Abundance Discrepancy Factor Research Articles

The kinematical behavior of ORLs and CELs in PNe with [WC] central star

AbstractIn 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.

Imaging the elusive H-poor gas in planetary nebulae with large abundance discrepancy factors

AbstractThe 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 [Oiii] CELs and, therefore, that the abundance discrepancy may be closely linked to binary evolution.

Very Large Telescope deep echelle spectroscopy of Galactic planetary nebulae NGC 6153, M 1-42 and Hf 2-2

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.

The chemical composition of Galactic ring nebulae around massive stars

We present deep spectra of ring nebulae associated with Wolf-Rayet (WR) and O-type stars: NGC 6888, G2.4+1.4, RCW 58, S 308, NGC 7635 and RCW 52. The data have been taken with the 10m Gran Telescopio Canarias and the 6.5m Clay Telescope. We extract spectra of several apertures in some of the objects. We derive C$^{++}$ and O$^{++}$ abundances from faint recombination lines in NGC 6888 and NGC 7635, permitting to derive their C/H and C/O ratios and estimate the abundance discrepancy factor (ADF) of O$^{++}$. The ADFs are larger than the typical ones of normal HII regions but similar to those found in the ionised gas of star-forming dwarf galaxies. We find that chemical abundances are rather homogeneous in the nebulae where we have spectra of several apertures: NGC 6888, NGC 7635 and G2.4+1.4. We obtain very high values of electron temperature in a peripheral zone of NGC 6888, finding that shock excitation can reproduce its spectral properties. We find that all the objects associated with WR stars show N enrichment. Some of them also show He enrichment and O deficiency as well as a lower Ne/O than expected, this may indicate the strong action of the ON and NeNa cycles. We have compared the chemical composition of NGC 6888, G2.4+1.4, RCW 58 and S 308 with the nucleosynthesis predicted by stellar evolution models of massive stars. We find that non-rotational models of stars of initial masses between 25 and 40 solar masses seem to reproduce the observed abundance ratios of most of the nebulae.

NGC 6778: strengthening the link between extreme abundance discrepancy factors and central star binarity in planetary nebulae

We present new optical spectra of the nearby, bright, planetary nebula NGC 6778. The nebula has been known to emit strong recombination lines for more than 40 years but this is the first detailed study of its abundances. Heavy element abundances derived from recombination lines are found to exceed those from collisionally excited lines by a factor of ~20 in an integrated spectrum of the nebula, which is among the largest known abundance discrepancy factors. Spatial analysis of the spectra shows that the abundance discrepancy factor is strongly, centrally peaked, reaching ~40 close to the central star. The central star of NGC 6778 is known to be a short period binary, further strengthening the link between high nebular abundance discrepancy factors and central star binarity.

PHYSICAL AND CHEMICAL PROPERTIES OF PLANETARY NEBULAE WITH WR-TYPE NUCLEI

We have carried out optical spectroscopic measurements of emission lines for a sample of Galactic planetary nebulae with Wolf-Rayet (WR) stars and weak emission-line stars (wels). The plasma diagnostics and elemental abundance analysis have been done using both collisionally excited lines (CELs) and optical recombination lines (ORLs). It is found that the abundance discrepancy factors (ADF=ORL/CEL) are closely correlated with the dichotomy between temperatures derived from forbidden lines and those from He I recombination lines, implying the existence of H-deficient materials embedded in the nebula. The H$\beta$ surface brightness correlations suggest that they might be also related to the nebular evolution.

