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标题：The Seahorse Nebula: New views of the filamentary infrared dark cloud G304.74+01.32 from SABOCA, Herschel, and WISE ⋆ ⋆⋆
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作者：O. Miettinen
期刊名称：Astronomy & Astrophysics
印刷版ISSN：0004-6361
电子版ISSN：1432-0746
出版年度：2018
卷号：609
DOI：10.1051/0004-6361/201731704
语种：English
出版社：EDP Sciences
摘要：Context.Filamentary molecular clouds, such as many of the infrared dark clouds (IRDCs), can undergo hierarchical fragmentation into substructures (clumps and cores) that can eventually collapse to form stars.Aims.We aim to determine the occurrence of fragmentation into cores in the clumps of the filamentary IRDC G304.74+01.32 (hereafter, G304.74). We also aim to determine the basic physical characteristics (e.g. mass, density, and young stellar object (YSO) content) of the clumps and cores in G304.74.Methods.We mapped the G304.74 filament at 350μm using the Submillimetre APEX Bolometer Camera (SABOCA) bolometer. The new SABOCA data have a factor of 2.2 times higher resolution than our previous Large APEX BOlometer CAmera (LABOCA) 870μm map of the cloud (9″vs.\hbox{$19\farcs86$}). We also employed theHerschelfar-infrared (IR) and submillimetre, and Wide-field Infrared Survey Explorer (WISE) IR imaging data available for G304.74. The WISE data allowed us to trace the IR emission of the YSOs associated with the cloud.Results.The SABOCA 350μm data show that G304.74 is composed of a dense filamentary structure with a mean width of only0.18 ± 0.05pc. The percentage of LABOCA clumps that are found to be fragmented into SABOCA cores is36% ± 16%, but the irregular morphology of some of the cores suggests that this multiplicity fraction could be higher. The WISE data suggest that65% ± 18%of the SABOCA cores host YSOs. The mean dust temperature of the clumps, derived by comparing theHerschel250, 350, and 500μm flux densities, was found to be15.0 ± 0.8K. The mean mass, beam-averaged H2column density, and H2number density of the LABOCA clumps are estimated to be55 ± 10M⊙,(2.0 ± 0.2) × 1022cm-2, and(3.1 ± 0.2) × 104cm-3. The corresponding values for the SABOCA cores are29 ± 3M⊙,(2.9 ± 0.3) × 1022cm-2, and(7.9 ± 1.2) × 104cm-3. The G304.74 filament is estimated to be thermally supercritical by a factor of≳ 3.5on the scale probed by LABOCA, and by a factor of≳ 1.5for the SABOCA filament.Conclusions.Our data strongly suggest that the IRDC G304.74 has undergone hierarchical fragmentation. On the scale where the clumps have fragmented into cores, the process can be explained in terms of gravitational Jeans instability. Besides the filament being fragmented, the finding of embedded YSOs in G304.74 indicates its thermally supercritical state, although the potential non-thermal (turbulent) motions can render the cloud a virial equilibrium system on scale traced by LABOCA. The IRDC G304.74 has a seahorse-like morphology in theHerschelimages, and the filament appears to be attached by elongated, perpendicular striations. This is potentially evidence that G304.74 is still accreting mass from the surrounding medium, and the accretion process can contribute to the dynamical evolution of the main filament. One of the clumps in G304.74, IRAS 13039-6108, is already known to be associated with high-mass star formation, but the remaining clumps and cores in this filament might preferentially form low and intermediate-mass stars owing to their mass reservoirs and sizes. Besides the presence of perpendicularly oriented, dusty striations and potential embedded intermediate-mass YSOs, G304.74 is a relatively nearby (d~ 2.5kpc) IRDC, which makes it a useful target for future star formation studies. Owing to its observed morphology, we propose that G304.74 could be nicknamed the Seahorse Nebula.
关键词：enstars: formationISM: cloudsinfrared: ISMsubmillimeter: ISMISM: individual objects: IRDC G304.74+01.32