High-pressure, low-abundance water in bipolar outflows

Results from a Herschel-WISH survey^⋆

¹ Observatorio Astronómico Nacional (IGN), Alfonso XII 3, 28014 Madrid, Spain
e-mail: m.tafalla@oan.es
² Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
³ INAF − Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Italy
⁴ Instituto de Radioastronomía Milimétrica (IRAM), Avenida Divina Pastora 7, Núcleo Central, 18012 Granada, Spain
⁵ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁶ Max-Planck Institut für Extraterrestrische Physik (MPE), Giessenbachstr. 1, 85748 Garching, Germany
⁷ Kavli Institute for Astronomy and Astrophysics, Peking University, Yi He Yuan Lu 5, Hai Dian Qu, 100871 Beijing, PR China

Received: 20 September 2012
Accepted: 15 January 2013

Abstract

Context. Water is a potential tracer of outflow activity because it is heavily depleted in cold ambient gas and is copiously produced in shocks.

Aims. We present a survey of the water emission in a sample of more than 20 outflows from low-mass young stellar objects with the goal of characterizing the physical and chemical conditions of the emitting gas.

Methods. We used the HIFI and PACS instruments on board the Herschel Space Observatory to observe the two fundamental lines of ortho-water at 557 and 1670 GHz. These observations were part of the “Water In Star-forming regions with Herschel” (WISH) key program, and have been complemented with CO and H₂ data.

Results. The emission of water has a different spatial and velocity distribution from that of the J = 1−0 and 2−1 transitions of CO. On the other hand, it has a similar spatial distribution to H₂, and its intensity follows the H₂ intensity derived from IRAC images. This suggests that water traces the outflow gas at hundreds of kelvins that is responsible for the H₂ emission, and not the component at tens of kelvins typical of low-J CO emission. A warm origin of the water emission is confirmed by a remarkable correlation between the intensities of the 557 and 1670 GHz lines, which also indicates that the emitting gas has a narrow range of excitations. A radiative transfer analysis shows that while there is some ambiguity in the exact combination of density and temperature values, the gas thermal pressure nT is constrained within less than a factor of 2. The typical nT over the sample is 4 × 10⁹ cm^-3K, which represents an increase of 10⁴ with respect to the ambient value. The data also constrain the water column density within a factor of 2 and indicate values in the sample between 2 × 10¹² and 10¹⁴ cm^-2. When these values are combined with estimates of the H₂ column density, the typical water abundance is only 3 × 10^-7, with an uncertainty of a factor of 3.

Conclusions. Our data challenge current C-shock models of water production through the combination of wing-line profiles, high gas compressions, and low abundances.