arXiv subjects

P. J. Storey

Publications and source records attributed to P. J. Storey.

36 records · Page 2Linked to original sources

arXiv2009

Electron impact excitation of OII fine-structure levels

Effective collision strengths for forbidden transitions among the 5 energetically lowest finestructure levels of O II are calculated in the Breit-Pauli approximation using the R-matrix method. Results are presented for the electron temperature range 100 to 100 000 K. The accuracy of the calculations is evaluated via the use of different types of radial orbital sets and a different configuration expansion basis for the target wavefunctions. A detailed assessment of previous available data is given, and erroneous results are highlighted. Our results reconfirm the validity of the original Seaton and Osterbrock scaling for the optical O II ratio, a matter of some recent controversy. Finally we present plasma diagnostic diagrams using the best collision strengths and transition probabilities.

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arXiv2008

Uncertainties in Theoretical HeI Emissivities: HII Regions, Primordial Abundance, and Cosmological Recombination

A number of recent works in astronomy and cosmology have relied upon theoretical He I emissivities, but we know of no effort to quantify the uncertainties in the atomic data. We analyze and assign uncertainties to all relevant atomic data, perform Monte Carlo analyses, and report standard deviations in the line emissivities. We consider two sets of errors, which we call "optimistic" and "pessimistic." We also consider three different conditions, corresponding to prototypical Galactic and extragalactic H II regions and the epoch of cosmological recombination. In the extragalactic H II case, the errors we obtain are comparable to or larger than the errors in some recent $Y_p$ calculations, including those derived from CMB observations. We demonstrate a systematic effect on primordial abundance calculations; this effect cannot be reduced by observing a large number of objects. In the cosmological recombination case, the errors are comparable to many of the effects considered in recent calculations.

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arXiv2007

The hydrogen-deficient knot of the `born again' planetary nebula Abell 58 (V605 Aql)

We have analysed deep optical spectra of the `born-again' planetary nebula Abell 58 and its hydrogen-deficient knot, surrounding V605 Aql, which underwent a nova-like eruption in 1919. The electron temperature we derive for the central knot varies widely depending on the diagnostic used. The [O III] nebular-to-auroral transition ratio gives a temperature of 20800 K, while the ratio of the [N II] nebular and auroral lines give Te=15200 K. The helium line ratios 5876/4471 and 6678/4471 imply temperatures of 350 K and 550 K respectively. Weakly temperature-sensitive O II recombination line ratios imply similarly low electron temperatures. Abundances derived from recombination lines are vastly higher than those found from collisionally excited lines, with the abundance discrepancy factor (adf) for O2+ reaching 89 -- the second highest known value after that found for the hydrogen deficient knots in Abell 30. The observed temperature diagnostics and abundances support the idea that, like Abell 30, the knot of Abell 58 contains some very cold ionised material. Although the central star is carbon-rich (C/O>1), the knot is found to be oxygen-rich, a situation not predicted by the single-star `born again' theory of its formation. We argue that the abundances in the ejecta observed in A 30 and A 58 have more in common with neon novae than with Sakurai's Object, which is believed to have undergone a final helium flash. In particular, the C/O ratio of less than unity and presence of substantial quantities of neon in the ejecta of both Abell 30 and Abell 58 are not predicted by very late thermal pulse models.

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arXiv2006

Theoretical calculations of the HI, HeI and HeII free-bound continuou emission spectra

We present coefficients for the calculation of the continuous emission spectra of HI, HeI and HeII due to electron-ion recombination. Coefficients are given for photon energies from the first ionization threshold for each ion to the n=20 threshold of hydrogen (36.5um), and for temperatures 100 K <= Te <=10^5 K. The emission coefficients for HeI are derived from accurate ab initio photoionization data. The coefficients are scaled in such a way that they may be interpolated by a simple scheme with uncertainties less than 1% in the whole temperature and wavelength domain. The data are suitable for incorporation into photoionisation/plasma codes and should aid with the interpretation of spectra from the very cold ionised gas phase inferred to exist in a number of gaseous clouds.

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arXiv2006

Three-Dimensional Ionisation, Dust RT and Chemical Modelling of Planetary Nebulae

The assumption of spherical symmetry is not justified for the vast majority of PNe. The interpretation of spatially-resolved observations cannot rely solely on the application of 1D codes, which may yield incorrect abundances determinations resulting in misleading conclusions. The 3D photoionisation code MOCASSIN (Monte CAarlo SimulationS of ionised Nebulae) is designed to remedy these shortcomings. The 3D transfer of both primary and secondary radiation is treated self-consistently without the need of approximations. The code was benchmarked and has been applied to the study of several PNe. The current version includes a fully self-consistent radiative transfer treatment for dust grains mixed within the gas, taking into account the microphysics of dust-gas interactions within the geometry-independent Monte Carlo transfer. The new code provides an excellent tool for the self-consistent analysis of dusty ionised regions showing asymmetries and/or density and chemical inhomogeneities. Work is currently in progress to incorporate the processes that dominate the thermal balance of photo-dissociation regions (PDRs), as well as the formation and destruction processes for all the main molecular species.

