Collisional excitation of sulfur dioxide in cold molecular clouds

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 2(02) level with the 1(11) one is in absorption against the cosmic background for a wide range of densities. The 4(04)-3(13) line is found to be inverted for densities below a few 10(4) cm(-3). We observed the 1(11)-2(02) 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.