The N2D+/N2H+ ratio as an evolutionary tracer of Class 0 protostars M. Emprechtinger, P. Caselli, N. H. Volgenau, J. Stutzki and M. C. Wiedner A&A, 493 1 (2009) 89-105 Published online: 27 October 2008 DOI: 10.1051/0004-6361:200810324