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

摘要

Shock modelling predicts an electron density enhancement within the magneticprecursor of C-shocks. Previous observations of SiO, H13CO+, HN13C and H13CNtoward the young L1448-mm outflow showed an over-excitation of the ion fluidthat was attributed to an electron density enhancement in the precursor. Were-visit this interpretation and test if it still holds when we considerdifferent source morphologies and kinetic temperatures for the observedmolecules, and also give some insight on the spatial extent of the electrondensity enhancement around L1448-mm. We estimate the opacities of H13CO+ and HN13C by observing the J=3\to2 linesof rarer isotopologues to confirm that the emission is optically thin. To modelthe excitation of the molecules, we use the large velocity gradient (LVG)approximation with updated collisional coefficients to i) re- analyse theobservations toward the positions where the over-excitation of H13CO+ haspreviously been observed [i.e. toward L1448- mm at offsets (0,0) and (0,-10)],and ii) to investigate if the electron density enhancement is still requiredfor the cases of extended and compact emission, and for kinetic temperatures ofup to 400 K. We also report several lines of SiO, HN13C and H13CO+ toward newpositions around this outflow, to investigate the spatial extent of theover-excitation of the ions in L1448-mm. From the isotopologue observations, wefind that the emission of H13CO+ and HN13C from the precursor is optically thinif this emission is extended. Using the new collisional coefficients, anelectron density enhancement is still needed to explain the excitation ofH13CO+ for extended emission and for gas temperatures of\le 400 K towardL1448-mm (0,-10), and possibly also toward L1448-mm (0,0). For compact emissionthe data cannot be fitted. We do not find any evidence for the over-excitationof the ion fluid toward the newly observed positions around L1448-mm. 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 the precursorcomponent, if this emission is spatially extended. This is also true for thecase of high gas temperatures (\le400 K) toward the (0,-10) offset. Theelectron density enhancement seems to be restricted to the southern, redshiftedlobe of the L1448-mm outflow. Interferometric images of the line emission ofthese molecules are needed to confirm the spatial extent of the over-excitationof the ions and thus, of the electron density enhancement in the magneticprecursor of L1448-mm.