Introduction Stars form from dense pockets of gas that have undergone gravitational collapse, and young stars are still embedded in this molecular material. There is no lack of evidence that the outflows from these stars, manifested in the form of Herbig-Haro objects and jets, interact dynamically with the dense gas either in the form of shock excitation or the imparting of momentum; molecular outflows, H2 emission, and perhaps also CO and SiO bullets and jets, are tracers of this phenomenon.

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... wever there are a set of radio observations of molecules showing emission near HH objects that are remarkable for their apparent lack of signatures of such dynamic interaction. A likely cause of this 'quiescent' emission is a radiative interaction between the HH shock system emission and ambient gas. The 'cores' from which stars form are localised (~ O.lpc) density enhancements within the molecular clouds traced by the CO molecule, and there is a great deal of interest in their chemistry since many molecules, some rather complex, have been observed in them (Williams 1994). These molecules are able to form because they are shielded from the ambient interstellar radiation field, but a number of uncertainties remain, amongst them the effect of accretion of gas molecules onto the surfaces of dust grains to form ice mantles, and the possibility of a subsequent surface chemistry. These ices are observed in infrared absorption in a number of molecules (H 2 0, CO, CH3OH, CH 4 , C0 2 ; Whittet et al. 1996) when suitable background sources are present, but must exist to some extent in all gas exposed to a greatly attenuated UV flux. In this contribution I will describe a model in which HH jets propagate into a clumpy medium, and each of these clumps contains dust that can accrete ice mantles. If a clump finds itself in the vicinity of the jet it may be 213 B. Reipurth and C. Bertout (eds.), Herbig-Haro Flows and the Birth of Low Mass Stars, 213-222. © 1997 IAU. Printed in the Netherlands. available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0074180900061660 Downloaded from https://www.cambridge.org/core. IP address: 207.241.231.83, on 27 Jul 2018 at 03:26:42, subject to the Cambridge Core terms of use, 214 S. D. TAYLOR exposed to radiation from one of its working surfaces, and if this can remove the mantles the ensuing gas phase chemistry can produce large abundance enhancements in many molecules, perhaps rendering them observable. I will then discuss the nature of these clumps, and how the illumination of the clumps by the jet 'searchlight' can help us to understand the dustrgas interaction in dense regions. Observations of Quiescent Emission Cores are detected by means of molecular transitions that trace gas denser than that of the ambient cloud, (n# > 10 4 cm -3 against n# ~ 10 3 cm -3 , η H is the number of hydrogen nuclei per unit volume). In order to study dense gas around HH objects it is also necessary to have high angular resolution to determine the position relative to the shocks, and so such observations have tended to be made using interferometers, although submm single dish observations can also achieve high resolution. For these reasons the HCO + (J=l-0) and NH3 (1,1) and (2,2) inversion transitions have mostly been used. Rudolph and Welch (1988, 1992) mapped HH7-11 and HH34 in HCO+, and Rudolph (1992) also looked at the jet in L1551 in the same molecule, all using the Hat Creek interferometer. HH1 and HH2 were imaged by Torrelles et al. (1992,1993) in ammonia using the VLA, as was HH80 (North) by Girart et al. (1994) . In addition Davis, Dent and Bell Burnell (1990) observed HCO+ (J=4-3) near HH1 and HH2 using JCMT and Davis & Dent (1993) found NH 3 in HH34 with the Effelsberg telescope. All of these observations share the following characteristics; (i) The emission is confined to patches of emission close to, but downwind of, the optical shock emission. These localised areas of emission, which we term clumps to distinguish from the core emission that surrounds the source of the outflow, have sizes of ~ 10" -20", which is typically ~ 0.02pc. (ii) The clumps appear dynamically unaffected, in the sense that the lines have widths of < 1 km s -1 , and centres within 1-2 km s _1 of the core emission surrounding the outflow source. At this conference, Dent (1997) has presented further observations around HH2 using JCMT, and detected HCO+ (J=3-2 and J=4-3), but also found weak emission from the new molecules H2CO (5ΐ5-4χ4), N2H+ (J=4-3), and HCS + (J=8-7), together with the non-detection of transitions from several other molecules. Once again the HCO+ shares the above characteristics.