Symmetrical chemi-ionization and chemi-recombination processes in low-temperature layers of helium-rich DB white dwarf atmospheres

Abstract

The influence of symmetrical chemi-ionization and chemi-recombination processes on the helium atom Rydberg states' population in weakly ionized layers of helium-rich DB white dwarfs has been investigated. The ionization processes in He*(n) + He(1s2) collisions and their inverse recombination processes He2+ + e and He(1s2) + He+ + e have been considered in domains of principal quantum numbers n ≥ 3 and temperatures 12,000 K ≤ Teff ≤ 30,000 K. These processes have been treated within the frame of the semiclassical theory developed earlier. Their contributions to the Rydberg state populations have been compared with electron-electron-ion recombination, electron-excited atom ionization, and electron-ion photorecombination processes. Results showed that these processes can be dominant ionization/recombination mechanisms in helium-rich DB white dwarf atmosphere layers for log g = 7 and 8 and Teff ≤ 20,000 K and have to be implemented in relevant models of weakly ionized helium plasmas.