Balmer ratios, the abundance discrepancy factor and a two temperature component nebula

Balmer ratios, the abundance discrepancy factor and a two temperature component nebula

The post-common envelope central stars of the planetary nebulae Henize 2-155 and Henize 2-161

We present a study of Hen 2-155 and Hen 2-161, two planetary nebulae which bear striking morphological similarities to other planetary nebulae known to host close-binary central stars. Both central stars are revealed to be photometric variables while spectroscopic observations confirm that Hen 2-155 is host to a double-eclipsing, post-common-envelope system with an orbital period of 3h33m making it one of the shortest period binary central stars known. The observations of Hen 2-161 are found to be consistent with a post-common-envelope binary of period ~1 day. A detailed model of central star of Hen 2-155, is produced, showing the nebular progenitor to be a hot, post-AGB remnant of approximately 0.62 Msol, consistent with the age of the nebula, and the secondary star to be an M dwarf whose radius is almost twice the expected ZAMS radius for its mass. In spite of the small numbers, all main-sequence companions, of planetary nebulae central stars, to have had their masses and radii constrained by both photometric and spectroscopic observations have also been found to display this "inflation". The cause of the "inflation" is uncertain but is probably related to rapid accretion, immediately before the recent common-envelope phase, to which the star has not yet thermally adjusted. The chemical composition of both nebulae is also analysed, showing both to display elevated abundance discrepancy factors. This strengthens the link between elevated abundance discrepancy factors and close binarity in the nebular progenitor.

BINARITY AND THE ABUNDANCE DISCREPANCY PROBLEM IN PLANETARY NEBULAE

The discrepancy between abundances computed using optical recombination lines (ORLs) and collisionally excited lines (CELs) is a major unresolved problem in nebular astrophysics. We show here that the largest abundance discrepancies are reached in planetary nebulae with close binary central stars. This is illustrated by deep spectroscopy of three nebulae with a post common-envelope (CE) binary star. Abell 46 and Ou5 have O++/H+ abundance discrepancy factors larger than 50, and as high as 300 in the inner regions of Abell 46. Abell 63 has a smaller discrepancy factor around 10, but still above the typical values in ionized nebulae. Our spectroscopic analysis supports previous conclusions that, in addition to "standard" hot (Te~10000 K) gas, a colder (Te~1000 K) ionized component that is highly enriched in heavy elements also exists. These nebulae have low ionized masses, between 0.001 and 0.1 solar masses depending on the adopted electron densities and temperatures. Since the much more massive red-giant envelope is expected to be entirely ejected in the CE phase, the currently observed nebulae would be produced much later, in post-CE mass loss episodes when the envelope has already dispersed. These observations add constraints to the abundance discrepancy problem. Possible explanations are revised. Some are naturally linked to binarity, such as for instance high-metallicity nova ejecta, but it is difficult at this stage to depict an evolutionary scenario consistent with all the observed properties. The hypothesis that these nebulae are the result of tidal destruction, accretion and ejection of Jupiter-like planets is also introduced.

DENSITIES, TEMPERATURES, PRESSURES, AND ABUNDANCES DERIVED FROM O II RECOMBINATION LINES IN H II REGIONS AND THEIR IMPLICATIONS

Based on high-quality observations of multiplet V1 of O ii and the NLTE atomic computations of O ii, we study the density and temperature of a sample of H ii regions. We find that the signature for oxygen-rich clumps of high density and low temperature is absent in all objects of our sample: one extragalactic and eight Galactic H ii regions. The temperatures derived from (1) recombination lines (RLs) of O ii, and (2) RLs of H i together with Balmer continua are lower than those derived from forbidden lines, while the densities derived from RLs of O ii are similar or smaller than densities derived from forbidden lines. Electron pressures derived from collisionally excited lines are about two times larger than those derived from RLs. These results imply that the proper abundances are those derived from RLs and suggest that other processes in addition to direct photoionization, such as dissipation of turbulent energy in shocks, magnetic reconnection, and shadowed regions, might be responsible for the large abundance discrepancy factor and t2 values observed in H ii regions.

Ionized gas diagnostics from protoplanetary discs in the Orion nebula and the abundance discrepancy problem

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 (3.8\times10^5 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.

Abundances and ADFs in planetary nebulae with [WC] central stars

AbstractWe present preliminary results obtained from the analysis of very deep echelle spectra of a dozen planetary nebulae with [WC] or weak emission lines (wels) central stars. The computed abundance discrepancy factors (ADFs) are moderate, with values lower than 4. In principle, no evidence of the H-poor metal enriched inclusions proposed by Liu et al. (2000) have been found. However, a detailed analysis of the data is in progress.

