Three-fluid plasmas in star formation

We investigate whether or not the low ionisation fractions in molecular cloud cores can solve the 'magnetic braking catastrophe', where magnetic fields prevent the formation of circumstellar discs around young stars. We perform three-dimensional smoothed particle non-ideal magnetohydrodynamics (MHD) simulations of the gravitational collapse of one solar mass molecular cloud cores, incorporating the effects of ambipolar diffusion, Ohmic resistivity and the Hall effect alongside a self-consistent calculation of the ionisation chemistry assuming 0.1µm grains. When including only ambipolar diffusion or Ohmic resistivity, discs do not form in the presence of strong magnetic fields, similar to the cases using ideal MHD. With the Hall effect included, disc formation depends on the direction of the magnetic field with respect to the rotation vector of the gas cloud. When the vectors are aligned, strong magnetic braking occurs and no disc is formed. When the vectors are anti-aligned, a disc with radius of 13 AU can form even in strong magnetic when all three non-ideal terms are present, and a disc of 38 AU can form when only the Hall effect is present; in both cases, a counter-rotating envelope forms around the first hydrostatic core. For weaker, anti-aligned fields, the Hall effect produces massive discs comparable to those produced in the absence of magnetic fields, suggesting that planet formation via gravitational instability may depend on the sign of the magnetic field in the precursor molecular cloud core.

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“…For grain-neutral collisions, the rate coefficient is given by (Wardle & Ng 1999;Pinto & Galli 2008) …”

Section: Conductivitiesmentioning

confidence: 99%

Can non-ideal magnetohydrodynamics solve the magnetic braking catastrophe?

Wurster

Price

Bate

2016

Mon. Not. R. Astron. Soc.

153

235

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“…where n(a) is the number density of species a, μ ab is the reduced mass of particles a and b, and σv ab is their elastic collision rate coefficient; the latter is evaluated via the recent analytical fits to quantum-mechanical calculations provided in Table 2 of Pinto & Galli (2008a), when available, given as a function of the effective relative speedv:…”

Section: Ambipolar Diffusion Coefficientsmentioning

confidence: 99%

“…with the σv ea also evaluated from the recent analytical fits of Pinto & Galli (2008a). The ion-electron momentum transfer rate entering ohmic heating is calculated with the classical formula of Schunk (1975;see Eq.…”

Section: Ambipolar Diffusion Coefficientsmentioning

confidence: 99%

Molecule survival in magnetized protostellar disk winds

Panoglou¹,

Cabrit²,

Forêts³

et al. 2012

A&A

104

144

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Context. Molecular counterparts to atomic jets have recently been detected within 1000 AU of young stars at early evolutionary stages. Reproducing these counterparts is an important new challenge for proposed ejection models. Aims. We explore whether molecules may survive in the magneto-hydrodynamic (MHD) disk wind solution currently invoked to reproduce the kinematics and tentative rotation signatures of atomic jets in T Tauri stars. Methods. The coupled ionization, chemical, and thermal evolution along dusty flow streamlines is computed for the prescribed MHD disk wind solution, using a method developed for magnetized shocks in the interstellar medium. Irradiation by (wind-attenuated) coronal X-rays and far-ultraviolet photons from accretion hot spots is included, with an approximate self-shielding of H 2 and CO. Disk accretion rates of 5 × 10 −6 , 10 −6 and 10 −7 M yr −1 are considered, representative of low-mass young protostars (so-called "Class 0"), evolved protostars ("Class I") and very active T Tauri stars ("Class II") respectively. Results. The disk wind has an "onion-like" thermo-chemical structure, with streamlines launched from larger radii having lower temperature and ionization, and higher H 2 abundance. The coupling between charged and neutral fluids is sufficient to eject molecules from the disk out to at least 9 AU. The launch radius beyond which most H 2 survives moves outward with evolutionary stage, from 0.2 AU (sublimation radius) in the Class 0 disk wind, to 1 AU in the Class I, and >1 AU in the Class II. In this molecular wind region, CO survives in the Class 0 but is significantly photodissociated in the Class I/II. Balance between ambipolar heating and molecular cooling establishes a moderate asymptotic temperature 700−3000 K, with cooler jets at earlier protostellar stages. As a result, endothermic formation of H 2 O is efficient, with abundances up to 10 −4 , while CH + and SH + can reach ≥10 −6 in the hotter and more ionised Class I/II winds. Conclusions. A centrifugal MHD disk wind launched from beyond 0.2−1 AU can produce molecular jets/winds up to speeds 100 km s −1 in young low-mass stars ranging from Class 0 to active Class II. The model predicts a high abundance ratio of H 2 to CO and an increase of molecular launch radius, temperature, and flow width as the source evolves, in promising agreement with current observed trends. Calculations of synthetic maps and line profiles in H 2 , CO and H 2 O will allow detailed tests of the model against observations.

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“…In our energy range the cross-section for charged-neutral collisions tends to that given by the polarization approximation ("the Langevin cross section", see Pinto & Galli 2008 for comparison with full quantum mechanical calculations):…”

Section: Charged Componentsmentioning

confidence: 95%

Helium diffusion during formation of the first galaxies

Медведев

Sazonov

Gilfanov

2016

Mon. Not. R. Astron. Soc.

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We investigate the possible impact of diffusion on the abundance of helium and other primordial elements during formation of the first structures in the early Universe. We consider the primary collapse of a perturbation and subsequent accretion of matter onto the virialized halo, restricting our consideration to halos with masses considerably above the Jeans limit. We find that diffusion in the cold and nearly neutral primordial gas at the end of the Dark Ages could raise the abundance of primordial elements relative to hydrogen in the first virialized halos: helium enrichment could reach δY p /Y p ∼ 10 −4 in the first star-forming minihalos of ∼ 10 5 -10 6 M . A moderate (to ∼ 100 K) preheating of the primordial gas at the beginning of cosmic reionization could increase this effect to δY p /Y p ∼ 3×10 −4 for ∼ 10 6 M halos. Even stronger abundance enhancements, δY p /Y p ∼ a few 10 −3 , may arise at much later, post-reionization epochs, z ∼ 2, in protogroups of galaxies (∼ 10 13 M ) as a result of accretion of warmhot intergalactic medium with T ∼ 10 6 K. The diffusion-induced abundance changes discussed here are small but comparable to the already achieved ∼ 0.1% precision of cosmological predictions of the primordial He abundance. If direct helium abundance measurements (in particular, in low-metallicity HII regions in dwarf galaxies) achieve the same level of precision in the future, their comparison with the BBN predictions may require consideration of the effects discussed here.

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Three-fluid plasmas in star formation

Cited by 76 publications

References 71 publications

Can non-ideal magnetohydrodynamics solve the magnetic braking catastrophe?

Can non-ideal magnetohydrodynamics solve the magnetic braking catastrophe?

Molecule survival in magnetized protostellar disk winds

Helium diffusion during formation of the first galaxies

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