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

¹ Université Grenoble Alpes, IRAM, 38000 Grenoble, France
e-mail: jan.orkisz@chalmers.se
² IRAM, 300 rue de la Piscine, 38406 Saint Martin d’Hères, France
³ Observatoire de Paris, Sorbonne Université, École normale supérieure, CNRS, LERMA, Université PSL, 75005 Paris, France
⁴ School of Physics and Astronomy, Cardiff University, Queen’s buildings, Cardiff CF24 3AA, UK
⁵ Observatoire de Paris, Sorbonne Université, CNRS, LERMA, Université PSL, 92190 Meudon, France
⁶ Grupo de Astrofísica Molecular, IFF-CSIC, Calle Serrano 121-123, 28006 Madrid, Spain
⁷ Laboratoire d’astrophysique de Bordeaux, Université de Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
⁸ Instituto de Astrofísica, Ponticia Universidad Católica de Chile, Avenida Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
⁹ CNRS, IRAP, 9 avenue colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
¹⁰ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
¹¹ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
¹² IRAM, Avenida Divina Pastora, 7, Núcleo Central, 18012 Granada, España
¹³ Maison de la Simulation, CEA-CNRS-INRIA-UPS-UVSQ, USR 3441, Centre d’étude de Saclay, 91191 Gif-Sur-Yvette, France

Received: 11 May 2018
Accepted: 6 February 2019

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″ 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.