A dynamically young, gravitationally stable network of filaments in Orion B

Abstract

Context. Filaments are a key step on the path that leads from molecular clouds to star formation. However, their characteristics, for instance their width, are heavily debated and the exact processes that lead to their formation and fragmentation into dense cores still remain to be fully understood. Aims. We aim at characterising the mass, kinematics, and stability against gravitational collapse of a statistically significant sample of filaments in the Orion B molecular cloud, which is renown for its very low star formation efficiency. Methods. We characterised the gas column densities and kinematics over a field of 1.9 deg2 , using C18O (J = 1−0) data from the IRAM 30 m large programme ORION-B at angular and spectral resolutions of 23.5 00 and 49.5 kHz, respectively. Using two different Hessian-based filters, we extracted and compared two filamentary networks, each containing over 100 filaments. Results. Independent of the extraction method, the filament networks have consistent characteristics. The filaments have widths of ∼0.12 ± 0.04 pc and show a wide range of linear (∼1−100 M pc−1 ) and volume densities (∼2 × 103−2 × 105 cm−3 ). Compared to previous studies, the filament population is dominated by low-density, thermally sub-critical structures, suggesting that most of the identified filaments are not collapsing to form stars. In fact, only ∼1% of the Orion B cloud mass covered by our observations can be found in super-critical, star-forming filaments, explaining the low star formation efficiency of the region. The velocity profiles observed across the filaments show quiescence in the centre and coherency in the plane of the sky, even though these profiles are mostly supersonic. Conclusions. The filaments in Orion B apparently belong to a continuum which contains a few elements comparable to already studied star-forming filaments, for example in the IC 5146, Aquila or Taurus regions, as well as many lower density, gravitationally unbound structures. This comprehensive study of the Orion B filaments shows that the mass fraction in super-critical filaments is a key factor in determining star formation efficiency.

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