THz spectroscopy and first ISM detection of excited torsional states of 13C-methyl formate I. Haykal, M. Carvajal, B. Tercero, I. Kleiner, A. López, J. Cernicharo, R. A. Motiyenko, T. R. Huet, J. C. Guillemin and L. Margulès A&A, 568 (2014) A58 Published online: 14 August 2014 DOI: 10.1051/0004-6361/201322937