Abstract
The chains of aligned knots that define the beams of HH jets are sometimes observed both in atomic/ionic as well as in molecular (H[2]) emission lines. We model such objects as jets with an ejection velocity time-variability, which produces internal working surfaces that travel down the jet beam. If this interpretation is indeed correct, then the presence of aligned H[2] knots is a sensitive indicator of the amplitude of the ejection velocity variability. We find that for variabilities with an amplitude of ~ 10-1, the internal working surfaces have appropriate conditions for H[2] to be formed (via negative and positive ion gas phase chemistry). We present axisymmetric gasdynamic+chemical numerical simulations which show that H[2] fractions as high as ~ 1-10% be obtained, and that the resulting H[2] 1-0 S(1) emission can be substantially larger than the Hα emission. From our study, we conclude that the H[2] emission observed in the chains of knots along some HH jets could correspond to molecules formed in situ within the internal working surfaces that travel down the jet flow.
