We extend the range of validity of the artis 3D radiative transfer code up to hundreds of days after explosion, when Type Ia supernovae are in their nebular phase. To achieve this, we add a non-local thermodynamic equilibrium (non-LTE) population and ionisation solver, a new multi-frequency radiation field model, and a new atomic dataset with forbidden transitions. We treat collisions with non-thermal leptons resulting from nuclear decays to account for their contribution to excitation, ionisation, and heating. We validate our method with a variety of tests including comparing our synthetic nebular spectra for the well-known onedimensional W7 model with the results of other studies. As an illustrative application of the code, we present synthetic nebular spectra for the detonation of a sub-Chandrasekhar white dwarf in which the possible effects of gravitational settling of 22 Ne prior to explosion have been explored. Specifically, we compare synthetic nebular spectra for a 1.06 M white dwarf model obtained when 5.5 Gyr of very-efficient settling is assumed to a similar model without settling. We find that this degree of 22 Ne settling has only a modest effect on the resulting nebular spectra due to increased 58 Ni abundance. Due to the high ionisation in sub-Chandrasekhar models, the nebular [Ni ii] emission remains negligible, while the [Ni iii] line strengths are increased and the overall ionisation balance is slightly lowered in the model with 22 Ne settling.In common with previous studies of sub-Chandrasekhar models at nebular epochs, these models overproduce [Fe iii] emission relative to [Fe ii] in comparison to observations of normal Type Ia supernovae.A natural candidate for triggering the ignition is for the WD to

Photoionization cross sections are obtained using the relativistic Dirac Atomic Rmatrix Codes (darc) for all valence and L-shell energy ranges between 27-270eV. A total of 557 levels arising from the dominant configurations 3s 2 3p 4 , 3s3p 5 , 3p 6 , 3s 2 3p 3 [3d, 4s, 4p], 3p 5 3d, 3s 2 3p 2 3d 2 , 3s3p 4 3d, 3s3p 3 3d 2 and 2s 2 2p 5 3s 2 3p 5 have been included in the target wavefunction representation of the Ar iii ion, including up to 4p in the orbital basis. We also performed a smaller Breit-Pauli (bp) calculation containing the lowest 124 levels. Direct comparisons are made with previous theoretical and experimental work for both valence shell and L-shell photoionization. Excellent agreement was found for transitions involving the 2 P o initial state to all allowed final states for both calculations across a range of photon energies. A number of resonant states have been identified to help analyze and explain the nature of the spectra at photon energies between 250 and 270eV.

Recent, state-of-the-art calculations of A-values and electron impact excitation rates for Fe III are used in conjunction with the Cloudy modeling code to derive emission-line intensity ratios for optical transitions among the fine-structure levels of the 3d 6 configuration. A comparison of these with high-resolution, high signal-to-noise spectra of gaseous nebulae reveals that previous discrepancies found between theory and observation are not fully resolved by the latest atomic data. Blending is ruled out as a likely cause of the discrepancies, because temperatureand density-independent ratios (arising from lines with common upper levels) match well with those predicted by theory. For a typical nebular plasma with electron temperature = T 9000 e K and electron density = -

Modelling of massive stars and supernovae (SNe) plays a crucial role in understanding galaxies. From this modelling we can derive fundamental constraints on stellar evolution, mass-loss processes, mixing, and the products of nucleosynthesis. Proper account must be taken of all important processes that populate and depopulate the levels (collisional excitation, de-excitation, ionization, recombination, photoionization, bound-bound processes). For the analysis of Type Ia SNe and core collapse SNe (Types Ib, Ic and II) Fe group elements are particularly important. Unfortunately little data is currently available and most noticeably absent are the photoionization cross-sections for the Fe-peaks which have high abundances in SNe. Important interactions for both photoionization and electron-impact excitation are calculated using the relativistic Dirac Atomic R-matrix Codes (darc) for low ionization stages of cobalt. All results are calculated up to photon energies of 45 eV and electron energies up to 20 eV. The wavefunction representation of Co iii has been generated using grasp0 by including the dominant 3d 7 , 3d 6 [4s, 4p], 3p 4 3d 9 and 3p 6 3d 9 configurations, resulting in 292 fine structure levels. Electron-impact collision strengths and Maxwellian averaged effective collision strengths across a wide range of astrophysically relevant temperatures are computed for Co iii. In addition, statistically weighted level-resolved ground and metastable photoionization cross-sections are presented for Co ii and compared directly with existing work.

We present an analysis of the appearance of the Cooper Minimum in singly ionized argon in both the photoionization cross-section (PICS) and high-harmonic generation (HHG) spectrum. We employ two computational approaches based on the same R-matrix technique to provide a coherent description of the atomic structure of the Ar + system, finding that the PICS and HHG spectrum are affected differently by the inclusion of additional residual ion states and the improved description of correlation effects. Both the PICS and HHG spectrum possess a clear minimum for all atomic structure models used, with the centre of the minimum at 55 eV in the PICS and 60 eV in the HHG spectrum for the most complete description employed. The HHG minimum is systematically shifted to higher energies with respect to the PICS minimum. We also find that the initial magnetic alignment (magnetic quantum number) of the Ar + system does not affect substantially the position and shape of the HHG minimum (given a sufficiently detailed atomic structure description), but the harmonic yield is enhanced by two-orders of magnitude for ML = 1 over ML = 0. We also perform similar calculations for neutral argon, finding that this system is more sensitive to enhancements in the atomic structure description.PACS numbers: 32.80. Rm, 42.65.Ky