Atomic data from the IRON project - LI. Electron impact excitation of FeIX

We calculate collision strengths and thermally averaged collision strengths for electron excitation between the one hundred and forty energetically lowest levels of Fe8+. The scattering target is more elaborate than in any earlier work and large increases are found in the excitation rates among the levels of the 3s(2)3p(5)3d electron configuration due to resonance series that have not been considered previously. The implications for solar and stellar spectroscopy have been discussed elsewhere (Storey & Zeippen 2001). We correct some errors that were made in generating the figures given in that paper and present corrected versions

Helium line emissivities for nebular astrophysics

We present the results of several collisional-radiative models describing optically thin emissivities of the main lines in neutral helium formed by recombination, for a grid of electron temperatures and densities, typical of H II regions and planetary nebulae. Accurate emissivities are required for example to measure the helium abundance in nebulae and as a consequence its primordial value. We compare our results with those obtained by previous models, finding significant differences, well above the target accuracy of 1 per cent. We discuss in some detail our chosen set of atomic rates and the differences with those adopted by previous models. The main differences lie in the treatment of electron and proton collision rates and we discuss which transitions are least sensitive to the choice of these rates and therefore best suited to high-precision abundance determinations. We have focused our comparisons on the case B approximation where only He and He+ are considered, but also present results of full models including the bare nuclei, photoexcitation, and photoionization, and either black-body or observed illuminating spectrum in the case of the Orion nebula, to indicate which spectral lines are affected by opacity. For those transitions, accurate radiative transfer calculations should be performed. We provide tables of emissivities for all transitions within n ≤ 5 and all those between the n ≤ 5 and n′ ≤ 25 states, in the log Te (K) = 103.0(0.1)4.6 and log Ne (cm−3) = 102(0.5)6 ranges, and a FORTRAN code to interpolate to any Te, Ne within these ranges

The WC10 central stars CPD-56 degrees 8032 and He 2-113 .1. Distances and nebular parameters

We present the results of an analysis of the WC10 central stars CPD −56°8032 and He 2-113 and of their surrounding planetary nebulae (PNe). UCL Echelle Spectrograph spectra with a resolving power of R = 50 000, covering the wavelength range 3600–9500 Å, were obtained for both objects. Expansion and radial velocities are derived from the nebular Balmer lines. Reddenings were derived from the observed Hα/Hβ decrements, as well as from the ratio of the radio free-free and Hβ fluxes. We find that E(B—V) = 0.68 for CPD−56°8032 and 1.00 for He 2-113. The bolometric luminosity is found to be 2820 D2 (kpc) L⊙ for CPD — 56°8032 and 2290 D2 (kpc) L⊙ for He 2-113. We have used a calibration based on Magellanic Cloud Wolf-Rayet central stars to estimate a distance of 1.35 kpc to CPD — 56°8032 and 1.50 kpc to He 2-113. A comparison of the radial velocities of interstellar Na I D-line absorption components with Galactic rotation curve predictions for each line of sight yields distances which agree within the uncertainties with these values. We also present deconvolved pre-COSTAR HST Hβ images, from which we derive nebular angular sizes of 1.6 × 2.1 arcsec2 for CPD—56°8032 and 1.4 × 1.1 arcsec2 for He 2-113. From our spectra, nebular electron temperatures of 8800 and 8400 K are derived for CPD. 56°8032 and He 2-113 respectively, while an electron density of 6 × 104 cm−3 is determined for both nebulae. Sulphur is found to have a near-solar abundance in both nebulae, and the nebular nitrogen abundances are also close to solar. The nebular C/O abundance ratios (determined with the help of low-resolution IUE spectra) are found to be equal to 13 for CPD. 56°8032 and 10 for He 2-113, respectively, significantly higher than the values normally found for Galactic PNe. We find no detectable amounts of hydrogen in either stellar wind

