The nature of the Class I population in Ophiuchus as revealed through gas and dust mapping T. A. van Kempen, E. F. van Dishoeck, D. M. Salter, M. R. Hogerheijde, J. K. Jørgensen and A. C. A. Boogert A&A, 498 1 (2009) 167-194 Published online: 05 March 2009 DOI: 10.1051/0004-6361/200810445