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

¹ Centro de Astrobiología (CSIC-INTA), Ctra de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain e-mail: robertsj@inta.es
² Department of Physics & Astronomy, University College London, Gower Street, London WC1E 6BT, UK
³ School of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT, UK
⁴ Harvard-Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, MA 02138, USA
⁵ Escuela Universitaria de Óptica, Departamento de Matemática Aplicada (Biomatemática), Universidad Complutense de Madrid, Avda. Arcos de Jalón s/n, 28037 Madrid, Spain
⁶ Laboratoire d'Astrophysique de Grenoble, UMR 5571-CNRS, Université Joseph Fourier, Grenoble, France

Received: 7 August 2009
Accepted: 1 February 2010

Abstract

Context. Shock modelling predicts an electron density enhancement within the magnetic precursor of C-shocks. Previous observations of SiO, H¹³CO⁺, HN¹³C and H¹³CN toward the young L1448-mm outflow showed an over-excitation of the ion fluid that was attributed to an electron density enhancement in the precursor.

Aims. We re-visit this interpretation and test if it still holds when we consider different source morphologies and kinetic temperatures for the observed molecules. To do this, we use updated collisional coefficients of HN¹³C and SiO with electrons in our excitation model. We also aim to give some insight on the spatial extent of the electron density enhancement around L1448-mm.

Methods. We estimate the opacities of H¹³CO⁺ and HN¹³C by observing the J = 3 2 lines of rarer isotopologues. To model the excitation of the molecules, we use the large velocity gradient (LVG) approximation with updated collisional coefficients to i) re-analyse the observations toward the positions 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 electron density enhancement is still required for the cases of extended and compact emission, and for kinetic temperatures of up to 400 K. We also report several lines of SiO, HN¹³C and H¹³CO⁺ toward new positions around this outflow, to investigate the spatial extent of the over-excitation of the ions in L1448-mm.

Results. From the isotopologue observations, we find that the emission of H¹³CO⁺ and HN¹³C from the precursor is optically thin if this emission is extended. Using the new collisional coefficients, an electron density enhancement is still needed to explain the excitation of H¹³CO⁺ for extended emission and for gas temperatures of 400 K toward L1448-mm (0, –10), and possibly also toward L1448-mm (0, 0). For compact emission the data cannot be fitted. We do not find any evidence for the over-excitation of the ion fluid toward the newly observed positions around 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 the precursor component, if this emission is spatially extended. This is also true for the case of high gas temperatures (400 K) toward the (0, –10) offset. The electron density enhancement seems to be restricted to the southern, redshifted lobe of the L1448-mm outflow. Interferometric images of the line emission of these molecules are needed to confirm the spatial extent of the over-excitation of the ions and thus, of the electron density enhancement in the magnetic precursor of L1448-mm.