Abstract
Abstract Energy levels, Einstein A-values and oscillator strengths are calculated for doubly-ionised germanium (Ge iii). We have undertaken a study of the electron-impact excitation of Ge iii and have also completed a study of the photoionisation of Ge ii. Ge iii target structures were generated using the relativistic GRASP0 package. Several scattering models were investigated to assess the uncertainty quantification of our results. Initially, the lowest lying 64 jj fine-structure levels were included in the close-coupling expansion resulting from 14 non-relativistic configurations. A second larger model was subsequently generated using the GASP package and incorporated the lowest 589 jj fine-structure levels from 17 non-relativistic configurations. Both the DARC and Breit-Pauli parallel suite of R-matrix collision codes were utilised in the scattering calculations to generate the collision strengths for incident electron-energies between 0 and 5 Rydbergs and subsequently the Maxwellian averaged effective collision strengths for temperatures in the range 1000 to 200000 Kelvin. The photoionisation calculations comprised of two studies, a DARC and a Breit-Pauli calculation incorporating the 250 lowest lying levels. Photoionisation cross sections for Ge ii are presented in the range 0-5 Ryds. Due to the lack of data available in the literature, the present radiative data and electron-impact/photoionisation cross sections are compared between the models presented in this paper. To assess their significance for astrophysics, the data are incorporated into the stellar atmosphere package sterne. Test models for the heavy-metal subdwarf LSIV−14○116 are not affected by the new data, but models for cooler stars may be sensitive to them.
Highlights
Germanium has been detected within the Earth’s crust at an abundance of approximately 1.6 ppm (Holl, Kling & Schroll 2007)
The collision strengths are presented for electron energies in the range of 0–5 Ryds, and the effective collision strengths are presented for astrophysically important temperatures in the range of 1000–2000 000 K
Slight differences are evident in the positioning and magnitudes of the low-lying resonances predicted by the two calculations. This is not unexpected due to the differing configuration interaction (CI) terms included in the wavefunction expansion of the target ion in the two models. These differences are highlighted by the effective collision strengths for both transitions where we see some discrepancies at the lowest temperatures considered
Summary
Germanium has been detected within the Earth’s crust at an abundance of approximately 1.6 ppm (Holl, Kling & Schroll 2007). Naslim et al (2011) report the detection of doubly ionized yttrium Y 3 and germanium Ge III and triply ionized zirconium Zr IV in the spectrum of the hot subdwarf LS IV−14◦116 These represented the first detection of these ions in the optical spectrum of any star, and yielded measured abundances between 3 and 4.6 dex above the solar value and >10 ppm in absolute terms. These detections and overabundances were recently confirmed in LS IV−14◦116, and in the hot subdwarfs Feige 46 and PHL 417 (Dorsch et al 2020; Østensen et al 2020). Neither the overall Ge IV photoexcitation spectrum nor the photoionization of any germanium ions was extensively investigated
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