Dissecting the molecular structure of the Orion B cloud: insight from principal component analysis

Guzmán

et al. 2017

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190

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Context. Molecular lines and line ratios are commonly used to infer properties of extra-galactic star forming regions. The new generation of millimeter receivers almost turns every observation into a line survey. Full exploitation of this technical advancement in extra-galactic study requires detailed bench-marking of available line diagnostics. Aims. We aim to develop the Orion B giant molecular cloud (GMC) as a local template for interpreting extra-galactic molecular line observations. Methods. We use the wide-band receiver at the IRAM-30 m to spatially and spectrally resolve the Orion B GMC. The observations cover almost 1 square degree at 26 resolution with a bandwidth of 32 GHz from 84 to 116 GHz in only two tunings. Results. We introduce the molecular anatomy of the Orion B GMC, including relationships between line intensities and gas column density or far-UV radiation fields, and correlations between selected line and line ratios. We also obtain a dust-traced gas mass that is less than approximately one third the CO-traced mass, using the standard X CO conversion factor. The presence of over-luminous CO can be traced back to the dependence of the CO intensity on UV illumination. As a matter of fact, while most lines show some dependence on the UV radiation field, CN and C 2 H are the most sensitive. Moreover, dense cloud cores are almost exclusively traced by N 2 H + . Other traditional high-density tracers, such as HCN(1−0), are also easily detected in extended translucent regions at a typical density of ∼500 H 2 cm −3 . In general, we find no straightforward relationship between line critical density and the fraction of the line luminosity coming from dense gas regions. Conclusions. Our initial findings demonstrate that the relationships between line (ratio) intensities and environment in GMCs are more complicated than often assumed. Sensitivity (i.e., the molecular column density), excitation, and, above all, chemistry contribute to the observed line intensity distributions, and they must be considered together when developing the next generation of extra-galactic molecular line diagnostics of mass, density, temperature, and radiation field.

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

The anatomy of the Orion B giant molecular cloud: A local template for studies of nearby galaxies

Guzmán

et al. 2017

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190

319

“…This area has been extensively observed in the 3 mm range with the IRAM-30 m telescope (PI: J. Pety). This survey has led to a series of papers (Liszt & Pety 2016;Pety et al 2017;Gratier et al 2017) to which this article belongs.…”

Section: Introductionmentioning

confidence: 99%

Turbulence and star formation efficiency in molecular clouds: solenoidal versus compressive motions in Orion B

Gérin

et al. 2017

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Context. The nature of turbulence in molecular clouds is one of the key parameters that control star formation efficiency: compressive motions, as opposed to solenoidal motions, can trigger the collapse of cores, or mark the expansion of Hii regions. Aims. We try to observationally derive the fractions of momentum density (ρv) contained in the solenoidal and compressive modes of turbulence in the Orion B molecular cloud and relate these fractions to the star formation efficiency in the cloud. Methods. The implementation of a statistical method applied to a 13 CO(J = 1−0) datacube obtained with the IRAM-30 m telescope, enables us to retrieve 3-dimensional quantities from the projected quantities provided by the observations, which yields an estimate of the compressive versus solenoidal ratio in various regions of the cloud. Results. Despite the Orion B molecular cloud being highly supersonic (mean Mach number ∼6), the fractions of motion in each mode diverge significantly from equipartition. The cloud's motions are, on average, mostly solenoidal (excess >8% with respect to equipartition), which is consistent with its low star formation rate. On the other hand, the motions around the main star forming regions (NGC 2023 and NGC 2024) prove to be strongly compressive. Conclusions. We have successfully applied to observational data a method that has so far only been tested on simulations, and we have shown that there can be a strong intra-cloud variability of the compressive and solenoidal fractions, these fractions being in turn related to the star formation efficiency. This opens a new possibility for star formation diagnostics in galactic molecular clouds.

“…dense cores) that can dominate the fainter filamentary structures being are searched for. In our case, we find it better to use the asinh transform used in Gratier et al (2017) (Fig. B.2, panel 1), which is linear at small intensities and logarithmic at high intensities.…”

Section: B13 Adaptive Filament Extraction With Ridge Detectionmentioning

confidence: 67%

“…Thanks to the multi-tracer nature of the ORION-B project, the filamentary skeletons could be used to stack the observed spectra to reveal faint molecular lines and characterise the molecular signature of filamentary regions, in a manner similar to Gratier et al (2017) and Bron et al (2018). Conversely, classification tools such as the clustering used by Bron et al (2018) could possibly reveal the presence of different families of filaments, based on their properties derived in this paper, which could point to different evolutionary stages or scenarios for the filaments in Orion B. Conversely, the number of components can decrease to a minimum of one.…”

Section: Discussionmentioning

confidence: 99%

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

Peretto

et al. 2019

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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 deg 2 , using C 18 O (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 × 10 3 − 2 × 10 5 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.