Multi-generation massive star-formation in NGC 3576

Abstract

<i>Context. <i/>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 star-formation leading us to believe that the adjacent clumps may host massive protostellar objects at a very early stage of evolution.<i>Aims. <i/>We have investigated the physical and chemical conditions in the dusty clumps with the goal of characterising their star-forming content.<i>Methods. <i/>We have used the Australia Telescope Compact Array (ATCA) to image the cloud for the NH<sub>3<sub/> (1,1), (2,2) and (4,4) transitions, 22 GHz H<sub>2<sub/>O masers, and 23 GHz continuum emission. The 70-m Tidbinbilla dish was used to estimate the total integrated intensity of NH<sub>3<sub/>. We also utilised the 22-m Mopra antenna to map the region for the molecular lines <sup>13<sup/>CO (1 – 0), C<sup>18<sup/>O (1 – 0), HCO<sup>+<sup/> (1 – 0), H<sup>13<sup/>CO<sup>+<sup/> (1 – 0), CS (1 – 0) and N<sub>2<sub/>H<sup>+<sup/> (1 – 0). <i>Results. <i/>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<sub>3<sub/> 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<sub>2<sub/>O masers were detected in the arms of the filament, all associated with NH<sub>3<sub/> emission peaks, confirming that star-formation has begun within these cores. Core masses range from 5 to 516 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 . 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<sub>3<sub/> (1,1) line profiles. The same clump exhibits an enhanced abundance of N<sub>2<sub/>H<sup>+<sup/>, which coupled with an absence of CO indicates depletion onto the dust grain surface. <i>Conclusions. <i/>The HII region at the heart of NGC 3576 is potentially triggering the formation of massive stars in the bulk of the associated cloud.