Properties of the WO Wolf-Rayet stars

We present optical spectrophotometry for five WO Wolf-Rayet stars, three of them in our own Galaxy and one in each of the SMC and LMC. IUE ultraviolet spectrophotometry has also been obtained for the two Magellanic Cloud WO stars, including a high resolution spectrum for one of them. Quantitative spectral typing criteria are defined for WO subtypes spanning WO1 to WO5 and, for the case of the two WO stars in spectroscopic binaries, spectral types for the O-type primaries are derived. From our spectrophotometry we derive reddenings and magnitudes for each star. Absolute visual magnitudes of -2.5 and -1.8 are derived for a WO4 star and for a WO2 star, respectively, each star lying at a known distance. Wind terminal velocities ranging from 4200km/s to 5500km/s are derived from the black absorption edge of an ultraviolet P Cygni profile and from the FWZI of strong optical emission lines. The relative abundances of helium, carbon and oxygen in the winds of each of the WO stars are derived using a recombination theory analysis of selected ultraviolet and optical emission lines to determine the ionic abundances of He^2+^, C^4+^, O^4+^, O^5+^ and O^6+^. The derived abundance ratios show relatively narrow ranges. C/He number ratios of 0.51-0.52 are derived for two Galactic WO stars and one LMC WO star, with their C/O ratios ranging between 4.6 and 5.2, and their (C+O)/He ratios equal to 0.62. The one SMC WO star has a C/He ratio of 0.81, a C/O ratio of 2.7 and a (C+O)/He number ratio of 1.10. These abundance ratios are broadly consistent with evolutionary models for the advanced stages of massive stars, and promising agreement as a function of initial metallicity is found with the most recent evolutionary models

Modelling low charge ions in the solar atmosphere

Extensions have been made recently to the coronal approximation for the purpose of modelling line emission from carbon and oxygen in the lower solar atmosphere. The same modelling is used here for other elements routinely observed in the solar transition region: N, Ne, Mg, Si, and S. The modelling includes the effects of higher densities suppressing dielectronic recombination and populating long lived, metastable levels; the presence of metastable levels typically causes effective ionization rates to increase and recombination rates to decrease. Processes induced by the radiation field, namely photoionization and photoexcitation, have been included, along with charge transfer, which occurs when electrons are exchanged during atom-ion and ion-ion collisions. The resulting ion balances are shown, and indicate significant changes compared to the frequently employed coronal approximation. The effect on level populations within ions caused by photoexcitation is also assessed. To give an illustration of how line emission could be altered by these processes, selected line contribution functions are presented at the end

The WC10 central stars CPD−56° 8032 and He 2–113 –III. Wind electron temperatures and abundances

We present a direct spectroscopic measurement of the wind electron temperatures and a determination of the stellar wind abundances of the WC10 central stars of planetary nebulae CPD−56° 8032 and He 2–113, for which high-resolution (0.15-Å) UCLES echelle spectra have been obtained using the 3.9-m Anglo-Australian Telescope. The intensities of dielectronic recombination lines, originating from autoionizing resonance states situated in the C2++e− continuum, are sensitive to the electron temperature through the populations of these states, which are close to their LTE values. The high-resolution spectra allow the intensities of fine-structure components of the dielectronic multiplets to be measured. New atomic data for the autoionization and radiative transition probabilities of the resonance states are presented, and used to derive wind electron temperatures in the two stars of 21 300 K for CPD−56°8032 and 16 400 K for He 2–113. One of the dielectronic lines is shown to have an autoionization width in agreement with the theoretical predictions. Wind abundances of carbon with respect to helium are determined from bound–bound recombination lines, and are found to be C/He=0.44 for CPD−56° 8032 and C/He=0.29 for He 2–113 (by number). The oxygen abundances are determined to be O/He=0.24 for CPD−56° 8032 and 0.26 for He 2–113. The effect of optical depth on the temperature and abundance determinations is investigated by means of a Sobolev escape-probability model. We conclude that the optically thicker recombination lines can still be used for abundance determinations, provided that their upper levels are far from LTE

New atomic data for C i Rydberg states compared with solar UV spectra

We use the Breit-Pauli R-matrix method to calculate accurate energies and radiative data for states in C i up to n = 30 and with l ≤ 3. We provide the full data set of decays to the five 2s2 2p2 ground configuration states 3P0,1,21D2, and 1S0. This is the first complete set of data for transitions from n ≥ 5. We compare oscillator strengths and transition probabilities with the few previously calculated values for such transitions, finding generally good agreement (within 10 per cent) with the exception of values recently recommended by National Institute of Standards and Technology, where significant discrepancies are found. We then calculate spectral line intensities originating from the Rydberg states using typical chromospheric conditions and assuming local thermal equilibrium, and compare them with well-calibrated Solar and Heliospheric Observatory Solar Ultraviolet Measurements of Emitted Radiation ultraviolet (UV) spectra of the quiet Sun. The relative intensities of the Rydberg series are in excellent agreement with observation, which provides firm evidence for the identifications and blends of nearly 200 UV lines. Such comparison also resulted in a large number of new identifications of C i lines in the spectra. We also estimate optical depth effects and find that these can account for much of the absorption noted in the observations. The atomic data can be applied to model a wide range of solar and astrophysical observations

The rich O II recombination spectrum of the planetary nebula NGC 7009: new observations and atomic data

