[P-PS21_29PM1] Planetary Sciences

[PPS21-14] Dead zones by electric heating in protoplanetary disks

*Shoji MORI¹, Satoshi OKUZUMI¹ (1.Tokyo Institute of Technology)

Keywords:protoplanetary disk, ionization degree, dust grains, MHD turbulence, electric heating

Turbulence driven by magnetorotational instability (MRI) is a viablemechanism of angular momentum transport in accretion disks. Inprotoplanetary disks, however, there is a region where the ionization degree istoo low for MRI to be active (e.g., Gammie 1996; Sano et al. 2000).Whether turbulence is present or not strongly affects the growth ofdust particles to planetesimals. Therefore, a good knowledge of thesize of dead zones is essential to understanding planet formation.In this study, we focus on the heating of electrons by turbulentelectric fields and its effect on the ionization state ofprotoplanetary disks. Previous studies have assumed that electrons inthe disks have the same temperature as the neutral gas. However, thisis not necessarily the case in MRI-driven turbulence, in whichturbulent electric fields can significantly heat up electrons(Inutsuka & Sano 2005). Heated electrons efficiently adsorb onto dustgrains, and therefore electron heating leads to a reduction of theionization degree (Okuzumi & Inutsuka, in prep.). This couldeffectively increase the dead zone size by reducing the saturationlevel of MRI turbulence outside the conventional dead zone.The aim of this study is to show where in protoplanetary disks theeffect mentioned above becomes important. We calculate the ionizationdegree of disks assuming that MRI operates outside the dead zone. Fora minimum-mass solar nebula with the dust grain radius of 0.1um anddust-to-gas mass ratio of 0.01, we find that the effect becomessignificant in a region extending from the outer edge of the dead zone(at ~20 AU from the central star) out to 70 AU. Furthermore, ouranalytic estimate suggests that the saturation level of turbulence inthis region is significantly low.