| Abstract
| - Context. Young massive stars and stellar clusters continuously form in the Galactic disk, generating new H ii regions within their natal giant molecular clouds and subsequently enriching the interstellar medium via their winds and supernovae. Aims. Massive stars are among the brightest infrared stars in such regions; their identification permits the characterisation of the star formation history of the associated cloud as well as constraining the location of stellar aggregates and hence their occurrence as a function of global environment. Methods. We present a stellar spectroscopic survey in the direction of the giant molecular cloud G23.3 −0.3. This complex is located at a distance of ~4-5 kpc, and consists of several H ii regions and supernova remnants. Results. We discovered 11 Of K+ stars, one candidate luminous blue variable, several OB stars, and candidate red supergiants. Stars with K-band extinction from ~1.3-1.9 mag appear to be associated with the GMC G23.3 −0.3; O and B-type s satisfying this criterion have spectrophotometric distances consistent with that of the giant molecular cloud. Combining near-IR spectroscopic and photometric data allowed us to characterize the multiple sites of star formation within it. The O-type stars have masses from ~25-45 M⊙, and ages of 5-8 Myr. Two new red supergiants were detected with interstellar extinction typical of the cloud; along with the two RSGs within the cluster GLIMPSE9, they trace an older burst with an age of 20-30 Myr. Massive stars were also detected in the core of three supernova remnants - W41, G22.7 −0.2, and G22.7583 −0.4917. Conclusions. A large population of massive stars appears associated with the GMC G23.3 −0.3, with the properties inferred for them indicative of an extended history of stars formation.
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