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.
Highlights
Filaments of interstellar dust and molecular gas have been known to represent an important structural element of starforming regions in the Galaxy (e.g. Schneider & Elmegreen 1979)
We propose to use observations from the IRAM 30 m large programme Outstanding Imaging of OrioN-B (ORION-B) to analyse the properties of the Orion B filaments over an area of 1.9 deg2
This is visible when looking at the typical volume densities of the filaments, which are estimated by assigning the linear density to a uniform cylinder, the diameter of which would be the full width at half maximum (FWHM) of the individual filament profile. These volume densities range from 104 to 105 cm−3, again with a distribution close to a log-normal one, with a median value of ∼2 × 104 cm−3 (Fig. 11, bottom). This is consistent with the upper end of the volume density distribution in the whole western edge of the Orion B molecular cloud, as presented in Bron et al (2018)
Summary
Filaments of interstellar dust and molecular gas have been known to represent an important structural element of starforming regions in the Galaxy (e.g. Schneider & Elmegreen 1979). This suggests that most star-forming cores form as the result of gravitational instabilities occurring within unstable filaments (André et al 2014) Another key proposition that has emerged from Herschel observations of star-forming clouds in the Gould Belt is the potential universality of the width of interstellar filaments at ∼0.1 pc (Arzoumanian et al 2011). In the last few years, large surveys such as the Green Bank Ammonia Survey (GAS; Friesen et al 2017) and the CARMA-NRO Orion Survey (Kong et al 2018) have started to observe Gould Belt clouds in molecular lines over several square degrees Such large datasets are required to build statistically significant samples of filaments and characterise their dynamical properties. We present the datasets and steps applied to obtain an accurate map of the molecular hydrogen column density
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