The Young Massive Stellar Cluster Associated with RCW 121

Abstract

We report near-infrared (NIR) broadband and narrowband photometric observations in the direction of the IRAS 17149À3916 source that reveal the presence of a young cluster of massive stars embedded in an H ii region coin-cident with RCW 121. These observations, together with published radio data and Midcourse Space Experiment and Spitzer images, were used to determine some of the physical parameters of the region. We found 96 cluster member candidates in an area of about 1:5 ; 2:0 arcmin 2 , 30% of them showing excess emission in the near-IR. IRS 1, the strongest source in the cluster with an estimated spectral type of O5 V-O6 V zero-age main sequence based on the color-magnitude diagram, is probably the main ionizing source of the H ii region detected at radio wavelengths. Using the integrated Br and the 5 GHz flux densities, we derived a mean visual extinction A V ¼ 5:49AE 2:06 1:32 mag. From the observed size of the Br extended emission, we calculated the emission measure E ¼ 4:5 ; 10 24 cm À5 and the electron density n e ¼ 2:6 ; 10 3 cm À3 , characteristic of compact H ii regions.