Collisional excitation of sulfur dioxide in cold molecular clouds^*

¹ CAB. INTA-CSIC. Department of Astrophysics. Crta Torrejón km 4. Torrejón de Ardoz, Madrid, Spain
e-mail: jcernicharo@cab.inta-csic.es
² LERMA and UMR 8112 of CNRS, Observatoire de Paris-Meudon, 92195 Meudon Cedex, France
e-mail: annie.spielfiedel@obspm.fr
³ Laboratoire d’Astrophysique de Grenoble, UMR 5571-CNRS, Université Joseph Fourier, Grenoble, France
e-mail: alexandre.faure@obs.ujf-grenoble.fr

Received: 26 November 2010
Accepted: 5 April 2011

Abstract

We present collisional rate coefficients for SO₂ with ortho and para molecular hydrogen for the physical conditions prevailing in dark molecular clouds. Rate coefficients for the first 31 rotational levels of this species (energies up to 55 K) and for temperatures between 5 and 30 K are provided. We have found that these rate coefficients are about ten times more than those previously computed for SO₂ with helium. We calculated the expected emission of the centimeter wavelength lines of SO₂. We find that the transition connecting the metastable 2₀₂ level with the 1₁₁ one is in absorption against the cosmic background for a wide range of densities. The 4₀₄−3₁₃ line is found to be inverted for densities below a few 10⁴ cm^-3. We observed the 1₁₁−2₀₂ transition with the 100 m Green Bank Telescope towards some dark clouds. The line is observed, as expected, in absorption and provides an abundance of SO₂ in these objects of a few 10^-10. The potential use of millimeter lines of SO₂ as tracers of the physical conditions of dark clouds is discussed.