Launching jets from the boundary layer of accretion disks in young stellar objects

Abstract

We reexamine a previously proposed model for thermal pressure acceleration of collimated outflows in young stellar objects (YSO). We are motivated by new results from recent X-ray observations of YSO. These show that there is essentially no difference between the properties of X-ray emission from YSO with and without outflows, imposing quite severe constraints on models based on magnetic launching of jets. In our scenario the magnetic fields are weak, and serve only to recollimate the outflow at large distances from the source. We perform time scale estimates and an analytical calculation of the acceleration of gas originating in the boundary layer (BL) of an accretion disk. By applying global energy conservation considerations we find that the mass escaping the system is compatible with observations. A crucial ingredient of the proposed model is that the accreted material is strongly shocked, and then cools down on a time scale longer than its ejection time from the disk. By using appropriate properties of accretion disks boundary layers around YSO, we show that the conditions for the proposed scenario to work are reasonably met. We find also that the thermal acceleration mechanism works only when the accretion rate in YSO accretion disk is large enough and the `alpha' parameter of the disk small enough - otherwise the cooling time is too short and significant ejection does not take place. This result appears to be compatible with observations as well as theoretical considerations.