OH far-infrared emission from low- and intermediate-mass protostars surveyed with Herschel-PACS^⋆,⋆⋆

¹ Institute for Astronomy, ETH Zurich, 8093 Zurich, Switzerland
² Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5–7, 1350 København K, Denmark
e-mail: wampfler@nbi.dk
³ Max Planck Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
⁴ Kavli Institute for Astronomy and Astrophysics at Peking University, 100871 Beijing, PR China
⁵ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁶ Centro de Astrobiología, CSIC-INTA, Carretera de Ajalvir, Km 4, Torrejón de Ardoz, 28850 Madrid, Spain
⁷ Department of Physics and Astronomy, Denison University, Granville, OH, 43023, USA
⁸ Department of Physics and Astronomy, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada
⁹ Université de Bordeaux, Laboratoire d’Astrophysique de Bordeaux, CNRS/INSU, UMR 5804, Floirac, France
¹⁰ INAF – Osservatorio Astronomico di Roma, 00040 Monte Porzio Catone, Italy
¹¹ Department of Astronomy, Stockholm University, AlbaNova, 106 91 Stockholm, Sweden

Received: 1 July 2012
Accepted: 18 December 2012

Abstract

Context. The OH radical is a key species in the water chemistry network of star-forming regions, because its presence is tightly related to the formation and destruction of water. Previous studies of the OH far-infrared emission from low- and intermediate-mass protostars suggest that the OH emission mainly originates from shocked gas and not from the quiescent protostellar envelopes.

Aims. We aim to study the excitation of OH in embedded low- and intermediate-mass protostars, determine the influence of source parameters on the strength of the emission, investigate the spatial extent of the OH emission, and further constrain its origin.

Methods. This paper presents OH observations from 23 low- and intermediate-mass young stellar objects obtained with the PACS integral field spectrometer on-board Herschel in the context of the “Water In Star-forming regions with Herschel” (WISH) key program. Radiative transfer codes are used to model the OH excitation.

Results. Most low-mass sources have compact OH emission (≲5000  AU scale), whereas the OH lines in most intermediate-mass sources are extended over the whole 47.″0 × 47.″0 PACS detector field-of-view (≳20 000  AU). The strength of the OH emission is correlated with various source properties such as the bolometric luminosity and the envelope mass, but also with the [OI] and H₂O emission. Rotational diagrams for sources with many OH lines show that the level populations of OH can be approximated by a Boltzmann distribution with an excitation temperature at around 70 K. Radiative transfer models of spherically symmetric envelopes cannot reproduce the OH emission fluxes nor their broad line widths, strongly suggesting an outflow origin. Slab excitation models indicate that the observed excitation temperature can either be reached if the OH molecules are exposed to a strong far-infrared continuum radiation field or if the gas temperature and density are sufficiently high. Using realistic source parameters and radiation fields, it is shown for the case of Ser SMM1 that radiative pumping plays an important role in transitions arising from upper level energies higher than 300 K. The compact emission in the low-mass sources and the required presence of a strong radiation field and/or a high density to excite the OH molecules points toward an origin in shocks in the inner envelope close to the protostar.