The magnetic precursor of L1448-mm: excitation differences between ion and neutral fluids

摘要

Context. Shock modelling predicts an electrondensity enhancement within the magnetic precursor of C-shocks. Previousobservations of SiO, H13CO+,HN13C and H13CN towardthe young L1448-mm outflow showed an over-excitation of the ion fluidthat was attributed to an electron density enhancement in theprecursor. Aims. We re-visit this interpretation and test if itstill holds when we consider different source morphologies and kinetictemperatures for the observed molecules. To do this, we use updatedcollisional coefficients of HN13C and SiO withelectrons in our excitation model. We also aim to give some insight onthe spatial extent of the electron density enhancement around L1448-mm.Methods. We estimate the opacities of H13CO+and HN13C by observing thelinesof rarer isotopologues. To model the excitation of the molecules, weuse the large velocity gradient (LVG) approximation with updatedcollisional coefficients to i) re-analyse the observations toward thepositions where the over-excitation of H13CO+has previously been observed [i.e. toward L1448-mm at offsets(0,0) and (0, -10)]; and ii) to investigate if the electrondensity enhancement is still required for the cases of extended andcompact emission, and for kinetic temperatures of up to 400K.We also report several lines of SiO, HN13C and H13CO+toward new positions around this outflow, to investigate the spatialextent of the over-excitation of the ions in L1448-mm. Results. From the isotopologue observations, we findthat the emission of H13CO+and HN13C from the precursor is optically thinif this emission is extended. Using the new collisional coefficients,an electron density enhancement is still needed to explain theexcitation of H13CO+ forextended emission and for gas temperatures of 400K toward L1448-mm (0, -10), andpossibly also toward L1448-mm (0,0). For compact emission thedata cannot be fitted. We do not find any evidence for theover-excitation of the ion fluid toward the newly observed positionsaround L1448-mm. Conclusions. The observed line emission of SiO, H13CO+and HN13C toward L1448-mm (0,0) and(0, -10) is consistent with an electron density enhancement in theprecursor component, if this emission is spatially extended. This isalso true for the case of high gas temperatures (400K)toward the (0, -10) offset. The electron density enhancement seems tobe restricted to the southern, redshifted lobe of the L1448-mm outflow.Interferometric images of the line emission of these molecules areneeded to confirm the spatial extent of the over-excitation of the ionsand thus, of the electron density enhancement in the magnetic precursorof L1448-mm. Key words: ISM: individual objects: L1448- ISM: clouds - ISM: jets and outflows - ISM: molecules - shock waves