Resolving the shocked gas in HH 54 with Herschel^⋆

CO line mapping at high spatial and spectral resolution

¹ Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
e-mail: per.bjerkeli@nbi.dk
² Centre for Star and Planet Formation and Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5–7, 1350 Copenhagen K, Denmark
³ Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
⁴ Department of Astronomy, Stockholm University, AlbaNova, 106 91 Stockholm, Sweden
⁵ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁶ INAF – Osservatorio Astronomico di Roma, Via di Frascati 33, 00040 Monte Porzio Catone, Italy
⁷ LERMA, Observatoire de Paris, UMR CNRS 8112, 61 Av. de l’Observatoire, 75014 Paris, France
⁸ Kavli Institute for Astronomy and Astrophysics, Peking University, 100871 Beijing, PR China
⁹ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
¹⁰ Observatorio Astronómico Nacional (IGN), Alfonso XII 3, 28014 Madrid, Spain

Received: 11 August 2014
Accepted: 22 September 2014

Abstract

Context. The HH 54 shock is a Herbig-Haro object, located in the nearby Chamaeleon II cloud. Observed CO line profiles are due to a complex distribution in density, temperature, velocity, and geometry.

Aims. Resolving the HH 54 shock wave in the far-infrared (FIR) cooling lines of CO constrain the kinematics, morphology, and physical conditions of the shocked region.

Methods. We used the PACS and SPIRE instruments on board the Herschel space observatory to map the full FIR spectrum in a region covering the HH 54 shock wave. Complementary Herschel-HIFI, APEX, and Spitzer data are used in the analysis as well. The observed features in the line profiles are reproduced using a 3D radiative transfer model of a bow-shock, constructed with the Line Modeling Engine code (LIME).

Results. The FIR emission is confined to the HH 54 region and a coherent displacement of the location of the emission maximum of CO with increasing J is observed. The peak positions of the high-J CO lines are shifted upstream from the lower J CO lines and coincide with the position of the spectral feature identified previously in CO (10−9) profiles with HIFI. This indicates a hotter molecular component in the upstream gas with distinct dynamics. The coherent displacement with increasing J for CO is consistent with a scenario where IRAS12500 – 7658 is the exciting source of the flow, and the 180 K bow-shock is accompanied by a hot (800 K) molecular component located upstream from the apex of the shock and blueshifted by −7 km s^-1. The spatial proximity of this knot to the peaks of the atomic fine-structure emission lines observed with Spitzer and PACS ([O i]63, 145 μm) suggests that it may be associated with the dissociative shock as the jet impacts slower moving gas in the HH 54 bow-shock.