Cosmic-ray ionisation in collapsing clouds

Padovani, M.; Hennebelle, P.; Galli, Daniele

doi:10.1051/0004-6361/201322407

Cited by 104 publications

(127 citation statements)

References 43 publications

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“…Our measurements probe -assuming that all other factors are well known -the product σ gr n gr G cr rather than G cr alone. Therefore, low values of the CR-induced FUV field in our models could indeed be the result of a reduced cosmic ray flux deep inside dense cores (e.g., Padovani et al 2013). Alternatively, it could also be due to dust growth to micron-sized particles deep in the envelope (e.g., Pagani et al 2010;Steinacker et al 2010).…”

Section: Water Vapour Chemistrymentioning

confidence: 89%

Water in low-mass star-forming regions withHerschel

Schmalzl

Visser

Walsh

et al. 2014

A&A

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Aims. Our aim is to determine the critical parameters in water chemistry and the contribution of water to the oxygen budget by observing and modelling water gas and ice for a sample of eleven low-mass protostars, for which both forms of water have been observed. Methods. A simplified chemistry network, which is benchmarked against more sophisticated chemical networks, is developed that includes the necessary ingredients to determine the water vapour and ice abundance profiles in the cold, outer envelope in which the temperature increases towards the protostar. Comparing the results from this chemical network to observations of water emission lines and previously published water ice column densities, allows us to probe the influence of various agents (e.g., far-ultraviolet (FUV) field, initial abundances, timescales, and kinematics). Results. The observed water ice abundances with respect to hydrogen nuclei in our sample are 30−80 ppm, and therefore contain only 10-30% of the volatile oxygen budget of 320 ppm. The keys to reproduce this result are a low initial water ice abundance after the pre-collapse phase together with the fact that atomic oxygen cannot freeze-out and form water ice in regions with T dust > ∼ 15 K. This requires short prestellar core lifetimes < ∼ 0.1 Myr. The water vapour profile is shaped through the interplay of FUV photodesorption, photodissociation, and freeze-out. The water vapour line profiles are an invaluable tracer for the FUV photon flux and envelope kinematics. Conclusions. The finding that only a fraction of the oxygen budget is locked in water ice can be explained either by a short precollapse time of < ∼ 0.1 Myr at densities of n H ∼ 10 4 cm −3 , or by some other process that resets the initial water ice abundance for the post-collapse phase. A key for the understanding of the water ice abundance is the binding energy of atomic oxygen on ice.

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Section: Water Vapour Chemistrymentioning

confidence: 89%

Water in low-mass star-forming regions withHerschel

Schmalzl

Visser

Walsh

et al. 2014

A&A

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“…The resulting flux of UV photons F UV (in the energy range between 11.2 and 13.6 eV) 4 More advanced models should take into account the fact that molecular clouds are magnetized and CRs gyrate along magnetic field lines in addition to being scattered by magnetic fluctuations on the scale of the particle gyroradius. For a detailed treatment of these effects, see Padovani & Galli (2011), Padovani et al (2013Padovani et al ( , 2014, and Morlino & Gabici (2015). 5 We note that the results presented in Section 3 practically do not depend on the precise form of the formula for x e .…”

Section: Local Radiation Fieldmentioning

confidence: 89%

Interstellar Dust Charging in Dense Molecular Clouds: Cosmic Ray Effects

Ivlev

Padovani

Galli

et al. 2015

ApJ

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The local cosmic-ray (CR) spectra are calculated for typical characteristic regions of a cold, dense molecular cloud to investigate two mechanisms of dust charging that have, thus far, been neglected: the collection of suprathermal CR electrons and protons by grains and photoelectric emission from grains due to the UV radiation generated by CRs. These two mechanisms add to the conventional charging by ambient plasma, produced in the cloud by CRs. We show that the CR-induced photoemission can dramatically modify the charge distribution function for submicron grains. We demonstrate the importance of the obtained results for dust coagulation: while the charging by ambient plasma alone leads to a strong Coulomb repulsion between grains and inhibits their further coagulation, the combination with the photoemission provides optimum conditions for the growth of large dust aggregates in a certain region of the cloud, corresponding to the densities n H 2 ( ) between ∼10 4 and ∼10 6 cm −3 . The charging effect of CRs is of a generic nature, and is therefore expected to operate not only in dense molecular clouds but also in the upper layers and the outer parts of protoplanetary disks.

