The “Dragon” Nebula G28.34+0.06

Abstract

This chapter presents new SMA and JVLA observations of an infrared dark cloud G28.34+0.06. Located in the quiescent southern part of the G28.34 cloud, the region of interest is a massive ($>$ $10^3$ $M_{\odot }$) molecular clump P1 with a luminosity of $\sim$ $10^3$ $L_{\odot }$, where our previous SMA observations at 1.3 mm have revealed a string of five dust cores of 22–64 $M_{\odot }$ along the 1 pc IR-dark filament. The cores are well aligned at a position angle of 48$^{\circ }$ and regularly spaced at an average projected separation of 0.16 pc. The new high-resolution, high-sensitivity 0.88 mm image further resolves the five cores into ten compact condensations of 1.1–17.2 $M_{\odot }$, with sizes a few thousands AU. The spatial structure at clump ($\sim$ $1$ pc) and core ($\sim$ $0.1$ pc) scales indicates a hierarchical fragmentation. While the clump fragmentation is consistent with a cylindrical collapse, the observed fragment masses are much larger than the expected thermal Jeans masses. All the cores are driving CO (3–2) outflows up to 38 km s$^{-1}$, majority of which are bipolar, jet-like outflows. Multiple $\mathrm {NH_{3}}$ transitions reveal the heated gas widely spread in the 1 pc clump. The temperature distribution is highly structured; the heated gas is offset from the protostars, and matches the outflows in morphology. Hot spots of spatially compact, spectrally broad $\mathrm {NH_{3}}$ (3,3) emission features are also found coincident with the outflows. A weak $\mathrm {NH_{3}}$ (3,3) maser is discovered at the interface between an outflow jet and the ambient gas. The moderate luminosity of the P1 clump sets a limit on the mass of protostars of 3–7 $M_{\odot }$. Because of the large reservoir of dense molecular gas in the immediate medium and ongoing accretion as evident by the jet-like outflows, we speculate that P1 will grow and eventually form a massive star cluster. This study provides a first glimpse of massive, clustered star formation that currently undergoes through an intermediate-mass stage. The findings suggest that protostellar heating may not be effective in suppressing fragmentation during the formation of massive cores.