A Near-Infrared/Millimeter-Wave Study of Six Fourth-Quadrant High-Mass Star Formation Regions

We present near-infrared and millimeter-wave observations of six high-mass star-forming regions in the fourth quadrant: RCW 108, G333.6, RCW 117, RCW 122, NGC 6334 I, and G351.6. These regions have heliocentric distances of 1.3–3.0 kpc and total luminosities ranging from 0.5 to 3 × 10 5 L ⊙ . Millimeter maps taken in transitions of C 18 O, HC 3 N, 12 C 16 O, and SiO with the Swedish-ESO Submillimetre Telescope 15 m telescope detail the structure and kinematics of the clouds. Near-infrared K -band images of the same regions obtained with the MPG/ESO 2.2 m telescope are used to map the stellar surface density. The C 18 O (1 → 0) line shows extended high column density gas, with column densities ranging from 10 22 to 3 × 10 23 cm -2 ; in contrast, the HC 3 N (15 → 14) emission shows eight sharply peaked dense cores. We examine these cores for recent and ongoing star formation by searching for high-velocity, non-Gaussian wings on the CO (1 → 0) and SiO (2 → 1) lines, peaks in the surface density of stars, and positional coincidences with IRAS point sources, as well as masers and H II regions reported in the literature. All of the cores show evidence of ongoing star formation. NGC 6334 I(N) stands out as a unique case in which an outflow has formed and masers have appeared, but no IRAS source, K -band cluster, or UC H II region has yet appeared; our data support previous claims that this is a rare example of a core in the earliest stages of forming a cluster of high- and low-mass stars.