Gaia Data Release 2 The celestial reference frame (Gaia-CRF2)

F. Mignard, S.A. Klioner, L. Lindegren, J. Hernandez, U. Bastian, A. Bombrun, Dick Hobbs, U. Lammers, Dirk Michalik, M. Ramos-Lerate, M. Biermann, J. Fernandez, R. Geyer, Hans Erik Peter Lindstrøm