We present new spectrophotometric observations of the rich O II optical recombination line spectrum of the planetary nubula NGC 7009, obtained at a spectral resolution of about 1 Å (FWHM). New intermediate coupling quantal calculations of O II radiative recombination coefficients for the 3d-3p and 4f-3d transitions are presented. The effect of departure from pure LS-coupling is shown to be important. Excellent agreement is found between the observed relative intensities of the O II lines and those calculated from recombination theory allowing for intermediate coupling effects. C, N and O abundances based on our recombination line measurements are derived. In all cases, they are about a factor of 5 higher than the corresponding values deduced from collisionally excited lines, indicating that the discrepancy between the abundances derived from these two different types of emission lines, previously known to exist for C2+, is a common phenomenon, and is probably caused by the same physical process. The nature of this process is still not known. If the discrepancy is due to temperature fluctuations, the implied rms temperature fluctuation prameter t2 is about a factor of 2 larger than that derived by comparing the temperatures deduced from the [O III] forbidden line ratio and from the ratio of the nebular continuum Balmer discontinuity to Hβ. However, if we adopted the electron temperature derived from nebular contiuum Balmer discontinuity instead of that from the [O III]forbidden line ratio, the C and N abundances deduced from ultraviolet collisionally excited lines would come into agreement with those deduced from the optical recombination lines, although the abundance of oxygen deduced from the optical forbidden lines would still be a factor of 2 lower than the corresponding value obtained from the optical recombination lines. The O/H abundance ratio derived from our recombination line analysis of NGC 7009 is more than a factor of 2 higher than the solar oxygen abundance

Balmer Discontinuity Temperatures in the Orion Nebula

We have mapped the Balmer discontinuity temperature Te (BJ) along two long-slit positions in the Orion Nebula, using high-quality spectra obtained with the ESO 1.52 m telescope and with the 3.9 m AAT. One slit was oriented north-south and positioned 40” west of Ö1 C Ori. The second slit was oriented east-west, with its eastern end 10" west of Ö1 C Ori, identical to the slit position previously studied by Walter & Dufour (1994). For the NS slit, both the 1.52 m and the AAT data yield a constant temperature of about 9000 K, with variations of only a few hundred K over a total slit length of about 3#5. For the EW-slit, our data reveal two distinct regions of nearly constant temperatures of 8900 and 7200 K, respectively, possibly resulting from two separate H II regions. No evidence is found for the anomalously low temperatures reported by Walter and Dufour for the same region. © 1995, IOP Publishing Ltd

Nebular spectra from Type Ia supernova explosion models compared to JWST observations of SN 2021aefx

Context: Recent JWST observations of the Type Ia supernova (SN Ia) 2021aefx in the nebular phase have paved the way for late-time studies covering the full optical to mid-infrared (MIR) wavelength range, and with it the hope to better constrain SN Ia explosion mechanisms. Aims: We investigate whether public SN Ia models covering a broad range of progenitor scenarios and explosion mechanisms (Chandrasekhar-mass, or MCh, delayed detonations, pulsationally assisted gravitationally confined detonations, sub-MCh double detonations, and violent mergers) can reproduce the full optical-MIR spectrum of SN 2021aefx at ∼270 days post explosion. Methods: We consider spherically averaged 3D models available from the Heidelberg Supernova Model Archive with a 56Ni yield in the range 0.5–0.8 M⊙. We performed 1D steady-state non-local thermodynamic equilibrium simulations with the radiative-transfer code CMFGEN, and compared the predicted spectra to SN 2021aefx. Results: The models can explain the main features of SN 2021aefx over the full wavelength range. However, no single model, or mechanism, emerges as a preferred match, and the predicted spectra are similar to each other despite the very different explosion mechanisms. We discuss possible causes for the mismatch of the models, including ejecta asymmetries and ionisation effects. Our new calculations of the collisional strengths for Ni III have a major impact on the two prominent lines at 7.35 μm and 11.00 μm, and highlight the need for more accurate collisional data for forbidden transitions. Using updated atomic data, we identify a strong feature due to [Ca IV] 3.21 μm, attributed to [Ni I] in previous studies. We also provide a tentative identification of a forbidden line due to [Ne II] 12.81 μm, whose peaked profile indicates the presence of neon all the way to the innermost region of the ejecta, as predicted for instance in violent merger models. Contrary to previous claims, we show that the [Ar III] 8.99 μm line can be broader in sub-MCh models compared to near-MCh models. Last, the total luminosity in lines of Ni is found to correlate strongly with the stable nickel yield, although ionisation effects can bias the inferred abundance. Conclusions: Our models suggest that key physical ingredients are missing from either the explosion models, or the radiative-transfer post-processing, or both. Nonetheless, they also show the potential of the near- and MIR to uncover new spectroscopic diagnostics of SN Ia explosion mechanisms