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

Abstract : 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. We try to observationally derive the fractions of momentum density ($\rho v$) 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. The implementation of a statistical method developed by Brunt & Federrath (2014), applied to a $^{13}$CO(J=1-0) datacube obtained with the IRAM-30m telescope, allows us to retrieve 3-dimensional quantities from the projected quantities provided by the observations, yielding an estimate of the compressive versus solenoidal ratio in various regions of the cloud. Despite the Orion B molecular cloud being highly supersonic (mean Mach number $\sim$ 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. We have successfully applied to observational data a method that was so far only tested on simulations, and 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.
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Astronomy and Astrophysics - A&A, EDP Sciences, 2017, 599, pp.id.A99 〈10.1051/0004-6361/201629220〉
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Soumis le : vendredi 13 janvier 2017 - 15:09:26
Dernière modification le : mardi 24 octobre 2017 - 01:15:35

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Jan H. Orkisz, Jérôme Pety, Maryvonne Gerin, Emeric Bron, Viviana V. Guzmán, et al.. Turbulence and star formation efficiency in molecular clouds: solenoidal versus compressive motions in Orion B. Astronomy and Astrophysics - A&A, EDP Sciences, 2017, 599, pp.id.A99 〈10.1051/0004-6361/201629220〉. 〈hal-01434996〉

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