Fossil magnetic field of accretion disks of young stars

Abstract

We elaborate the model of accretion disks of young stars with the fossil large-scale magnetic field in the frame of Shakura and Sunyaev approximation. Equations of the MHD model include Shakura and Sunyaev equations, induction equation and equations of ionization balance. Magnetic field is determined taking into account ohmic diffusion, magnetic ambipolar diffusion and buoyancy. Ionization fraction is calculated considering ionization by cosmic rays and X-rays, thermal ionization, radiative recombinations and recombinations on the dust grains.Analytical solution and numerical investigations show that the magnetic field is coupled to the gas in the case of radiative recombinations. Magnetic field is quasi-azimuthal close to accretion disk inner boundary and quasi-radial in the outer regions.Magnetic field is quasi-poloidal in the dusty "dead" zones with low ionization degree, where ohmic diffusion is efficient. Magnetic ambipolar diffusion reduces vertical magnetic field in 10 times comparing to the frozenin field in this region. Magnetic field is quasi-azimuthal close to the outer boundary of accretion disks for standard ionization rates and dust grain size a d = 0.1 µm. In the case of large dust grains (a d > 0.1 µm) or enhanced ionization rates, the magnetic field is quasiradial in the outer regions.It is shown that the inner boundary of dusty "dead" zone is placed at r = (0.1 − 0.6) AU for accretion disks of stars with M = (0.5 − 2) M ⊙ . Outer boundary of "dead" zone is placed at r = (3 − 21) AU and it is determined by magnetic ambipolar diffusion. Mass of solid material in the "dead" zone is more than 3 M ⊕ for stars with M ≥ 1 M ⊙ .

Magnetic fields in circumstellar disks

“…Our magnetic field strength distribution depends on the density and reproduces the same radial behavior as shown in Fig. 4 of the work from Dudorov & Khaibrakhmanov (2014) as follows:…”

“…The impact of magnetic fields on the formation and evolution of circumstellar disks is a matter of ongoing discussions (e.g., Turner et al 2014;Dudorov & Khaibrakhmanov 2014;Li et al 2016;Khaibrakhmanov et al 2017). For instance, magnetically induced viscosity such as that caused by magnetorotational instability (MRI) depends on the degree of ionization and the structure and strength of the magnetic field inside these disks (Balbus 2009;Dzyurkevich et al 2013;Dudorov & Khaibrakhmanov 2014;Khaibrakhmanov et al 2017).…”

“…For instance, magnetically induced viscosity such as that caused by magnetorotational instability (MRI) depends on the degree of ionization and the structure and strength of the magnetic field inside these disks (Balbus 2009;Dzyurkevich et al 2013;Dudorov & Khaibrakhmanov 2014;Khaibrakhmanov et al 2017).…”

A concordant scenario to explain FU Orionis from deep centimeter and millimeter interferometric observations

“…Dust sublimation on its own can also increase the ionization fraction because dust particles may have electrons stuck to their surface. A line-of-sight averaged ionization fraction of 10 −15 , which may be plausible (e.g., Dudorov & Khaibrakhmanov 2014), is already adequate for explaining the assumed EMs in our SED models. Our SED models are optically thick at 8-10 GHz, and therefore the fluxes observed in this frequency range is not sensitive to the exact ionization fraction, but are more sensitive to T e and Ω.…”

Magnetic field buoyancy in accretion disks of young stars

“…In paper [14] (DK14, hereafter), we found that extremely strong azimuthal magnetic field is generated in the inner regions of the accretion disks of T Tauri stars, where thermal ionization operates. Therefore, the problem is what process limits further generation of the azimuthal magnetic field.…”