Multi-generation massive star-formation in NGC 3576

¹ Jodrell Bank Centre for Astrophysics, Alan Turing Building, School of Physics and Astronomy, The University of Manchester, Oxford Road, Manchester M13 9PL, UK e-mail: cormac.purcell@manchester.ac.uk
² School of Physics, University of New South Wales, Sydney, NSW 2052, Australia
³ CEA, DSM, IRFU, Service d'Astrophysique, 91191 Gif-sur-Yvette, France
⁴ Laboratoire AIM, CEA/DSM - CNRS - Université Paris Diderot, IRFU/Service d'Astrophysique, CEA-Saclay, 91191 Gif-sur-Yvette, France
⁵ Harvard-Smithsonian Centre For Astrophysics, 60 Garden Street, Cambridge, MA, 02138, USA
⁶ CSIRO Australia Telescope National Facillity, PO Box 76, Epping, NSW 1710, Australia
⁷ Centre for Astronomy, James Cook University, Townsville, QLD 4811, Australia
⁸ Departmento de Astronoma, Universidad de Chile, Casilla 36-D, Santiago, Chile
⁹ Université de Bordeaux, Laboratoire d'Astrophysique de Bordeaux, 33000 Bordeaux, France
¹⁰ CNRS/INSU, UMR 5804, BP 89, 33271 Floirac Cedex, France
¹¹ School of Physics, University of Exeter, Stocker Road, EX4 4QL, Exeter, UK
¹² Leiden Observatory, Leiden University, PO BOX 9513, 2300 RA Leiden, the Netherlands

Received: 16 November 2008
Accepted: 3 July 2009

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 star-formation 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 NH₃ (1,1), (2,2) and (4,4) transitions, 22 GHz H₂O masers, and 23 GHz continuum emission. The 70-m Tidbinbilla dish was used to estimate the total integrated intensity of NH₃. We also utilised the 22-m Mopra antenna to map the region for the molecular lines ¹³CO (1 – 0), C¹⁸O (1 – 0), HCO⁺ (1 – 0), H¹³CO⁺ (1 – 0), CS (1 – 0) and N₂H⁺ (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 NH₃ 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 H₂O masers were detected in the arms of the filament, all associated with NH₃ 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 NH₃ (1,1) line profiles. The same clump exhibits an enhanced abundance of N₂H⁺, 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.