The Herschel HIFI water line survey in the low-mass proto-stellar outflow L1448^⋆

¹ Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monteporzio Catone Italy
e-mail: gina.santangelo@oa-roma.inaf.it
² Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Florence, Italy
³ Observatorio Astronómico Nacional (IGN), Calle Alfonso XII 3, 28014 Madrid, Spain
⁴ Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
⁵ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁶ Max Planck Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany

Received: 16 September 2011
Accepted: 24 November 2011

Abstract

Aims. As part of the WISH (Water In Star-forming regions with Herschel) key project, systematic observations of H₂O transitions in young outflows are being carried out with the aim of understanding the role of water in shock chemistry and its physical and dynamical properties. We report on the observations of several ortho- and para-H₂O lines performed with the HIFI instrument toward two bright shock spots (R4 and B2) along the outflow driven by the L1448 low-mass proto-stellar system, located in the Perseus cloud. These data are used to identify the physical conditions giving rise to the H₂O emission and to infer any dependence on velocity.

Methods. We used a large velocity gradient (LVG) analysis to derive the main physical parameters of the emitting regions, namely n(H₂), T_kin, N(H₂O) and emitting-region size. We compared these with other main shock tracers, such as CO, SiO and H₂ and with shock models available in the literature.

Results. These observations provide evidence that the observed water lines probe a warm (T_kin ~ 400−600 K) and very dense (n ~ 10⁶−10⁷ cm^-3) gas that is not traced by other molecules, such as low-J CO and SiO, but is traced by mid-IR H₂ emission. In particular, H₂O shows strong differences with SiO in the excitation conditions and in the line profiles in the two observed shocked positions, pointing to chemical variations across the various velocity regimes and chemical evolution in the different shock spots. Physical and kinematical differences can be seen at the two shocked positions. At the R4 position, two velocity components with different excitation can be distinguished, of which the component at higher velocity (R4-HV) is less extended and less dense than the low velocity component (R4-LV). H₂O column densities of about 2 × 10¹³ and 4 × 10¹⁴ cm^-2 were derived for the R4-LV and the R4-HV components, respectively. The conditions inferred for the B2 position are similar to those of the R4-HV component, with H₂O column density in the range 10¹⁴−5 × 10¹⁴ cm^-2, corresponding to H₂O/H₂ abundances in the range 0.5−1 × 10^-5. The observed line ratios and the derived physical conditions seem to be more consistent with excitation in a low-velocity J-type shock with strong compression rather than in a stationary C-shock, although none of these stationary models seems able to reproduce the whole characteristics of the observed emission.