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Description
We present new deep near-infrared broad- and narrow-band imaging and low-resolution spectroscopy of the star formation region RCW 121 (IRAS17149-3916) which we analyse in combination with Herschel (70, 160, 250, 350 and 500 microns) and Spitzer (3.6, 4.5, 5.8 and 8 microns) images. The near-infrared photometry reveals the presence of a stellar cluster of approximate size of 92" which is composed of at least 264 members, approximately 25 per cent of these showing excess emission at wavelengths >2.0 microns, indicative of circumstellar discs. Isochrones corresponding to ages 0.5 to 1.0Myr and A_V_=7.8 fit well the position of a large fraction of likely cluster members in the Ks versus H-Ks diagram. We find three massive star-forming cores located in the boundaries of an expanding HII region ionized by a central O-type star. From their far-infrared spectral energy distributions (SEDs) we derive masses and temperatures of the dense cores. When these young stellar objects (YSOs) have warm emission components, the 1.2 to 500 microns SEDs are fitted with Robitaille et al.'s (2006ApJS..167..256R, 2007ApJS..169..328R) star-disc-envelope model to obtain their physical parameters. The masses of the three YSOs are between 8 and 10M_{sun}_. The youngest site (core I) is undetected at wavelengths <100 microns and is at the earliest evolutionary stage that can currently be detected. The other two cores (II and III) contain YSOs of similar masses and have near-infrared counterparts, which imply a more advanced evolutionary stage. The YSO at core II has been found to have associated a jet, with strong molecular hydrogen line emission, coexisting with a water maser source. RCW 121 is another example of multiple star formation being triggered by the expansion of a single HII region.
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