Chemical limit cycles for models of a region of low-mass star formation

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Abstract
Barnard 5 (B5) is a nearby, clumpy region in which low-mass star formation is occurring. We have calculated the chemical evolution for three variants of the dynamics of B5. In all of these models, interclump gas collapses to produce clumps which are dispersed by the stars which form; molecular gas cycles repeatedly between clump and interclump phases and during the dispersal interval may mix with ionized gas in the stellar winds. We find that the general chemical structure attains an approximate limit cycle in each of these models. During each dynamical cycle in each model, the molecular abundances vary substantially as functions of time; the time variations differ markedly between the models. The results of high resolution mapping of chemical abundances in B5, which because of its proximity is an ideal source for such studies, should reflect these substantial abundance variations and permit the determination of the dynamical cycling time-scales including those associated with the survival of clumps and the interclump medium. The data should also show whether mixing of molecular gas and ionized wind can occur in turbulent, magnetic boundary layers between clumps and wind. Such studies are important for the general question of the response of and feedback in a gravitationally bound, fragmenting medium in which internal energy sources are formed; it is far more feasible to address such a problem in the context of B5 than in that of galaxy cluster formation, say. Also, they are relevant for the general investigation of clump-high velocity flow interactions which occur in a very wide variety of astrophysical sources. A comparison of model abundances with measured abundances in regions other than B5 suggest tentatively that mixing of H+ and He+ with neutral gas, which is then shocked or which has been shocked shortly before the mixing occurs, affects the chemistry in dark clouds.

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