SOFIA observations of the planetary nebula NGC 7009

AbstractWe report spectrophotometric observations made with SOFIA/FORCAST on 2011 June 2 UT. Optical measurements have previously shown that the abundance discrepancy factor (adf) varies with position in several high-adf PNe, and is highest close to the central star. The very low electron temperature inclusions postulated to explain the abundance discrepancy, must be cooled predominantly by fine structure IR lines. These SOFIA data will map mid-IR FS lines (and our related Herschel program will add several far-IR FS lines) in the bright, well-characterized, high-adf PN NGC 7009. We will compare these IR results with FS optical line measurements in order to correlate ratios of IR to optical fluxes with position, and thus correlate with where the adf peaks.

On the Metallicity of Blue Compact Dwarf Galaxies from Tailor-Made Models

Blue compact dwarf galaxies take up the low end of the distribution of metallicity as derived using collisional emission lines in ionized gaseous nebulae. In fact, the most metal poor objects in the Local Universe belong to this category. Nevertheless, the abundance discrepancy factor (ADF) found in some local HII regions whose metallicity have been derived by means of recombination lines and the offset claimed by some authors in relation to the predictions of some photoionization models question the real chemical status of these objects. In this contribution, we summarize the results of Perez-Montero (2010, MNRAS, 404, 2037) and we show how tailor-made models of a subsample of HII galaxies with a good determination of the thermal inner structure can be used to shed some light on this issue, helping also to study their star formation history and the properties of their ionizing stellar clusters.

Small-spatial-scale variations of nebular properties and the abundance discrepancy in three Galactic H ii regions★

We present the results of long-slit spectroscopy in several slit positions that cover different morphological structures of the central parts of three bright Galactic H ii regions: M8, M17 and NGC 7635. We study the spatial distributions of a large number of nebular parameters, such as the extinction coefficient, line fluxes, physical conditions and ionic abundances at the maximum spatial resolution attainable with our instrumentation. Particularly, our goal is to study the behaviour of the abundance discrepancy factor of O2+[ADF(O2+)], defined as the logarithmic difference of the O2+ abundances derived from collisionally excited and recombination lines. We find that ADF(O2+) remains fairly constant along the slit positions of M8 and M17. For NGC 7635, we only detect the O ii recombination lines in the integrated spectrum along the whole slit, where ADF(O2+) reaches a remarkably high value of about 0.59 dex. We compare our results with previous results obtained for the Orion Nebula. We find much evidence to suggest the presence of a candidate Herbig–Haro object in M8.

Disentangling the metallicity and star formation history of H ii galaxies through tailor-made models

We present a self-consistent study of the stellar populations and the ionized gas in a sample of 10 HII galaxies with, at least, four measured electron temperatures and a precise determination of ionic abundances following the "direct method". We fitted the spectral energy distribution of the galaxies using the program STARLIGHT in order to quantify the contribution of the underlying stellar population to EW(Hbeta), which amounts to about 10% for most of the objects. We then studied the Wolf-Rayet stellar populations detected in seven of the galaxies. The presence of these populations and the corrected EW(Hbeta) values indicate that the ionizing stellar populations were created following a continuous star formation episode of 10 Myr duration, hence WR stars may be present in all of objects even if they are not detected in some of them. The derived stellar features, the number of ionizing photons and the relative intensities of the strongest emission lines were used as input parameters to compute tailored models with the photoionization code CLOUDY. Our models are able to y reproduce their thermal and ionization structure as deduced from their collisionally excited emission lines and, hence, no abundance discrepancy factors are implied for this kind of objects. Only the electron temperature of S+ is overestimated by the models, pointing to the possible presence of outer shells of diffuse gas in these objects. This kind of geometrical effects can affect the determination of the equivalent effective temperature of the ionizing cluster using calibrators which depend on low-excitation emission lines.

Can X-rays provide a solution to the abundance discrepancy problem in photoionized nebulae?

