Discovery of a Young Massive Stellar Cluster near HESS J1813–178

We present the serendipitous discovery of a young stellar cluster in the Galactic disk at l = 12°. Using Keck/NIRSPEC, we obtained high- and low-resolution spectroscopy of several stars in the cluster, and we identified one red supergiant and two blue supergiants. The radial velocity of the red supergiant provides a kinematic cluster distance of 4.7 ± 0.4 kpc, implying luminosities of the stars consistent with their spectral types. Together with the known Wolf-Rayet star located 2.4^' from the cluster center, the presence of the red supergiant and the blue supergiants suggests a cluster age of 6-8 Myr and an initial mass of ≳2000 M_☉. Several stars in the cluster are coincident with X-ray sources, including the blue supergiants and the Wolf-Rayet star. This is indicative of a high binary fraction and is reminiscent of the massive young cluster Westerlund 1. The cluster is coincident with two supernova remnants, SNR G12.72–0.0 and G12.82–0.02, and the highly magnetized pulsar associated with the TeV γ-ray source HESS J1813–178. The mixture of spectral types suggests that the progenitors of these objects had initial masses of 20-30 M_☉.