[C II] 158 μm self-absorption and optical depth effects C. Guevara, J. Stutzki, V. Ossenkopf-Okada, R. Simon, J. P. Pérez-Beaupuits, H. Beuther, S. Bihr, R. Higgins, U. Graf and R. Güsten A&A, 636 (2020) A16 Published online: 07 April 2020 DOI: 10.1051/0004-6361/201834380