Collisional effects on the formation of the second solar spectrum of the Sr {\sc ii} $\lambda$4078 line

This work aims to provide key elements to gain better understanding of the formation of the second solar spectrum of the Sr II $\lambda$4078 line. We show that 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 $\lambda$4078 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 II $\lambda$4078 line.