The ionization fraction gradient across the Horsehead edge: an archetype for molecular cloudsJ. R. Goicoechea, J. Pety, M. Gerin, P. Hily-Blant and J. Le BourlotA&A, 498 3 (2009) 771-783DOI: https://doi.org/10.1051/0004-6361/200811496
