature and abundance discrepancy problems. Here we describe the rationale and initial results from a detailed spectroscopic study of three Galactic PNe undertaken with the VLT FLAMES integral-field unit spectrograph, which advances our knowledge about the small-scale physical properties, chemical abundances and velocity structure of these objects across a two-dimensional field of view, and opens up for exploration an uncharted territory in the study and modelling of PNe and photoionized nebulae in general.Comment: 4 pages; 3 figures; invited paper to appear in proceedings of IAU Symp. No. 234, 2006, Planetary Nebulae in our Galaxy and Beyond (held in Hawaii, April 2006

Chemical abundances in the protoplanetary disk LV2 (Orion): clues to the causes of the abundance anomaly in HII regions

Optical integral field spectroscopy of the archetype protoplanetary disk LV2 in the Orion Nebula is presented, taken with the VLT FLAMES/Argus fibre array. The detection of recombination lines of CII and OII from this class of objects is reported, and the lines are utilized as abundance diagnostics. The study is complemented with the analysis of HST Faint Object Spectrograph ultraviolet and optical spectra of the target contained within the Argus field of view. By subtracting the local nebula background the intrinsic spectrum of the proplyd is obtained and its elemental composition is derived for the first time. The proplyd is found to be overabundant in carbon, oxygen and neon compared to the Orion Nebula and the sun. The simultaneous coverage over LV2 of the CIII] 1908-A and [OIII] 5007-A collisionally excited lines (CELs) and CII and OII recombination lines (RLs) has enabled us to measure the abundances of C++ and O++ for LV2 with both sets of lines. The two methods yield consistent results for the intrinsic proplyd spectrum, but not for the proplyd spectrum contaminated by the generic nebula spectrum, thus providing one example where the long-standing abundance anomaly plaguing metallicity studies of HII regions has been resolved. These results would indicate that the standard forbidden-line methods used in the derivation of light metal abundances in HII regions in our own and other galaxies underestimate the true gas metallicity.Comment: Accepted by MNRAS November 8; 16 pages, 9 figs; typos corrected, error in FWHMs in table 4 corrected in this versio

Detection of the Central Star of the Planetary Nebula NGC 6302

NGC 6302 is one of the highest ionization planetary nebulae known and shows emission from species with ionization potential >300eV. The temperature of the central star must be >200,000K to photoionize the nebula, and has been suggested to be up to ~ 400,000K. On account of the dense dust and molecular disc, the central star has not convincingly been directly imaged until now. NGC 6302 was imaged in six narrow band filters by Wide Field Camera 3 on HST as part of the Servicing Mission 4 Early Release Observations. The central star is directly detected for the first time, and is situated at the nebula centre on the foreground side of the tilted equatorial disc. The magnitudes of the central star have been reliably measured in two filters(F469N and F673N). Assuming a hot black body, the reddening has been measured from the (4688-6766\AA) colour and a value of c=3.1, A_v=6.6 mag determined. A G-K main sequence binary companion can be excluded. The position of the star on the HR diagram suggests a fairly massive PN central star of about 0.64,M_sun close to the white dwarf cooling track. A fit to the evolutionary tracks for (T,L,t)=(200,000K, 2000L_sun, 2200yr), where t is the nebular age, is obtained; however the luminosity and temperature remain uncertain. The model tracks predict that the star is rapidly evolving, and fading at a rate of almost 1 % per year. Future observations could test this prediction.Comment: 13 pages, 5 figures, submitted to ApJ Letters on 25.09.2009 accepted on 19.10.200

Protoplanetary disks in the hostile environment of Carina

We report the first direct imaging of protoplanetary disks in the star-forming region of Carina, the most distant massive cluster in which disks have been imaged. Using the Atacama Large Millimeter/sub-millimeter Array (ALMA), the disks are observed around two young stellar objects (YSOs) that are embedded inside evaporating gaseous globules and exhibit jet activity. The disks have an average radius of 60 au and total masses of 30 and 50 ${M}_{\mathrm{Jup}}$. Given the measured masses, the minimum timescale required for planet formation (~1–2 Myr) and the average age of the Carina population (~1–4 Myr), it is plausible that young planets are present or their formation is currently ongoing in these disks. The non-detection of millimeter emission above the 4σ threshold ($\sim 7{M}_{\mathrm{Jup}}$) in the core of the massive cluster Trumpler 14, an area containing previously identified proplyd candidates, suggests evidence for rapid photo-evaporative disk destruction in the cluster's harsh radiation field. This would prevent the formation of giant gas planets in disks located in the cores of Carina's dense subclusters, whereas the majority of YSO disks in the wider Carina region remain unaffected by external photoevaporation

A deep survey of heavy element lines in planetary nebulae -- II. Recombination line abundances and evidence for ultra-cold plasma

