Herschel/HIFI spectral line survey of the Orion Bar

Temperature and density differentiation near the PDR surface

¹ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
² Department of Physics and Astronomy, University of Toledo, 2801 West Bancroft Street, Toledo, OH 43606, USA
e-mail: zsofia.nagy.astro@gmail.com
³ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
⁴ SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands
⁵ School of Space Research, Kyung Hee University, 1732, Deogyeong-daero, Giheung-gu, Yongin-si, 17104 Gyeonggi-do, Republic of Korea
⁶ University of Michigan, Ann Arbor, MI 48197, USA
⁷ LERMA, UMR 8112 du CNRS, Observatoire de Paris, École Normale Supérieure, 75014 Paris, France
⁸ Université de Toulouse, UPS-OMP, IRAP, 31400 Toulouse, France
⁹ CNRS, IRAP, 9 avenue Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France

Received: 13 May 2016
Accepted: 21 November 2016

Abstract

Context. Photon dominated regions (PDRs) are interfaces between the mainly ionized and mainly molecular material around young massive stars. Analysis of the physical and chemical structure of such regions traces the impact of far-ultraviolet radiation of young massive stars on their environment.

Aims. We present results on the physical and chemical structure of the prototypical high UV-illumination edge-on Orion Bar PDR from an unbiased spectral line survey with a wide spectral coverage which includes lines of many important gas coolants such as [Cii], [Ci], and CO and other key molecules such as H₂CO, H₂O, HCN, HCO⁺, and SO.

Methods. A spectral scan from 480–1250 GHz and 1410–1910 GHz at 1.1 MHz resolution was obtained by the HIFI instrument on board the Herschel Space Observatory. We obtained physical parameters for the observed molecules. For molecules with multiple transitions we used rotational diagrams to obtain excitation temperatures and column densities. For species with a single detected transition we used an optically thin LTE approximation. In the case of species with available collisional rates, we also performed a non-LTE analysis to obtain kinetic temperatures, H₂ volume densities, and column densities.

Results. About 120 lines corresponding to 29 molecules (including isotopologues) have been detected in the Herschel/HIFI line survey, including 11 transitions of CO, 7 transitions of ¹³CO, 6 transitions of C¹⁸O, 10 transitions of H₂CO, and 6 transitions of H₂O. The rotational temperatures are in the range between ~22 and ~146 K and the column densities are in the range between 1.8 × 10¹² cm^-2 and 4.5 × 10¹⁷ cm^-2. For species with at least three detected transitions and available collisional excitation rates we derived a best fit kinetic temperature and H₂ volume density. Most species trace kinetic temperatures in the range between 100 and 150 K and H₂ volume densities in the range between 10⁵ and 10⁶ cm^-3. The species with temperatures and/or densities outside this range include the H₂CO transitions tracing a very high temperature (315 K) and density (1.4 × 10⁶ cm^-3) component and SO corresponding to the lowest temperature (56 K) measured as a part of this line survey.

Conclusions. The observed lines/species reveal a range of physical conditions (gas density/temperature) involving structures at high density/high pressure, making the traditional clump/interclump picture of the Orion Bar obsolete.