Herschel, Spitzer and Magellan infrared observations of the star-forming region RCW 121 (IRAS 17149-3916)

Abstract

We present new deep near-infrared broad- and narrow-band imaging and low-resolution spectroscopy of the star formation region RCW 121 (IRAS 17149-3916) which we analyse in combination with Herschel (70, 160, 250, 350 and 500 μm) and Spitzer (3.6, 4.5, 5.8 and 8 μm) images. The near-infrared photometry reveals the presence of a stellar cluster of approximate size of 92 arcsec which is composed of at least 264 members, approximately 25 per cent of these showing excess emission at λ > 2.0 μm, indicative of circumstellar discs. Isochrones corresponding to ages 0.5-1.0 Myr and A_V = 7.8 fit well the position of a large fraction of likely cluster members in the K_s versus H - K_s diagram. We find three massive star-forming cores located in the boundaries of an expanding H II 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 1.2-500 μm SEDs are fitted with Robitaille et al.'s star-disc-envelope model to obtain their physical parameters. The masses of the three YSOs are between 8 and 10 M_⊙. The youngest site (core I) is undetected at λ < 100 μm 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 H₂ line emission, co-existing with an H₂O maser source. RCW 121 is another example of multiple star formation being triggered by the expansion of a single H II region.