Observational evidence for dissociative shocks in the inner 100 AU of low-mass protostars using Herschel-HIFI^⋆

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: lkristensen@cfa.harvard.edu
² Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
³ Max Planck Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
⁴ Institute for Astronomy, ETH Zurich, 8093 Zurich, Switzerland
⁵ Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109-1042, USA
⁶ Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, 1350 Copenhagen K, Denmark
⁷ Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark

Received: 1 April 2013
Accepted: 24 June 2013

Abstract

Aims. Herschel-HIFI spectra of H₂O towards low-mass protostars show a distinct velocity component not seen in observations from the ground of CO or other species. The aim is to characterise this component in terms of excitation conditions and physical origin.

Methods. A velocity component with an offset of ~10 km s^-1 detected in spectra of the H₂O 1₁₀–1₀₁ 557 GHz transition towards six low-mass protostars in the “Water in star-forming regions with Herschel” (WISH) programme is also seen in higher-excited H₂O lines. The emission from this component is quantified and local excitation conditions are inferred using 1D slab models. Data are compared to observations of hydrides (high-J CO, OH⁺, CH⁺, C⁺, OH) where the same component is uniquely detected.

Results. The velocity component is detected in all six targeted H₂O transitions (E_up ~ 50–250 K), as well as in CO 16–15 towards one source, Ser SMM1. Inferred excitation conditions imply that the emission arises in dense (n ~ 5 × 10⁶–10⁸ cm^-3) and hot (T ~ 750 K) gas. The H₂O and CO column densities are ≳10¹⁶ and 10¹⁸ cm^-2, respectively, implying a low H₂O abundance of ~10^-2 with respect to CO. The high column densities of ions such as OH⁺ and CH⁺ (both ≳10¹³ cm^-2) indicate an origin close to the protostar where the UV field is strong enough that these species are abundant. The estimated radius of the emitting region is 100 AU. This component likely arises in dissociative shocks close to the protostar, an interpretation corroborated by a comparison with models of such shocks. Furthermore, one of the sources, IRAS 4A, shows temporal variability in the offset component over a period of two years which is expected from shocks in dense media. High-J CO gas detected with Herschel-PACS with T_rot ~ 700 K is identified as arising in the same component and traces the part of the shock where H₂ reforms. Thus, H₂O reveals new dynamical components, even on small spatial scales in low-mass protostars.