10. Acknowledgements

We would like to thank Egidio Landi Degl’Innocenti and Marco Landolfi for sharing with us their deep knowledge on the physics of the spectral line polarization, which they have described in great detail in their rigorous monograph on “Polarization in Spectral Lines”. Financial support was provided by the AYA2007-63881, AYA2010-18029 and AYA2014-60476-P projects of the Spanish MICINN and MINECO.