Multi-generation massive star-formation in NGC 3576

Abstract

Context. Recent 1.2-mm continuum observations have shown the giant H II region NGC 3576 to be embedded in the centre of an extended filamentary dust-cloud. The bulk of the filament away from the H II region contains a number of clumps seen only at (sub-)millimetre wavelengths. Infrared and radio observations of the central star cluster have uncovered evidence of sequential starformation leading us to believe that the adjacent clumps may host massive protostellar objects at a very early stage of evolution. Aims. We have investigated the physical and chemical conditions in the dusty clumps with the goal of characterising their star-forming content. Methods. We have used the Australia Telescope Compact Array (ATCA) to image the cloud for the NH 3 (1,1), (2,2) and (4,4) transitions, 22 GHz H 2 O masers, and 23 GHz continuum emission. The 70-m Tidbinbilla dish was used to estimate the total integrated intensity of NH 3 . We also utilised the 22-m Mopra antenna to map the region for the molecular lines 13 CO (1 -0), C 18 O (1 -0), HCO + (1 -0), H 13 CO + (1 -0), CS (1 -0) and N 2 H + (1 -0). Results. Emission from dense molecular gas follows the morphology of the 1.2-mm dust emission, except towards the central ionised region. The H II region is observed to be expanding into the molecular cloud, sweeping up a clumpy shell of gas, while the central star cluster is dispersing the molecular gas to the east. Analysis of the NH 3 data indicates that temperature and linewidth gradients exist in the western arm of the filament. Temperatures are highest adjacent to the central H II region, indicating that the embedded cluster of young stars there is heating the gas. Six new H 2 O masers were detected in the arms of the filament, all associated with NH 3 emission peaks, confirming that star-formation has begun within these cores. Core masses range from 5 to 516 M and most appear to be gravitationally bound. Complementary results by André et al. (2008) imply that seven cores will go on to form massive stars between 15 and 50 M . The large scale velocity structure of the filament is smooth, but at least one clump shows the signature of inward gas motions via asymmetries in the NH 3 (1,1) line profiles. The same clump exhibits an enhanced abundance of N 2 H + , which coupled with an absence of CO indicates depletion onto the dust grain surface. Conclusions. The HII region at the heart of NGC 3576 is potentially triggering the formation of massive stars in the bulk of the associated cloud.

Initial phases of massive star formation in high infrared extinction clouds

“…The CO depletion in a clump can therefore be used as a time marker. Observations of massive protostellar objects (Purcell et al 2009) and clumps in IRDCs (Pillai et al 2007) show depletion of CO, indicating that these objects are already in a state of collapse and not in the chemically Fig. 11.…”

Ionisation impact of high-mass stars on interstellar filaments

“…Deharveng et al 2010;Zavagno et al 2010;Purcell et al 2009;Minier et al 2009). However, the actual impact of high-mass star radiation remains a debated scientific controversy in the literature (e.g.…”

Evidence of triggered star formation in G327.3-0.6

“…Possible phenomena responsible for their formation include external events such as expanding HII regions and propagating shocks, which are common in the environment of young highmass star-forming complexes (e.g., Zavagno et al 2008). The ionised front of an HII region can compress a pre-existing molecular cloud and trigger the formation of dense cores (e.g., Purcell et al 2009). Stellar winds or shocks from supernovae can also lead to the formation of similar objects (e.g., Koo et al 2008).…”