Large Fluctuations within 1 AU in Protoplanetary Disks

Abstract

Protoplanetary disks are often assumed to change slowly and smoothly during planet formation. Here, we investigate the time evolution of isolated disks subject to viscosity and a disk wind. The viscosity is assumed to increase rapidly at around 900 K due to thermal ionization of alkali metals, or thermionic and ion emission from dust, and the onset of magneto-rotational instability (MRI). The disks generally undergo large, rapid fluctuations for a wide range of time-averaged mass accretion rates. Fluctuations involve coupled waves in temperature and surface density that move radially in either direction through the inner 1.5 AU of the disk. Two types of wave are seen with radial speeds of roughly 50 and 1000 cm/s respectively. The pattern of waves repeats with a period of roughly 10,000 years that depends weakly on the average mass accretion rate. Viscous transport due to MRI is confined to the inner disk. This region is resupplied by mass flux from the outer disk driven by the disk wind. Interior to 1 AU, the temperature and surface density can vary by a factor of 2--10 on timescales of years to ky. The stellar mass accretion rate varies by 3 orders of magnitude on a similar timescale. This behavior lasts for at least 1 My for initial disks comparable to the minimum-mass solar nebula.