Abstract 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 > 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).

Properties of the ionized gas in HH 202 - II. Results from echelle spectrophotometry with Ultraviolet Visual Echelle Spectrograph

We present results of deep echelle spectrophotometry of the brightest knot of the Herbig–Haro object HH 202 in the Orion Nebula – HH 202-S – using the Ultraviolet Visual Echelle Spectrograph in the spectral range from 3100 to 10 400 Å. The high spectral resolution of the observations has permitted to separate the component associated with the ambient gas from that associated with the gas flow. We derive electron densities and temperatures from different diagnostics for both components, as well as the chemical abundances of several ions and elements from collisionally excited lines, including the first determinations of Ca+ and Cr+ abundances in the Orion Nebula. We also calculate the He+, C2+, O+ and O2+ abundances from recombination lines. The difference between the O2+ abundances determined from collisionally excited and recombination lines – the so-called abundance discrepancy factor – is 0.35 and 0.11 dex for the shock and nebular components, respectively. Assuming that the abundance discrepancy is produced by spatial variations in the electron temperature, we derive values of the temperature fluctuation parameter, t2, of 0.050 and 0.016 for the shock and nebular components, respectively. Interestingly, we obtain almost coincident t2 values for both components from the analysis of the intensity ratios of He i lines. We find significant departures from case B predictions in the Balmer and Paschen flux ratios of lines of high principal quantum number n. We analyse the ionization structure of HH 202-S, finding enough evidence to conclude that the flow of HH 202-S has compressed the ambient gas inside the nebula trapping the ionization front. We measure a strong increase of the total abundances of nickel and iron in the shock component, the abundance pattern and the results of photoionization models for both components are consistent with the partial destruction of dust after the passage of the shock wave in HH 202-S.

Properties of the ionized gas in HH 202 - I. Results from integral field spectroscopy with PMAS

We present results from integral field spectroscopy with the Potsdam Multi-Aperture Spectrograph of the head of the Herbig-Haro (HH) object HH 202 with a spatial sampling of 1 x 1 arcsec 2 . We have obtained maps of different emission lines, physical conditions - such as electron temperature and density - and ionic abundances from recombination and collision-ally excited lines. We present the first map of the Balmer temperature and of the temperature fluctuation parameter, t 2 . We have calculated the t 2 in the plane of the sky, which is substantially smaller than that determined along the line of sight. We have mapped the abundance discrepancy factor (ADF) of O 2+ , ADF(O 2+ ), finding its maximum value at the HH 202-S position. We have explored the relations between the ADF(O 2+ ) and the electron density, the Balmer and [O III] temperatures, the ionization degree as well as the t 2 parameter. We do not find clear correlations between these properties and the results seem to support that the ADF and t 2 are independent phenomena. We have found a weak negative correlation between the O 2+ abundance determined from recombination lines and the temperature, which is the expected behaviour in an ionized nebula, hence it seems that there is no evidence for the presence of supermetal-rich droplets in H II regions.

Independent Emission and Absorption Abundances for Planetary Nebulae1

Emission-line abundances have been uncertain for more than a decade due to unexplained discrepancies in the relative intensities of the forbidden lines and weak permitted recombination lines in planetary nebulae (PNe) and H II regions. The observed intensities of forbidden and recombination lines originating from the same parent ion differ from their theoretical values by factors of more than an order of magnitude in some of these nebulae. In this study we observe UV resonance line absorption in the central stars of PNe produced by the nebular gas, and from the same ions that emit optical forbidden lines. We then compare the derived absorption column densities with the emission measures determined from ground-based observations of the nebular forbidden lines. We find for our sample of PNe that the collisionally excited forbidden lines yield column densities that are in basic agreement with the column densities derived for the same ions from the UV absorption lines. A similar comparison involving recombination line column densities produces poorer agreement, although near the limits of the formal uncertainties of the analyses. An additional sample of objects with larger abundance discrepancy factors will need to be studied before a stronger statement can be made that recombination line abundances are not correct.