H₂O line mapping at high spatial and spectral resolution

Herschel observations of the VLA 1623 outflow^⋆

¹ Department of Earth and Space SciencesChalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
e-mail: per.bjerkeli@chalmers.se
² Department of Astronomy, Stockholm University, AlbaNova, 10691 Stockholm, Sweden
³ INAF – Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Italy
⁴ Observatorio Astronómico Nacional (IGN), Calle Alfonso XII,3. 28014 Madrid, Spain
⁵ INAF – Instituto di Astrofisica e Planetologia Spaziali, via Fosso del Cavaliere 100, 00133 Roma, Italy
⁶ Onsala Space Observatory, Chalmers University of Technology, 43992 Onsala, Sweden
⁷ LERMA, Observatoire de Paris, UMR 8112 of the CNRS, 61 Av. de l’Observatoire, 75014 Paris, France
⁸ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁹ Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
¹⁰ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹¹ Max Planck Institut für Extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching, Germany

Received: 8 June 2012
Accepted: 30 August 2012

Abstract

Context. Apart from being an important coolant, water is known to be a tracer of high-velocity molecular gas. Recent models predict relatively high abundances behind interstellar shockwaves. The dynamical and physical conditions of the water emitting gas, however, are not fully understood yet. Using the Herschel Space Observatory, it is now possible to observe water emission from supersonic molecular outflows at high spectral and spatial resolution. Several molecular outflows from young stars are currently being observed as part of the WISH (Water In Star-forming regions with Herschel) key program.

Aims. We aim to determine the abundance and distribution of water, its kinematics, and the physical conditions of the gas responsible for the water emission. The observed line profile shapes help us understand the dynamics in molecular outflows.

Methods. We mapped the VLA 1623 outflow, in the ground-state transitions of o-H₂O, with the HIFI and PACS instruments. We also present observations of higher energy transitions of o-H₂O and p-H₂O obtained with HIFI and PACS towards selected outflow positions. From comparison with non-LTE radiative transfer calculations, we estimate the physical parameters of the water emitting regions.

Results. The observed water emission line profiles vary over the mapped area. Spectral features and components, tracing gas in different excitation conditions, allow us to constrain the density and temperature of the gas. The water emission originates in a region where temperatures are comparable to that of the warm H₂ gas (T ≳ 200 K). Thus, the water emission traces a gas component significantly warmer than the gas responsible for the low-J CO emission. The water column densities at the CO peak positions are low, i.e. N(H₂O) ≃ (0.03−10) × 10¹⁴ cm^-2.

Conclusions. The water abundance with respect to H₂ in the extended outflow is estimated at X(H₂O) < 1  ×  10^-6, significantly lower than what would be expected from most recent shock models. The H₂O emission traces a gas component moving at relatively high velocity compared to the low-J CO emitting gas. However, other dynamical quantities such as the momentum rate, energy, and mechanical luminosity are estimated to be the same, independent of the molecular tracer used, CO or H₂O.