Far-infrared molecular lines from low- to high-mass star forming regions observed with Herschel^⋆

¹ Max-Planck Institut für Extraterrestrische Physik (MPE), Giessenbachstr. 1, 85748 Garching, Germany
e-mail: karska@mpe.mpg.de
² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
³ Université de Bordeaux, Observatoire Aquitain des Sciences de l’Univers, 2 rue de l’Observatoire, BP 89, 33271 Floirac Cedex, France
⁴ CNRS, LAB, UMR 5804, 33271 Floirac Cedex, France
⁵ Centro de Astrobiología. Departamento de Astrofísica. CSIC-INTA. Carretera de Ajalvir, Km 4, 28850 Torrejón de Ardoz., Madrid, Spain
⁶ Kavli Institut for Astronomy and Astrophysics, Yi He Yuan Lu 5, HaiDian Qu, Peking University, 100871 Beijing, PR China
⁷ SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands
⁸ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
⁹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 24 May 2013
Accepted: 25 November 2013

Abstract

Aims. Our aim is to study the response of the gas-to-energetic processes associated with high-mass star formation and compare it with previously published studies on low- and intermediate-mass young stellar objects (YSOs) using the same methods. The quantified far-IR line emission and absorption of CO, H₂O, OH, and [O i] reveals the excitation and the relative contribution of different atomic and molecular species to the gas cooling budget.

Methods. Herschel/PACS spectra covering 55–190 μm are analyzed for ten high-mass star forming regions of luminosities L_bol ~ 10⁴−10⁶  L_⊙ and various evolutionary stages on spatial scales of ~10⁴ AU. Radiative transfer models are used to determine the contribution of the quiescent envelope to the far-IR CO emission.

Results. The close environments of high-mass protostars show strong far-IR emission from molecules, atoms, and ions. Water is detected in all 10 objects even up to high excitation lines, often in absorption at the shorter wavelengths and in emission at the longer wavelengths. CO transitions from J = 14 − 13 up to typically 29 − 28 (E_u/k_B ~ 580−2400 K) show a single temperature component with a rotational temperature of T_rot ~ 300 K. Typical H₂O excitation temperatures are T_rot ~250 K, while OH has T_rot ~ 80 K. Far-IR line cooling is dominated by CO (~75%) and, to a smaller extent, by [O i] (~20%), which becomes more important for the most evolved sources. H₂O is less important as a coolant for high-mass sources because many lines are in absorption.

Conclusions. Emission from the quiescent envelope is responsible for ~45–85% of the total CO luminosity in high-mass sources compared with only ~10% for low-mass YSOs. The highest− J lines (J_up ≥ 20) originate most likely in shocks, based on the strong correlation of CO and H₂O with physical parameters (L_bol, M_env) of the sources from low- to high-mass YSOs. The excitation of warm CO described by T_rot ~ 300 K is very similar for all mass regimes, whereas H₂O temperatures are ~100 K high for high-mass sources compared with low-mass YSOs. The total far-IR cooling in lines correlates strongly with bolometric luminosity, consistent with previous studies restricted to low-mass YSOs. Molecular cooling (CO, H₂O, and OH) is ~4 times greater than cooling by oxygen atoms for all mass regimes. The total far-IR line luminosity is about 10^-3 and 10^-5 times lower than the dust luminosity for the low- and high-mass star forming regions, respectively.