The dense gas mass fraction in the W51 cloud and its protoclusters Adam Ginsburg, John Bally, Cara Battersby, Allison Youngblood, Jeremy Darling, Erik Rosolowsky, Héctor Arce and Mayra E. Lebrón Santos A&A, 573 (2015) A106 Published online: 06 January 2015 DOI: 10.1051/0004-6361/201424979