Overview
Quantitative spectroscopy of hot subdwarfs necessitates accurate oscillator strengths for doubly and triply ionized heavy elements. While existing data for these elements remain sparse, new radiative data have been developed for As III, Se III, Hf IV, and Tl IV. These calculations employed two distinct theoretical approaches: the pseudo-relativistic Hartree-Fock (HFR) method, which incorporated core-polarization effects and semi-empirical adjustment of radial parameters, and the multiconfiguration Dirac-Hartree-Fock (MCDHF) method. The HFR data's reliability was assessed through comparison with independent MCDHF computations for As III, Hf IV, and Tl IV, and with previously published MCDHF data for Se III. The resulting data have been utilized in the spectroscopic analysis of heavy-metal hot subdwarfs, specifically LS IV-14°116 and EC 22536-5304, facilitating abundance determinations for arsenic, selenium, hafnium, and thallium, and contributing additional spectral lines for future studies.
Research Context
The field of quantitative spectroscopy, particularly concerning hot subdwarfs, requires precise radiative data. A specific requirement exists for accurate oscillator strengths pertaining to heavy elements that are doubly and triply ionized. A notable challenge in this area is the current scarcity of available data for these particular species. The generation of new, reliable radiative data directly addresses this identified gap.
Approach
The research involved the calculation of new radiative data for four specific ionic species: As III, Se III, Hf IV, and Tl IV. Two primary computational methods were employed for these calculations:
- Pseudo-relativistic Hartree-Fock (HFR) Method: This approach included core-polarization effects. Radial parameters were subjected to semi-empirical adjustment based on experimental energy levels.
- Multiconfiguration Dirac-Hartree-Fock (MCDHF) Method: Independent MCDHF computations were performed for As III, Hf IV, and Tl IV. For Se III, previously published MCDHF data were adopted for comparative purposes.
The reliability of the HFR-derived oscillator strengths was evaluated by comparing them with the MCDHF data. Discrepancies between the two methods were primarily associated with weak transitions, the presence of strong cancellation effects, or significant gauge disagreement. Conversely, for strong transitions that met the established reliability criteria, a good agreement was observed between the oscillator strengths derived from both the HFR and MCDHF methods.
Findings
New radiative data for As III, Se III, Hf IV, and Tl IV have been generated. These data were computed using the HFR method, which incorporated core-polarization effects and semi-empirical adjustment to experimental energy levels. Independent MCDHF computations were also performed for As III, Hf IV, and Tl IV, while existing MCDHF data for Se III were adopted for comparison. This comparison indicated that the largest discrepancies between the HFR and MCDHF approaches were predominantly linked to weak transitions, strong cancellation effects, or substantial gauge disagreement. However, for strong transitions that satisfied specified reliability criteria, good agreement was found between the oscillator strengths obtained by both methodologies. The newly developed data have already been applied to the spectra of the heavy-metal hot subdwarfs LS IV-14°116 and EC 22536-5304, which facilitated abundance determinations for As, Se, Hf, and Tl, and provided additional spectral lines.
Why This Matters
The provision of accurate oscillator strengths for doubly and triply ionized heavy elements is essential for quantitative spectroscopic analyses of hot subdwarfs. The new data directly enable the determination of elemental abundances for arsenic, selenium, hafnium, and thallium in specific hot subdwarf stars like LS IV-14°116 and EC 22536-5304. This also contributes additional spectral lines that can be used for subsequent spectroscopic investigations.