Letter to the Editor

Excitation and abundance of C₃ in star forming cores ^*,**

Herschel/HIFI observations of the sight-lines to W31C and W49N

¹ Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India e-mail: bhaswati@tifr.res.in
² I. Physikalisches Institut, University of Cologne, Germany
³ Centro de Astrobiología, CSIC-INTA, 28850, Madrid, Spain
⁴ LERMA, CNRS, Observatoire de Paris and ENS, France
⁵ California Institute of Technology, Pasadena, CA 91125, USA
⁶ JPL, California Institute of Technology, Pasadena, USA
⁷ Onsala Space Observatory, Chalmers University of Technology, 43992 Onsala, Sweden
⁸ The Johns Hopkins University, Baltimore, MD 21218, USA
⁹ Institut d'Astrophysique Spatiale (IAS), Orsay, France
¹⁰ Université Toulouse; UPS; CESR; and CNRS; UMR5187, 9 avenue du colonel Roche, 31028 Toulouse Cedex 4, France
¹¹ Gemini telescope, Hilo, Hawaii, USA
¹² Laboratoire d'Astrophysique de Marseille (LAM), France
¹³ Depts. of Physics, Astronomy & Chemistry, Ohio State Univ., USA
¹⁴ Laboratoire d'Astrophysique de Grenoble, France
¹⁵ Nicolaus Copernicus University, Toruń, Poland
¹⁶ Institute of Physical Chemistry, PAS, Warsaw, Poland
¹⁷ MPI für Radioastronomie, Bonn, Germany
¹⁸ Dept. of Physics & Astronomy, University of Calgary, Canada
¹⁹ Nicolaus Copernicus Astronomical Center (CMAK), Toruń, Poland
²⁰ European Space Astronomy Centre, ESA, Madrid, Spain
²¹ SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands
²² Infrared Processing and Analysis Center, California Institute of Technology, MS 100-22, Pasadena, CA 91125, USA
²³ Sterrewacht Leiden, University of Leiden, Leiden, The Netherlands 


Received: 31 May 2010
Accepted: 2 July 2010

Abstract

We present spectrally resolved observations of triatomic carbon (C₃) in several ro-vibrational transitions between the vibrational ground state and the low-energy ν₂ bending mode at frequencies between 1654–1897 GHz along the sight-lines to the submillimeter continuum sources W31C and W49N, using Herschel's HIFI instrument. We detect C₃ in absorption arising from the warm envelope surrounding the hot core, as indicated by the velocity peak position and shape of the line profile. The sensitivity does not allow to detect C₃ absorption due to diffuse foreground clouds. From the column densities of the rotational levels in the vibrational ground state probed by the absorption we derive a rotation temperature (T_rot) of ~50-70 K, which is a good measure of the kinetic temperature of the absorbing gas, as radiative transitions within the vibrational ground state are forbidden. It is also in good agreement with the dust temperatures for W31C and W49N. Applying the partition function correction based on the derived T_rot, we get column densities N(C₃) ~ 7–9 × 10¹⁴ cm^-2 and abundance x(C₃) ~ 10^-8 with respect to H₂. For W31C, using a radiative transfer model including far-infrared pumping by the dust continuum and a temperature gradient within the source along the line of sight we find that a model with x(C₃) = 10^-8, T_kin = 30–50 K, N(C₃) = 1.5 × 10¹⁵ cm^-2 fits the observations reasonably well and provides parameters in very good agreement with the simple excitation analysis.