2012A&A...544A.150R

The detection of very narrow SiO emission and the higher degree of excitation of the ion fluid compared to the neutral one towards the protostar L1448-mm have been interpreted as signatures of the magnetic precursor of C-shocks. Since this is the only source where these features have been observed together, we carried out a survey to map the J=2->1 and 8->7 lines of SiO, and the J=1->0 and 4->3 lines of H¹³CO⁺ (as the representative ion) and HN¹³C (as the representative neutral species) towards four class 0/I low-mass protostars known to have young outflows (L1448-IRS3, NGC 1333-IRAS 4A/B and SSV 13, VLA 1623 and IRAS 20353). As previously reported, towards L1448-IRS3 and NGC 1333 strong, narrow SiO 2->1 is detected, and we also find a velocity component towards each outflow which shows high a H¹³CO⁺ 4->3/1->0 integrated intensity ratio compared to that of HN¹³C. Towards VLA 1623, we find very weak, narrow SiO, and a component that is visible in H¹³CO⁺ but not at all in HN¹³C. For IRAS 20353, we find neither narrow SiO nor high H¹³CO⁺ 4->3/1->0 flux ratios. Previous work explained the higher degree of excitation of H¹³CO⁺ by the selective excitation of ions by electrons, whose abundance is expected to increase at the precursor stage. Here, however, by considering a range of kinetic temperatures for the gas, we find that it is possible to fit the observations with a single temperature and density, without an increased electron abundance, despite the large differences in 4->3/1->0 ratio between the two molecules. The high H¹³CO⁺ 4->3/1->0 flux ratios may simply be due to protostellar heating. However, the hypothesis that narrow SiO is formed at the precursor stage of shocks remains plausible, since other explanations require either the presence of shocks with larger dynamical timescales, or predict the narrow SiO to have a larger spatial extent than it appears to have towards the sources L1448-IRS3 and VLA 1623. Larger scale maps of the narrow SiO emission are needed to confirm that it is confined to the regions around protostars (as would be expected in the precursor scenario), and mapping of different ion and neutral species is necessary to confirm whether there are indeed precursor regions surrounding young outflows where the ion and neutral species behave as separate fluids.