EDP Sciences
Free access
Volume 406, Number 2, August I 2003
Page(s) 603 - 611
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20030809

A&A 406, 603-611 (2003)
DOI: 10.1051/0004-6361:20030809

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

N. Soker1, 2 and O. Regev2, 3

1  Department of Physics, Oranim, 36006 Tivon, Israel
2  Department of Physics, Technion-Israel Institute of Technology, 32000 Haifa, Israel
3  Department of Astrophysics, American Museum of Natural History, New York, NY 10024, USA

(Received 28 April 2003 / Accepted 20 May 2003)

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.

Key words: accretion, accretion disks -- ISM: jets and outflows -- stars: pre-main sequence

Offprint request: N. Soker, soker@physics.technion.ac.il

© ESO 2003