Herschel/PACS observations of young sources in Taurus: the far-infrared counterpart of optical jets^⋆

¹ Institut de Planétologie et d’Astrophysique de Grenoble, 414 rue de la Piscine, 38400 St. Martin-d’Hères France
e-mail: linda.podio@obs.ujf-grenoble.fr
² Kapteyn Institute, Landleven 12, 9747 AD Groningen, The Netherlands
³ Physics Department, The University of Durham, Durham, DH1 3LE, UK
⁴ SOFIA-USRA, NASA Ames Research Center, USA
⁵ ESA-ESAC Gaia SOC, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
⁶ Department of Physics & Astronomy, 118 Kinard Laboratory, Clemson University, Clemson, SC 29634, USA
⁷ ALMA, Avda Apoquindo 3846, Piso 19, Edificio Alsacia, Las Condes, Santiago, Chile
⁸ Department of Physical Sciences, The Open University, Milton Keynes, MK7 6AA, UK
⁹ RALSpace, The Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, UK

Received: 25 April 2012
Accepted: 10 July 2012

Abstract

Context. Observations of the atomic and molecular line emission associated with jets and outflows emitted by young stellar objects provide sensitive diagnostics of the excitation conditions, and can be used to trace the various evolutionary stages they pass through as they evolve to become main sequence stars.

Aims. To understand the relevance of atomic and molecular cooling in shocks, and how accretion and ejection efficiency evolves with the evolutionary state of the sources, we will study the far-infrared counterparts of bright optical jets associated with Class I and II sources in Taurus (T Tau, DG Tau A, DG Tau B, FS Tau A+B, and RW Aur).

Methods. We have analysed Herschel/PACS observations of a number of atomic ([O i]63 μm, 145 μm, [C ii]158 μm) and molecular (high-J CO, H₂O, OH) lines, collected within the open time key project GASPS (PI: W. R. F. Dent). To constrain the origin of the detected lines we have compared the obtained FIR emission maps with the emission from optical-jets and millimetre-outflows, and the measured line fluxes and ratios with predictions from shock and disk models.

Results. All of the targets are associated with extended emission in the atomic lines; in particular, the strong [O i] 63 μm  emission is correlated with the direction of the optical jet/mm-outflow. The line ratios suggest that the atomic lines can be excited in fast dissociative J-shocks occurring along the jet. The molecular emission, on the contrary, originates from a compact region, that is spatially and spectrally unresolved, and lines from highly excited levels are detected (e.g., the o-H₂O 8₁₈–7₀₇ line, and the CO J = 36−35 line). Disk models are unable to explain the brightness of the observed lines (CO and H₂O line fluxes up to 10^-15−6 × 10^-16 W m^-2). Slow C- or J-shocks with high pre-shock densities reproduce the observed H₂O  and high-J CO lines; however, the disk and/or UV-heated outflow cavities may contribute to the observed emission.

Conclusions. Similarly to Class 0 sources, the FIR emission associated with Class I and II jet-sources is likely to be shock-excited. While the cooling is dominated by CO and H₂O lines in Class 0 sources, [O i]  becomes an important coolant as the source evolves and the environment is cleared. The cooling and mass loss rates estimated for Class II and I sources are one to four orders of magnitude lower than for Class 0 sources. This provides strong evidence to indicate that the outflow activity decreases as the source evolves.