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“…Padovani et al (2013Padovani et al ( , 2014 studied the propagation of CR along magnetic fields lines in collapsing cores, accounting for the effect of CR energy loss and magnetic mirroring. They show how CR ionisation rate is efficiently reduced in the central region because of the complex magnetic field lines configurations found in numerical models of protostellar collapse, making magnetic resistivities larger towards the cloud center.…”

Section: Limitationsmentioning

confidence: 99%

Magnetic diffusivities in 3D radiative chemo-hydrodynamic simulations of protostellar collapse

Dzyurkevich

Commerçon

Lesaffre

et al. 2017

A&A

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Context. Both theory and observations of star-forming clouds require the simulations which combine the co-evolving chemistry, magneto-hydrodynamics and radiative transfer in protostellar collapse simulation. A detailed knowledge of self-consistent chemical evolution for the main charge carriers (both gas species and dust grains) allows to correctly estimate the rate and nature of magnetic dissipation in the collapsing core. Last is of crucial importance for answering the grand question of star and planet formation: the magnitude and spatial distribution of magnetic flux as the initial condition to protoplanetary disk evolution. Aims. We use a chemo-dynamical version of RAMSES, described in a companion publication, to follow the chemo-dynamical evolution of collapsing dense cores with the various dust properties and interpret the occuring differences in the magnetic diffusivity terms. Later are of crucial importance for the circumstellar disk formation. Methods. We perform 3D chemo-dynamical simulations of 1 M⊙ isolated dense core collapse for a range in the dust size assumptions. The number density of dust and it's mean size are affecting the efficiency of charge capturing and the formation of ices. The radiative hydrodynamics and dynamical evolution of chemical abundances are used to reconstruct the magnetic diffusivity terms for clouds with various magnetisation. Results. The simulations are performed for a mean dust size ranging from 0.017µm to 1µm, and we adopt both a fixed dust size and a dust size distribution. The chemical abundances for this range of dust sizes are produced by RAMSES and serve as an input to calculations of Ohmic, ambipolar and Hall diffusivity terms. Ohmic resistivity only play a role at the late stage of the collapse, in the innermost region of the cloud where gas density exceeds few times 10 13 cm −3 . Ambipolar diffusion is a dominant magnetic diffusivity term in cases where mean dust size is a typical ISM value or larger. We demonstrate that the assumption of a fixed 'dominant ion' mass can lead to one order of magnitude mismatch in the ambipolar diffusion magnitude. 'Negative' Hall effect is dominant during the collapse in case of small dust, i.e. for the mean dust size of 0.02 µm and smaller, the effect which we connect to the dominance of negatively charged grains. We find that the Hall effect reverses its sign for mean dust size of 0.1µm and smaller. The phenomenon of the sign reversal is strongly depending on the number of negatively charged dust relative to the ions, and quality of coupling of the last to the magnetic fields. We have adopted different strength of magnetic fields, β = Pgas/Pmag = 2, 5, 25. We observe that the variation on the field strength only shifts the Hall effect reversal along the radius of the collapsing cloud, but do not prevent it. Conclusions. The dust grain mean size appears to be the parameter with the strongest impact on the magnitude of the magnetic diffusivity, dividing the collapsing clouds in Hall-dominated and ambipolar-dominated cloud...

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Cosmic-ray ionisation in collapsing clouds

Cited by 104 publications

References 43 publications

Water in low-mass star-forming regions withHerschel

Water in low-mass star-forming regions withHerschel

Interstellar Dust Charging in Dense Molecular Clouds: Cosmic Ray Effects

Magnetic diffusivities in 3D radiative chemo-hydrodynamic simulations of protostellar collapse

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