Massive Stars in the SDSS-IV/APOGEE-2 Survey. II. OB-stars in the W345 Complexes

In this work, we have applied a semi-empirical spectral classification method for OB-stars using the APOGEE spectrograph to a sample of candidates in the W3-W4-W5 (W345) complexes. These massive star-forming regions span over 200 pc across the Perseus arm and have a notorious population of massive stars, from which a large fraction are members of various embedded and young open clusters. From 288 APOGEE spectra showing H-band spectral features typical of O- and B-type sources, 46 probably correspond to previously unknown O-type stars. Therefore, we confirm that Br11-Br13 together with He ii λ16923 (7-12) and He ii λ15723 (7-13) lines contained in the APOGEE spectral bands are useful in providing spectral classification down to one spectral sub-class for massive stars in regions as distant as d ≈ 2 kpc. The large number of newly found O-type stars as well as the numerous intermediate-mass population confirm that W345 is a very efficient massive star factory, with an integral stellar population probably amounting several thousand solar masses.