Collisional Excitation of SO₂ in Cold Molecular Clouds

Department of Astrophysics. CAB. INTA-CSIC. Crta TorrejOn km 4.28850 Torrejon de Ardoz., Madrid, Spain

^a e-mail: jcernicharo@cab.inta-csic.es

Abstract

Using recently calculated collisional rates for SO₂ with ortho and para molecular hydrogen, we have calculated the expected emission of the centimeter wavelength lines of SO₂ for the physical conditions previling in dark molecular clouds. We have found that the transition connecting the metastable 2₀₂ level with the 1₁₁ one will be in absorption against the cosmic background for a large range of densities. The 4₀₄-3₁₃ line is found to be masering for densities below a few 10⁴ cm⁻³. We have observed the 1₁₁-2₀₂ transition with the GBT 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.