Gas-phase SO 2 in absorption towards massive protostars

Abstract

We present the first detection of the nu₃ ro-vibrational band of gas-phase SO₂ in absorption in the mid-infrared spectral region around 7.3μm of a sample of deeply embedded massive protostars. Comparison with model spectra shows that the derived excitation temperatures correlate with previous C₂H₂ and HCN studies, indicating that the same warm gas component is probed. The SO₂ column densities are similar along all lines of sight suggesting that the SO₂ formation has saturated, but not destroyed, and the absolute abundances of SO₂ are high ( ~ 10^-7). Both the high temperature and the high abundance of the detected SO₂ are not easily explained by standard hot core chemistry models. Likewise, indicators of shock induced chemistry are lacking. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands and the UK) and with the participation of ISAS and NASA.