摘要：Context. W40 is a heavily obscured bipolar HII region projected in the direction of the Aquila Rift and ionized by hot stars in a central, partly embedded cluster. The study of the cluster and its surroundings has been greatly hampered thus far by the strong extinction in the region. Aims. Our aim is to improve the characterization of the W40 central cluster in terms of the census of its members and their spectral classification. We also search for other members of the region outside the central cluster, with a particular interest in previously unidentified massive members that may contribute to the energy budget powering the expansion of the HII region. Methods. We used the Gaia eDR3 catalog to establish astrometric membership criteria based on the population of the W40 central cluster, reassess the distance of the region, and use this information to identify new members, both inside and outside the cluster. We obtained visible spectroscopy in the red spectral region to classify both known and new members, complemented with Gaia and Spitzer photometry to assess the evolutionary status of the stellar population. Results. Based on stars with high quality Gaia astrometry, we derive a high-confidence geometric distance to the W40 region of 502 pc ± 4 pc and we confirm the presence of a comoving extended population of stars at the same distance, spreading over the whole projected area of the HII region and beyond. Spectral classifications are presented for 21 members of the W40 region, 10 of them belonging to the central cluster. One of the newly identified B stars in the extended population is clearly interacting with the shell surrounding the HII region, giving rise to a small arc-shaped nebula that traces a bow shock. The infrared excess properties suggest that the extended population is significantly older (∼3 Myr) than the W40 central cluster (<1 Myr). Conclusions. The area currently occupied by the W40 HII region and its surroundings has a history of star formation extending at least several million years in the past. The formation of the W40 central cluster and the subsequent HII region is one of the latest episodes of these processes. The newly determined distance suggests that W40 is behind, as well as physically detached from, a pervasive large dust layer, which is some 60 pc foreground with respect to it, as determined by previous studies.
关键词：stars: early-type;ISM: bubbles;HII regions;open clusters and associations: individual: W40