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arXiv2006

bHROS high spectral resolution observations of PN forbidden and recombination line profiles

We have acquired high spectral resolution observations (R=150,000) of the planetary nebulae NGC 7009 and NGC 6153, using bHROS on Gemini South. Observations of this type may provide a key to understanding why optical recombination lines (ORLs) yield systematically higher heavy element abundances for photoionized nebulae than do the classical forbidden collisionally excited lines (CELs) emitted by the same ions; NGC 7009 and NGC 6153 have notably high ORL/CEL abundance discrepancy factors (ADFs) of 5 and 10, respectively. Due to the opposite temperature dependences of ORLs and CELs, ORLs should be preferentially emitted by colder plasma. Our bHROS observations of NGC 7009 reveal that the [O III] 4363A CEL has a FWHM linewidth that is 1.5 times larger than that shown by O II ORLs in the same spectrum, despite the fact that all of these lines are emitted by the O2+ ion. The bHROS spectra of NGC 6153 also show that its O II ORLs have significantly narrower linewidths than do the [O III] 4363A and 5007A lines but, in addition, the [O III] 4363A and 5007A lines show very different velocity profiles, implying the presence of large temperature variations in the nebula.

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arXiv2006

Chemical abundances for Hf 2-2, a planetary nebula with the strongest known heavy element recombination lines

We present high quality optical spectroscopic observations of the planetary nebula (PN) Hf 2-2. The spectrum exhibits many prominent optical recombination lines (ORLs) from heavy element ions. Analysis of the H {\sc i} and He {\sc i} recombination spectrum yields an electron temperature of $\sim 900$ K, a factor of ten lower than given by the collisionally excited [O {\sc iii}] forbidden lines. The ionic abundances of heavy elements relative to hydrogen derived from ORLs are about a factor of 70 higher than those deduced from collisionally excited lines (CELs) from the same ions, the largest abundance discrepancy factor (adf) ever measured for a PN. By comparing the observed O {\sc ii} $λ$4089/$λ$4649 ORL ratio to theoretical value as a function of electron temperature, we show that the O {\sc ii} ORLs arise from ionized regions with an electron temperature of only $\sim 630$ K. The current observations thus provide the strongest evidence that the nebula contains another previously unknown component of cold, high metallicity gas, which is too cool to excite any significant optical or UV CELs and is thus invisible via such lines. The existence of such a plasma component in PNe provides a natural solution to the long-standing dichotomy between nebular plasma diagnostics and abundance determinations using CELs on the one hand and ORLs on the other.

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arXiv2005

The dusty MOCASSIN: fully self-consistent 3D photoionisation and dust radiative transfer models

We present the first 3D Monte Carlo (MC) photoionisation code to include a fully self-consistent treatment of dust radiative transfer (RT) within a photoionised region. This is the latest development (Version 2.0) of the gas-only photoionisation code MOCASSIN (Ercolano et al., 2003a), and employs a stochastic approach to the transport of radiation, allowing both the primary and secondary components of the radiation field to be treated self-consistently, whilst accounting for the scattering of radiation by dust grains mixed with the gas, as well as the absorption and emission of radiation by both the gas and the dust components. A set of rigorous benchmark tests have been carried out for dust-only spherically symmetric geometries and 2D disk configurations. MOCASSIN's results are found to be in agreement with those obtained by well established dust-only RT codes that employ various approaches to the solution of the RT problem. A model of the dust and of the photoionised gas components of the planetary nebula (PN) NGC 3918 is also presented as a means of testing the correct functioning of the RT procedures in a case where both gas and dust opacities are present. The two components are coupled via the heating of dust grains by the absorption of both UV continuum photons and resonance line photons emitted by the gas. The MOCASSIN results show agreement with those of a 1D dust and gas model of this nebula published previously, showing the reliability of the new code, which can be applied to a variety of astrophysical environments.

