Strong Toroidal Recirculation Zones at the Inner Dust Rim as the Origin of Meteoric Chondrules and Calcium-Aluminum Inclusions

Abstract

We hypothesize strong (transonic) twin toroidal recirculation zones above and below the accretion disk midplane, rather close-in to the protosun, to be the source of chondrules and calcium-aluminum inclusions (CAIs). The recirculation zones act as centrifugal separators. In the case of chondrules, we suggest this happens during Class II (T~Tauri) stage of protostellar accretion, and in the case of CAIs, during an earlier higher-$\dot M$ phase of accretion. The recirculation zones advect and raise dust and solid aggregates above the midplane, making a ``mushroom-cap,'' and they also generate weak standing oblique shocks that heat and fuse protochondrules. We do not model the emission of these shocks, but point out that they will produce Doppler-broadened, possibly twin-peaked line emission with width of the order of $\simeq 200\ {\rm km\ s^{-1}}$. For concreteness, we focus on chondrules in the paper. Small ($\lessapprox 10 {\rm\ \mu m}$) diameter protochondrules are evaporated by the standing shocks, whereas large ($\gtrapprox 1\ {\rm cm}$) protochondrules are too heavy to be entrained and accelerated by the outer recirculation zone and outflow. Intermediate-size protochondrules, however, are centrifugally ejected and carried by high-speed diffuse gas outflow to the outer regions of the disk, where they rain down. The recirculation-induced dust mushroom caps will create significant IR continuum emission. We suggest they coincide with observed inner ``puffed-up'' dust rims. We also suggest that the recent interferometric inferences of arcs or ellipses in the sub-AU continuum IR of low-mass Class II protostars may be observations of corresponding chondrule-producing recirculation zones in those systems.