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Register a new userCan X-rays provide a solution to the abundance discrepancy problem in photoionised nebulae?
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Barbara Ercolano Dr
Publication date
2009
fields
Physics
and research's language is
English
Authors
Barbara Ercolano
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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 {it chemically homogeneous} medium by the presence of dense X-ray irradiated regions which present different ionisation and temperature structures from those of the more diffuse medium they are embedded in, which is predominantly ionised by extreme-ultraviolet radiation. The presence of X-ray ionised dense clumps or filaments also naturally explains the lower temperatures often measured from O {sc ii} recombination lines 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 upwards corrections for CEL-determined abundance. While we show that the abundance discrepancy factor (ADF) 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 Orions 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 we outline how the framework developed in this letter may be applied in the future to objects with better constrained geometries (e.g. planetary nebulae).
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In this paper, we will focus on the advances made in the last few years regarding the abundance discrepancy problem in ionized nebulae. We will show the importance of collecting deep, high-quality data of H II regions and planetary nebulae taken with the most advanced instruments attached to the largest ground-based telescopes. We will also present a sketch of some new scenarios proposed to explain the abundance discrepancy.
We present results from integral field spectroscopy with PMAS. The observed field contains: five 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{beta}) coefficient, electron densities and temperatures, ionic abundances of different ions from collisionally excited lines (CELs), C2+ and O2+ abundances from recombination lines (RLs) and the abundance discrepancy factor of O2+, ADF(O2+). 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 reveals the presence of high-density gas (3.8times10^5 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. 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(O2+) about zero for the intrinsic spectra of 177-341, while the background emission presents the typical ADF(O2+) observed in the Orion Nebula. 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.
The discrepancy between abundances computed using optical recombination lines (ORLs) and collisionally excited lines (CELs) is a major, unresolved problem with significant implications for the determination of chemical abundances throughout the Universe. In planetary nebulae (PNe), the most common explanation for the discrepancy is that two different gas phases coexist: a hot component with standard metallicity, and a much colder plasma enhanced in heavy elements. This dual nature is not predicted by mass loss theories, and direct observational support for it is still weak. In this work, we present our recent findings that demonstrate that the largest abundance discrepancies are associated with close binary central stars. OSIRIS-GTC tunable filter imaging of the faint O II ORLs and MUSE-VLT deep 2D spectrophotometry confirm that O II ORL emission is more centrally concentrated than that of [O III] CELs and, therefore, that the abundance discrepancy may be closely linked to binary evolution.
(Abridged) We present the abundance analysis of 12 PNe ionized by [WC]-type stars and wels obtained from high-resolution spectrophotometric data. Our main aims are to determine the chemical composition of the PNe and to study the behaviour of the abundance discrepancy problem (ADF) in this type of planetary nebulae. The detection of a large number of optical recombination lines (ORLs) and collisionally excited lines (CELs) from different ions were presented previously. Most of the ORLs were reported for the first time in these PNe. Ionic abundances were derived from the available CELs and ORLs, using previously determined physical conditions. Based on these two sets of ionic abundances, we derived the total chemical abundances in the nebulae using suitable ICFs (when available). In spite of the [WC] nature of the central stars, moderate ADF(O^++), in the range from 1.2 to 4, were found for all the objects. We found that when the quality of the spectra is high enough the ORLs O^++/H^+ abundance ratios obtained from different multiplets excited mainly by recombination are very similar. Possible dependence of ADFs with some nebular characteristics were analysed, finding no correlation. Abundances derived from CELs were corrected by determining the t^2 parameter. O abundances for PNe, derived from ORLs, are in general larger than the solar abundance. We derived the C/O ratio from ORLs and N/O and alpha-element/O ratios from CELs and found that these PNe are, in average, N-and C-richer than the average of large PN samples. About half of our sample is C-rich (C/O>1). The alpha-elements grow in lockstep with O abundance. Comparing the N/O and C /O ratios with those derived from stellar evolution models, we estimate that about half of our PNe have progenitors with initial masses > 4 M_sun. No correlation was found between the stellar [WC]-type and the nebular abundances.
Photoionization produces supra-thermal electrons, electrons with much more energy than is found in a thermalized gas at electron temperatures characteristic of nebulae. The presence of these high energy electrons may solve the long-standing t^2/ADF puzzle, the observations that abundances obtained from recombination and collisionally excited lines do not agree, and that different temperature indicators give different results, if they survive long enough to affect diagnostic emission lines. The presence of these non-Maxwellian distribution electrons is usually designated by the term kappa. Here we use well-established methods to show that the distance over which heating rates change are much longer than the distance supra thermal electrons can travel, and that the timescale to thermalize these electrons are much shorter than the heating or cooling timescales. These estimates establish that supra thermal electrons will have disappeared into the Maxwellian velocity distribution long before they affect the collisionally excited forbidden and recombination lines that are used for deriving abundances relative to hydrogen. The electron velocity distribution in nebulae should be closely thermal.
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