[Abridged] Deep optical observations of the spectra of 12 Galactic planetary nebulae (PNe) and 3 Magellanic Cloud PNe were presented in Paper I by Tsamis et al. (2003b), who carried out an abundance analysis using the collisionally excited forbidden lines. Here, the relative intensities of faint optical recombination lines (ORLs) from ions of carbon, nitrogen and oxygen are analysed in order to derive the abundances of these ions relative to hydrogen. 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 ADF's that exceed unity. There is no dependence of the magnitude of the ADF upon the excitation energy of the UV, optical or IR 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. We have developed a new electron temperature diagnostic, based upon the relative intensities of the OII 4f-3d 4089A and 3p-3s 4649A 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 Balmer jump temperatures derived for the same nebulae. These results provide direct observational evidence for the presence of H-deficient, cold plasma regions within the nebulae, consistent with gas cooled largely by infrared fine structure and recombination transitions.Comment: 27 pages, 7 figures, submitted to the MNRA

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

We present spectrophotometry of 12 Galactic and 3 Magellanic Cloud planetary nebulae (PNe). Nine of the Galactic PNe were observed by scanning the slit across the PN. We use the fluxes of collisionally excited lines (CELs) to derive electron densities (D's) and temperatures (T's), and ionic abundances. We find that the D's derived from optical CEL ratios are systematically higher than those derived from the ratios of the IR fine-structure (FS) lines of [OIII], indicating the presence of significant density variations within the PNe. We also compare T's obtained from the ratio of optical nebular to auroral [OIII] lines with those obtained from the ratio of [OIII] optical to IR FS lines. We find that when the latter are derived using D's based on the [OIII] 52um/88um ratio, they yield values that are significantly higher than the optical [OIII] T's. Contrasting this, [OIII] optical/IR T's derived using the higher D's obtained from [ClIII] 5517A/5537A ratios show much closer agreement with optical [OIII] T's, implying that the observed [OIII] optical/IR ratios are significantly weighted by D's in excess of the critical densities of both [OIII] FS lines. Consistent with this, ionic abundances derived from [OIII] and [NIII] FS lines using D's from optical CELs show much better agreement with abundances derived for the same ions from optical and UV CELs than do abundances derived from the FS lines using the lower D's obtained from the 52um/88um ratios. The behaviour of T's obtained making use of the T-insensitive IR FS lines provides no support for significant T-fluctuations within the PNe that could be responsible for derived Balmer jump T's being lower than those obtained from the much more T-sensitive [OIII] optical lines.Comment: 36 pages; 3 figures; to be published in MNRA

The merging dwarf galaxy UM 448: chemodynamics of the ionized gas from VLT integral field spectroscopy

Using Very Large Telescope/Fibre Large Array Multi Element Spectrograph optical integral field unit observations, we present a detailed study of UM 448, a nearby blue compact galaxy (BCG) previously reported to have an anomalously high N/O abundance ratio. New Technology Telescope/Superb-Seeing Imager images reveal a morphology suggestive of a merger of two systems of contrasting colour, whilst our Hα  emission maps resolve UM 448 into three separate regions that do not coincide with the stellar continuum peaks. UM 448 exhibits complex emission line profiles, with most lines consisting of a narrow [full width at half-maximum (FWHM) ≲ 100 km s-1], central component, an underlying broad component (FWHM ∼ 150–300 km s-1) and a third, narrow blueshifted component. Radial velocity maps of all three components show signs of solid body rotation across UM 448, with a projected rotation axis that correlates with the continuum morphology of the galaxy. A spatially resolved, chemodynamical analysis, based on the [O III] λλ4363, 4959, [N II] λ6584, [S II] λλ6716, 6731 and [Ne III] λ3868 line maps, is presented. Whilst the eastern tail of UM 448 has electron temperatures (Te) that are typical of BCGs, we find a region within the main body of the galaxy where the narrow and broad [O III] λ4363 line components trace temperatures differing by 5000 K and oxygen abundances differing by 0.4 dex. We measure spatially resolved and integrated ionic and elemental abundances for O, N, S and Ne throughout UM 448, and find that they do not agree, possibly due the flux weighting of Te  from the integrated spectrum. This has significant implications for abundances derived from long-slit and integrated spectra of star-forming galaxies in the nearby and distant universe. A region of enhanced N/O ratio is indeed found, extended over a ∼0.6 kpc2 region within the main body of the galaxy. Contrary to previous studies, however, we do not find evidence for a large Wolf–Rayet (WR) population, and conclude that WR stars alone cannot be responsible for producing the observed N/O excess. Instead, the location and disturbed morphology of the N-enriched region suggest that interaction-induced inflow of metal-poor gas may be responsible