Letter to the Editor

The origin of the [C II] emission in the S140 photon-dominated regions. New insights from HIFI ^*,**

¹ Institute for Astronomy, ETH Zürich, 8093 Zürich, Switzerland
² I. Physikalisches Institut der Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany e-mail: carolin.dedes@astro.phys.ethz.ch
³ Tata Institute of Fundamental Research (TIFR), Homi Bhabha Road, Mumbai 400005, India
⁴ SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands
⁵ Institut für 4D-Technologien, FHNW, 5210 Windisch, Switzerland
⁶ Instituto de Radio Astronomía Milimétrica (IRAM), Avenida Divina Pastora 7, Local 20, 18012 Granada, Spain
⁷ LERMA, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France
⁸ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121, Bonn, Germany
⁹ Leiden Observatory, Universiteit Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
¹⁰ Institut d'Astrophysique Spatiale, Université Paris-Sud, Bât. 121, 91405 Orsay Cedex, France
¹¹ Department of Astronomy and Astrophysics, University of Toronto, 60 St. George Street, Toronto, ON M5S 3H8, Canada
¹² Observatorio Astronómico Nacional (OAN), Apdo. 112, 28803 Alcalá de Henares (Madrid), Spain
¹³ Centro de Astrobiología (INTA-CSIC), Ctra de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
¹⁴ Astronomy Department, University of Maryland, College Park, MD 20742, USA
¹⁵ Université de Toulouse, UPS, CESR, 9 avenue du colonel Roche, 31062 Toulouse Cedex 4, France
¹⁶ CNRS, UMR 5187, 31028 Toulouse, France
¹⁷ Observatoire de Paris, LUTH and Université Denis Diderot, Place J. Janssen, 92190 Meudon, France
¹⁸ IPAC/Caltech, MS 100-22, Pasadena, CA 91125, USA
¹⁹ Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
²⁰ California Institute of Technology, 320-47, Pasadena, CA 91125-4700, USA
²¹ Laboratoire d'Études du Rayonnement et de la Matière en Astrophysique, UMR 8112 CNRS/INSU, OP, ENS, UPMC, UCP, Paris, France
²² Kapteyn Astronomical Institute, University of Groningen, PO box 800, 9700 AV Groningen, The Netherlands
²³ European Space Astronomy Centre, Urb. Villafranca del Castillo, PO Box 50727, 28080 Madrid, Spain
²⁴ Jet Propulsion Laboratory, 4800 Oak Grove Drive, MC 302-231, Pasadena, CA 91109, USA

Received: 31 May 2010
Accepted: 28 July 2010

Abstract

Using Herschel's HIFI instrument, we observe C ii along a cut through S140, as well as high-J transitions of CO and HCO⁺ at two positions on the cut, corresponding to the externally irradiated ionization front and the embedded massive star-forming core IRS1. The HIFI data were combined with available ground-based observations and modeled using the KOSMA-τ model for photon-dominated regions (PDRs). We derive the physical conditions in S140 and in particular the origin of C ii emission around IRS1. We identify three distinct regions of C ii emission from the cut, one close to the embedded source IRS1, one associated with the ionization front, and one further into the cloud. The line emission can be understood in terms of a clumpy model of PDRs. At the position of IRS1, we identify at least two distinct components contributing to the [C ii] emission, one of them a small, hot component, which can possibly be identified with the irradiated outflow walls. This is consistent with the C ii peak at IRS1 coinciding with shocked H₂ emission at the edges of the outflow cavity. We note that previously available observations of IRS1 can be reproduced well by a single-component KOSMA-τ model. Thus, it is HIFI's unprecedented spatial and spectral resolution, as well as its sensitivity that has allowed us to uncover an additional hot gas component in the S140 region.