A long-period Cepheid variable in the starburst cluster VdBH222

Context. Galactic starburst clusters play a twin role in astrophysics, serving as laboratories for the study of stellar physics and also delineating the structure and recent star formation history of the Milky Way. Aims. In order to exploit these opportunities we have undertaken a spectroscopic survey of the red supergiant dominated young massive clusters thought to be present at both near and far ends of the Galactic Bar. Methods. Specifically, multi-epoch observations were employed to identify and investigate stellar variability and its potential role in initiating mass loss amongst the cool super-/hypergiant populations of these aggregates. Results. Significant spectroscopic variability suggestive of radial pulsations was found for the yellow supergiant VdBH 222 #505. Follow-up photometric investigations revealed modulation with a period of ~23.325 d; both timescale and pulsational profile are consistent with a Cepheid classification. Conclusions. #505 is one of the longest period Galactic cluster Cepheids identified to date and hence of considerable use in constraining the bright end of the period/luminosity relation at solar metallicities. In conjunction with extant photometry we infer a distance of ~6 kpc for VdBH222 and an age of ~20 Myr. This results in a moderate reduction in both the integrated cluster mass (~2×104M⊙) and the initial masses of the evolved cluster members (~10 M⊙). As such VdBH222 becomes an excellent test-bed for studying the properties of some of the lowest mass stars observed to undergo type-II supernovae. Moreover, the distance is in tension with a location of VdBH 222 at the far end of the Galactic Bar. Instead a birthsite in the near 3 kpc arm is suggested; providing compelling evidence of extensive recent star formation in a region of the inner Milky Way which has hitherto been thought to be devoid of such activity.This research is partially supported by the Spanish Ministerio de Economía y Competitividad under grant AYA2012-39364-C02-02, and the European Union