Abstract
. The presence of collimated outflows associated with young stellar objects is a common phenomenon in star-fortning regions. These jets frequently show a chain of emission-line regions several of which have long been known as Herbig-Haro (H-H) objects. In this paper we examine the possibility that these knots are condensations produced by thermal instability in an expanding plasma , recombination and emission-line radiation losses. We show that the minimum value of = P /PNo under isobaric conditions for the growth of a thermal instability 0is = (6/5)1 [9/(STcN)e) - 3/2]; where P0 is the particle pressure, NQ tWe magnetic pressure, N) the expansion rate and Tc the radiative cooling time of the ambient plasma flow.Performing nonlinear calculations, we found that collimated flows of temperature T0 %% iOS K, mass-loss rates N = iO-6 - 10-8 Ne yr-1, and flow velocities VJ = 100-400 km/s, are favorable to the formation of condensations by thermal instability with density contrasts Pp/P0 1.3 - 2.0 created in time irr%ervals shorter than the estimated expansion time scales of the jets, where Pp(Po) is the density at the perturbed (ambient) region. Key : HERBIG-HARO OBJECTS - STARS-MASS LOSS
