The Cygnus complex is one of the most powerful star forming regions at a close distance from the Sun (~1.4 kpc). Its richest OB association Cygnus OB2 is known to harbour many tens of O-type stars and hundreds of B-type stars, providing a large homogeneous population of OB stars that can be analysed. Many studies of its massive population have been developed in the last decades, although the total number of OB stars is still incomplete. We have obtained new blue intermediate-resolution spectra suitable for spectral classification of the 61 candidates presented by Comer\’on & Pasquali (2012) in Cygnus OB2 and surroundings. We thus performed a spectral classification of the sample using He I-II and metal lines rates, as well as the Marxist Ghost Buster (MGB) software for O-type stars and the IACOB standards catalogue for B-type stars. Out of the 61 candidates, we have classified 42 stars as new massive OB-type stars, earlier than B3, in Cygnus OB2 and surroundings, including 11 O-type stars. The other candidates are discarded as they display later spectral types inconsistent with membership in the association. However, the magnitude cutoff and dust extinction introduce an incompleteness. Many O and early B stars at B > 16 mag are still undiscovered in the region. Finally, we have studied the age and extinction distribution of our sample within the region, placing them in the Hertzsprung-Russell Diagram using both rotating and non-rotating models calculated by Ekstr\”om et al. (2012) and Brott et al. (2011) in order to assess age uncertainties. Massive star formation in Cygnus OB2 seems to have proceeded from lower to higher Galactic longitudes, regardless of the details of the models used. The correlation between age and Galactic longitude previously found in the region is now confirmed.
S. Berlanas, A. Herrero, F. Comeron, et. al.
Tue, 21 Nov 17
Comments: 20 pages, 12 figures, accepted for publication in Astronomy & Astrophysics