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arXiv2005

Hyperfine splitting of [Al VI]3.66um and the Al isotopic ratio in NGC6302

The core of planetary nebula NGC 6302 is filled with high-excitation photoionised gas at low expansion velocities. It represents a unique astrophysical situation in which to search for hyperfine structure (HFS) in coronal emission lines from highly ionised species. HFS is otherwise blended by thermal or velocity broadening. Spectra containing [AlVI]~3.66um 3P2-3P1, obtained with Phoenix on Gemini-South at resolving powers of up to 75000, resolve the line into five hyperfine components separated by 20 to 60km/s due to the coupling of the I=5/2 nuclear spin of 27Al with the total electronic angular momentum J. 26Al has a different nuclear spin of I=5, and a different HFS, which allows us to place a 3sigma upper limit on the 26Al/27Al abundance ratio of 1/33. We measure the HFS magnetic-dipole coupling constants for [AlVI], and provide the first estimates of the electric-quadrupole HFS coupling constants obtained through astronomical observations of an atomic transition.

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arXiv2004

MOCASSIN: 3D photoionisation and dust radiative transfer modelling of PNe

We present the first 3D Monte Carlo photoionisation code to include a fully self-consistent treatment of dust radiative transfer (RT) within the photoionised region. This is the latest development of the recently published pure photoionisation code MOCASSIN (Ercolano et al., 2003a) and it is currently undergoing several benchmarking tests. The preliminary results of these tests are presented in these conference proceedings. The new code provides the ideal tool for the analysis of dusty ionised regions showing asymmetries and/or density and chemical inhomogeneities

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arXiv2004

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.

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arXiv2004

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 3646A, we determine electron temperatures and densities for 48 Galactic PNe. The electron temperatures based on this method are found to be systematically lower than those derived from [O III] 4959/4363 and [O III] (88μ+52μ)/4959 ratios. And the electron densities based on this method are found to be systematically higher than those derived from forbidden line ratios. These results suggest that temperature and density fluctuations are generally present within nebulae. In addition, He/H abundances of PNe are found to be positively correlated with the difference between T_e([O III]) and T_e(Bal), suggesting that He/H abundances might have been overestimated generally because of the possible existence of H-deficient knots.

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arXiv2003

Three-dimensional photoionisation modelling of the hydrogen-deficient knots in the planetary nebula Abell 30

We have constructed a photoionization model, using the 3-D Monte Carlo code MOCASSIN for one of the hydrogen-deficient knots (J3) of the born-again planetary nebula Abell 30. The model consists of spherical knots, comprising a cold, dense, hydrogen-deficient core with very high metal abundances. The inner core is surrounded by a less dense hydrogen-deficient and metal-enriched gas envelope, with less extreme abundances. The envelope of the knot might have been formed by the mixing of the knot material with the surrounding nebular gas. This bi-chemistry, bi-density model did not produce enough heating to match the fluxes of the collisionally excited emission lines (CELs) and of the optical recombination lines (ORLs) observed in the spectrum of the knot. We therefore included heating by photoelectric emission from dust grains in the thermal equilibrium calculations, and found that dust-to-gas ratios of 0.077 and 0.107 by mass for the central core and the envelope of the knot, respectively, are sufficient to fit the spectrum. Surprisingly, photoelectric emission from grains is the dominant source of heating in the hot envelope of the knot, while heating by photoionization of helium and heavy elements dominates in the cold core. The chemical abundances inferred from our modelling indicate, in agreement with the empirical analysis of Wesson et al. (2003), that the C/O ratio in the two regions of the knot is less than unity, contrary to theoretical predictions for born-again nebulae.

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arXiv2003

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.

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arXiv2003

Three-dimensional photoionization modelling of the planetary nebula NGC 3918

The three-dimensional Monte Carlo photoionization code Mocassin has been applied to construct a realistic model of the planetary nebula NGC 3918. Three different geometric models were tried. The effects of the interaction of the diffuse fields coming from two adjacent regions of different densities were investigated. These are found to be non-negligible, even for the relatively uncomplicated case of a biconical geometry. We found that the ionization structure of low ionization species near the boundaries is particularly affected. It is found that all three models provided acceptable matches to the integrated nebular optical and ultraviolet spectrum. Large discrepancies were found between all of the model predictions of infrared fine-structure line fluxes and ISO SWS measurements. This was found to be largely due to an offset of ~14 arcsec from the centre of the nebula that affected all of the ISO observations of NGC 3918. For each model, we also produced projected emission-line maps and position-velocity diagrams from synthetic long-slit spectra, which could be compared to recent HST images and ground-based long-slit echelle spectra. This comparison showed that spindle-like model B provided the best match to the observations. We have therefore shown that although the integrated emission line spectrum of NGC 3918 can be reproduced by all three of the three-dimensional models investigated in this work, the capability of creating projected emission-line maps and position-velocity diagrams from synthetic long-slit spectra was crucial in allowing us to constrain the structure of this object.

