Collisional effects on the formation of the second solar spectrum of the Sr? II λ 4078 line

Abstract

Context. One of the challenging theoretical problems in solar physics is the modeling of polarimetric observations in view of diagnostics of magnetic fields in the solar atmosphere. Aims.This work aims to provide key elements to gain better understanding of the formation of the second solar spectrum of the Sr iiλ4078 line. Methods.The atomic states are quantified by the density matrix elements expressed on the basis of the irreducible tensorial operators. We perform accurate computation of the collisional depolarization and polarization transfer rates for all levels involved in the Sr ii 4078 Å line. We solve the statistical equilibrium equations to calculate the linear polarization degree where: (a) collisions are completely ignored; (b) collisions are taken into account in the framework of a simplified atomic model; and (c) collisions are taken into account in the framework of a 5-levels and 5-lines atomic model. Results.We provide all collisional rates needed for Sr ii λ4078 line modeling. Although Sr ii λ4078 line is the resonance line of Sr ii connecting the ground state 2S1/2 to the excited state 2P3/2, we show that its linear polarization is sensitive to collisions with neutral hydrogen mainly because the metastable D-levels are vulnerable to collisions. Thus, a 5-levels and 5-lines atomic model is needed to study this line. We determine a correction factor that one must apply to the value of the linear polarization derived from a simplified two-level model. Conclusions.In a certain range of hydrogen density, the effect of isotropic collisions between Sr ii ions with hydrogen atoms is important in determining the polarization of the Sr ii λ4078 line. The use of an atomic model that neglects the metastable level 4d of Sr ii can induce errors of up to 25% in the value of the scattering polarization of the solar Sr iiλ4078 line.