Disintegrating multiple systems in early stellar evolution

An analysis of the multiplicity of 14 sources driving giant Herbig-Haro flo ws has revealed an observed binary frequency between 79% and 86%, of which half are higher order multiples. These sources represent the hitherto young est sample of stars examined for binarity. I postulate that the dynamical d ecay of triple or multiple systems leads to strong outflow activity. It is well known that a large fraction of nonhierarchical triple systems rapidly break up and eject the lightest member. At the same time a closer binary in a highly eccentric orbit is formed. Massive disk truncation results, accom panied by large-scale accretion, with a consequent burst of outflow activit y, which produces the observed giant HH bow shocks. Some of the material cu lled from the individual circumstellar disks may settle into a circumbinary disk around the newly bound stellar pair. The small remaining and truncate d circumstellar disks are fed from the circumbinary disk through gas stream s, and this as well as other dynamical effects cause the binary orbit to sh rink. Gas streams together with disk interactions at periastron drive cycli c accretion modulated on an orbital timescale. As the stellar components gr adually spiral toward each other, the increasingly frequent mass-loss event s form chains of HH objects until eventually the binary has a semimajor axi s of only 9-12 AU, at which point the closely spaced shocked ejecta appear as a finely collimated jet. Thus, such HH flows can be read as a fossil rec ord of the evolution of orbital motions of a binary, newly formed in a trip le disintegration event, as it shrinks from a typical separation of 100 AU or more to 10 AU or less. When the triple system disintegrates and a single star is ejected, the newly formed binary recoils, and as a result both com ponents (star and close binary) leave their nascent envelope. While one com ponent becomes visible as a T Tauri star, the other will be obscured for a while by the envelope and will appear as a bright near-infrared object. For typical parameters, this geometry persists for only 5000 yr or so. If the ejected star does not escape, cyclic motion of a hierarchical triple begins . This explains the so-called IRC binaries that are infrequently found in s tar-forming regions. The standard model of early stellar evolution states t hat young stars gradually and smoothly make the transitions from Class 0 th rough Class I and II objects to eventually become Class III objects. In con trast, stars born in multiple systems can abruptly transit from a Class 0 o r I object to a visible T Tauri star. The main accretion phase may be termi nated by the stochastic process of triple decay. Depending on the moment of triple disintegration, the ejected objects can range from stellar embryos, which will emerge as very low mass stars or even brown dwarfs, to essentia lly fully built-up stars. In this picture, the initial mass function toward its low-mass end has an important stochastic component that can only be de scribed by the half-life of the decay processes. Because the ejected stars can take only limited circumstellar material with them, they will soon lose their classical T Tauri characteristics and join the halo of weak-line T T auri stars that surround star-forming clouds. Differences in ejection may e xplain why two apparently similar T Tauri stars of about the same age can h ave major differences in the size of their circumstellar disks.