4-8 October 2015
Hans Harnack Haus
Europe/Berlin timezone
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Contribution Poster

Poster session

Speakers

  • Mr. Bo ZHAO

Primary authors

Co-authors

Content

Dense, star-forming cores of molecular clouds are observed to be strongly magnetized. A realistic magnetic field of moderate strength has shown to suppress the formation of a rotationally supported disk (RSD) through magnetic braking, if the neutral gas are well-coupled to the magnetic field by Cosmic-Ray (CR) ionization. Here, we present the conditions for formation of RSDs through non-ideal MHD simulations including an equilibrium chemical network computed self-consistently at each point in space. We find that with large grain sizes ($>0.1 \mu m$), large RSDs ($\sim50-100$~AU in size) can form through efficient ambipolar diffusion (AD) decoupling of magnetic fields over the circumstellar region. The reason is that large grains ($>0.1 \mu m$) formed by compression and coagulation of small grains remove most of the grains that have Hall parameters of order unity (grain size of $\sim0.01\mu m$), which tends to reduce the magnitude of Hall conductivity ($|\sigma_H|$) and increase the ambipolar diffusion coefficient. Such enhancement of ambipolar diffusion takes place from $\sim10^2$ to $10^3$~AU scale, which helps the infalling gas flow to retain a large amount of angular momentum before they reach the inner $10^2$~AU scale. We also find that the Ohmic dissipation, which helps to sustain the RSD disk, only becomes important inside the disk where number densities are above $10^{11}$~cm$^{-3}$. Therefore, with large grain size, the problem of magnetic braking catastrophe in protostellar disk formation can be avoided through efficient ambipolar diffusion at large scale and Ohmic dissipation at disk scale. Our result also suggests that grain coagulation should be efficient at $\sim10^3$~AU scale around the protostar, where most small grains with size $<0.1$~$\mu m$ should disappear.