The origin of forbidden line emission from young stellar objects

Abstract

We present a model for the origin of blueshifted, optical forbidden line emission and jets in young stellar objects based on generic properties of hydromagnetic disk winds. Magnetic stresses recollimate hydromagnetic disk winds to magnetic focal regions under very general conditions. We demonstrate that conditions in MHD shocks at these points account for the observed emission. We find that for fiducial accretion rates of 10(-7) M. yr-1 and magnetic fields at the inner edge of the disk (congruent-to 10(12) cm, gas accelerated from the innermost parts of a Keplerian accretion disk focuses into regions greater-than-or-equal-to 0.4 AU in radius that lie congruent-to 16 AU above and below the disk. The shocked gas density ranges from congruent-to 10(4) to 10(8) cm-3 where the latter occurs in the innermost part of the flow and shock. Shocked gas speeds range up to 250 km s-1 (going from the outer part of the shock at congruent-to 2 AU, to the innermost region) under these conditions. The magnetic field is moderately amplified in the shock and diverges from the flow axis in the postshock flow. It is this feature of MHD shock that produces an expanding cone of shocked gas. The opening angle of the postshock gas with respect to the flow axis is 40-degrees, and this accounts for the double-peaked character of the line profiles. Our model also predicts that the slower velocity component is associated with shocked gas of lower density than that associated with the higher velocity component. We show that the wind is largely neutral with an electron fraction of 10(-1). The wind remains largely neutral in adiabatic MHD shocks because much of the preshock kinetic energy goes into an increased postshock magnetic field. Substantial fractions of the flow energy can be liberated in these shocks.