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arXiv2003

Mocassin: A fully three-dimensional Monte Carlo photoionization code

The study of photoionized environments is fundamental to many astrophysical problems. Up to the present most photoionization codes have numerically solved the equations of radiative transfer by making the extreme simplifying assumption of spherical symmetry. Unfortunately very few real astronomical nebulae satisfy this requirement. To remedy these shortcomings, a self-consistent, three-dimensional radiative transfer code has been developed using Monte Carlo techniques. The code, Mocassin, is designed to build realistic models of photoionized nebulae having arbitraries geometry and density distributions with both the stellar and diffuse radiation fields treated self-consistently. In addition, the code is capable of tretating on or more exciting stars located at non-central locations. The gaseous region is approximated by a cuboidal Cartesian grid composed of numerous cells. The physical conditions within each grid cell are determined by solving the thermal equilibrium and ionization balance equations This requires a knowledge of the local primary and secondary radiation fields, which are calculated self-consistently by locally simulating the individual processes of ionization and recombination. The main structure and computational methods used in the Mocassin code are described in this paper. Mocassin has been benchmarked against established one-dimensional spherically symmetric codes for a number of standard cases, as defined by the Lexington/Meudon photoionization workshops (Pequignot et al., 1986; Ferland et al., 1995; Pequignot et al., 2001)\citep{pequignot86,ferland95, pequignot01}. The results obtained for the benchmark cases are satisfactory and are presented in this paper. A performance analysis has also been carried out and is discussed here.

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arXiv2002

Heavy elements in Galactic and Magellanic Cloud HII regions: recombination-line versus forbidden-line abundances

We have obtained deep optical, long-slit spectrophotometry of the Galactic HII regions M 17, NGC 3576 and of the Magellanic Cloud HII regions 30 Doradus, LMC N11B and SMC N66, recording the optical recombination lines (ORLs) of CII, NII and OII. Temperature-insensitive ORL C2+/O2+ and N2+/O2 ratios are obtained for all nebulae except SMC N66. The ORL C2+/O2+ ratios show remarkable agreement within each galactic system, while also being in agreement with the corresponding CEL ratios. For all five nebulae, the O2+/H+ abundance derived from multiple OII ORLs is found to be higher than the corresponding value derived from the strong [OIII] 4959, 5007A CELs, by factors of 1.8--2.7 for four of the nebulae. The LMC N11B nebula exhibits a more extreme discrepancy factor for the O2+ ion, ~5. Thus these HII regions exhibit ORL/CEL abundance discrepancy factors that are similar to those previously encountered amongst planetary nebulae. Our optical CEL O2+/H+ abundances agree to within 20-30 per cent with published O2+/H+ abundances that were obtained from observations of infrared fine-structure lines. Since the low excitation energies of the latter make them insensitive to variations about typical nebular temperatures, fluctuations in temperature are ruled out as the cause of the observed ORL/CEL O2+ abundance discrepancies. We present evidence that the observed OII ORLs from these HII regions originate from gas of very similar density (<3500 cm-3) to that emitting the observed heavy-element optical and infrared CELs, ruling out models that employ high-density ionized inclusions in order to explain the abundance discrepancy. We consider a scenario whereby much of the heavy-element ORL emission originates from cold (<=500 K) metal-rich ionized regions.

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arXiv2002

Temperature Variations from HST Imagery and Spectroscopy of NGC 7009

We present new HST/WFPC2 imagery and STIS long-slit spectroscopy of the planetary nebula NGC 7009. The primary goal was to obtain high spatial resolution of the intrinsic line ratio [O III] 4364/5008 and thereby evaluate the electron temperature (T_e) and the fractional mean-square T_e variation (t_A^2) across the nebula. The WFPC2 T_e map is rather uniform; almost all values are between 9000 - 11,000 K, with the higher T_e's closely coinciding with the inner He^++-zone. The results indicate very small values - <~ 0.01 - for t_A^2 throughout. Our STIS data allow an even more direct determination of T_e and t_A^2, albeit for a much smaller area than with WFPC2. We present results from binning the data along the slit into tiles that are 0.5'' square (matching the slit width). The average [O III] temperature using 45 tiles (excluding the central star and STIS fiducial bars) is 10,139 K; t_A^2 is 0.0035. The measurements of T_e reported here are an average along each line of sight. Therefore, despite finding remarkably low t_A^2, we cannot completely rule out temperature fluctuations along the line of sight as the cause of the large abundance discrepancy between heavy element abundances inferred from collisionally excited emission lines compared to those derived from recombination lines.

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