Electron impact excitation of Te IV and V and Level Resolved R-matrix Photoionization of Te I - IV with application to modelling of AT2017gfo

TL;DR

This study tackles the problem of insufficient atomic data for heavy, low-ionization species in kilonova modelling by delivering Te IV and Te V electron-impact excitation data and Te I–IV level-resolved photoionization cross sections using R-matrix methods with MCDHF-based atomic structures. The authors build compact Te IV and Te V models with GRASP0, validate energy levels and A-values against the literature and FAC, and compute comprehensive collision data (Ω and Υ) for NLTE spectral synthesis via ColRadPy. They also perform fully level-resolved DARC photoionization calculations for Te I–IV to refine opacities used in radiative-transfer codes. A key finding is that Te IV could plausibly contribute to the 1.08 μm emission feature observed in AT2017gfo during intermediate epochs, broadening the potential role of Te in kilonova spectra. The resulting data products will be publicly released to support large-scale KN modelling and further investigations into non-thermal ionization and recombination processes.

Abstract

Spectral modelling of kilonovae (KNe) require large amounts of collisional excitation and photoionization atomic data for lowly ionised (neutral, singly and doubly ionised) species of heavy elements. Much of the data currently used is calculated using approximate hydrogenic results or adopts semi-empirical formulae. We present atomic data for ions of tellurium (Te) computed using the well-known $R$-matrix method. Results will also be presented for radiative and thermal collisions of Te IV and V, for which the required atomic data are also typically limited in the literature. The Multi-Configuration-Dirac-Hartree-Fock (MCDHF) method is used to produce model atomic structures and radiative rates. These model structures are then used to calculate electron-impact-excitation and photoionization cross-sections. The resulting excitation and radiative rates are further used in a collisional radiative model to produce synthetic spectra, which are compared with observations. We also investigate the possibility of Te IV contributing to the 1.08 $μ$m emission feature in the mid-epochs of AT2017gfo alongside the established P-Cygni feature of Sr II.

Figures (11)

• Figure 1: A comparison of fac and grasp$^0$ Einstein A-values (s$^{-1}$) for Te iv.
• Figure 2: A comparison of A-values (s$^{-1}$) for Te V. The present grasp$^0$ calculation (carried through to the darc codes) is compared with the theoretical calculations of ekman2013spectral as well as values generated in this work with the fac code gu2008flexible.
• Figure 3: The collision strengths as a function of incident electron energy for two Te iv transitions. The two top panels show the raw and Maxwellian averaged collision strengths for the 5s$^2$5p $^2$P$^{\rm{o}}_{1/2}$$\to$$^2$P$^{\rm{o}}_{3/2}$ ($\lambda = 1084.25$nm) forbidden transition. The bottom two panels show the raw and Maxwellian averaged collision strengths for the 5s$^2$5p $^2$P$^{\rm{o}}_{3/2}$$\to$ 5s5p$^2$$^2$D$_{5/2}$ ($\lambda = 116.84$nm) intercombination line.
• Figure 4: The collision strengths as a function of incident electron energy for two Te v transitions. The two top panels show the raw and Maxwellian averaged collision strengths for the 5s$^2$$^1$S$_{0}$$\to$ 5s5p $^3$P$^{\rm{o}}_{0}$ ($\lambda = 133.14$nm) intercombination transition. The bottom two panels show the raw and Maxwellian averaged collision strengths for the 5s$^2$$^1$S$_{0}$$\to$ 5s5p $^1$P$^{\rm{o}}_{1}$ ($\lambda = 89.52$nm) allowed line.
• Figure 5: Contour plots of constant luminosity, at values of $L = 1.0\times 10^{39}$ erg/s, $L = 2.0\times 10^{39}$ erg/s, $L = 3.0\times 10^{39}$ erg/s. The parameter space of Te iv, electron temperature and electron density are explored. On the top panel, the density is fixed at 2$\times 10^{7}$ cm$^{-3}$ and temperature and mass are varied. The dashed lines are fixed electron temperatures, namely 3,000K (0.26 eV, black) and 10,000K (0.86 eV, green). On the bottom panel, the electron temperature is fixed at 3,000K and density and mass are varied. The black dashed line is a density of 2$\times 10^{7}$ cm$^{-3}$. On both figures the black dashed lines correspond to a mass estimate of $2.5 \times 10^{-3}$M$_{\odot}$. On the top figure, the green dashed line corresponds to a mass of $1.5 \times 10^{-4}$M$_{\odot}$.