The total and relative contribution of the relevant absorption processes to the opacity of DB white dwarf atmospheres in the UV and VUV regions

Abstract

The main aim of this work is to estimate the total contribution of the processes of He + 2 molecular ion photodissociation and He + He + collisional absorption charge exchange to the opacity of DB white dwarf atmospheres, and compare this with the contribution of He- and other relevant radiative absorption processes included in standard models. The method for the calculations of the molecular ion He + 2 photodissociation cross-sections is based on the dipole approximation and quantum-mechanical treatment of the internuclear motion, while the quasi-classical method for describing absorption processes in He + He + collisions is based on the quasi-static approximation. Absorption coefficients are calculated in the region 50 nm ≤ λ ≤ 850 nm and compared with the corresponding coefficients of other relevant absorption processes; the calculations of the optical depth of the atmosphere layers considered are performed in the far-UV and VUV regions; the contribution of the relevant absorption processes to the opacity of DB white dwarf atmospheres is examined. We examined the spectral ranges in which the total He + 2 and He- absorption processes dominate in particular layers of DB white dwarf atmospheres. In addition, we show that in the region of λ ≤ 70 nm the process of H(1s) atom photoionization is also important, in spite of the fact that the ratio of hydrogen and helium abundances in the DB white dwarf atmosphere considered is 1:10 5 .