H2CO in the Horsehead PDR: photo-desorption of dust grain ice mantles V. Guzmán, J. Pety, J. R. Goicoechea, M. Gerin and E. Roueff A&A, 534 (2011) A49 Published online: 30 September 2011 DOI: 10.1051/0004-6361/201117257