Dielectronic recombination of trans-iron ions Sr4+, Se4+, Ga4+, and In4+ for NLTE modeling of hot white dwarf atmospheres

Dielectronic recombination (DR) is a fundamental resonance-enhanced kinetic process that governs the ionization balance and radiative properties of non-local thermodynamic equilibrium (NLTE) astrophysical plasmas. Within the Dirac–Fock–Slater framework, the flexible atomic code is employed to calculate radiative transition probabilities and autoionization rates. Adopting the independent-processes approximation and isolated-resonance approximation, we further derive the DR cross sections and rate coefficients for Sr4+, Se4+, Ga4+, and In4+. Convergence tests for Rydberg-state truncation are performed to support atomic parameter evaluations. Our results reveal a profound ion-dependent sensitivity in the DR rate coefficients, which we attribute to the distinct alignment of near-threshold resonances and dominant core-excitation channels in these intermediate-mass species. Among the ions considered, Sr4+ exhibits the largest calculated DR rate coefficients over most of the investigated temperature range. Sr4+, In4+, and Ga4+ show thermally driven double-peak structures, with maxima arising from the sequential activation of mid- to high-energy resonance manifolds. In contrast, Se4+ exhibits a single dominant peak centered near 105 K. These distinct ion-dependent variations in recombination efficiency can provide valuable physical inputs for collisional–radiative (CR) models of hot white-dwarf atmospheres. We provide analytic fits to the total DR rate coefficients to facilitate their integration into CR models, with fitted results consistent with original numerical data within finite deviations. The present zero-density DR data may serve as useful input for future NLTE and CR calculations of trans-iron elements in hot white-dwarf atmospheres.