Collisional Excitation of SO2in Cold Molecular Clouds

Jose Cernicharo

doi:10.1051/epjconf/20123404002

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.

Highlights

The observation of molecular emission at millimeter and infrared wavelengths, supplemented by careful and detailed modeling, is a powerful tool to investigate the physical and chemical conditions of astrophysical objects
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
For dark clouds it will be extremely difficult to extrapolate the rates from 25 K to 10 K, i.e. the typical temperature of these objects, as many resonances occur at low energies and may play an important contribution to collisional rates when the temperature decreases

Summary

Introduction

The observation of molecular emission at millimeter and infrared wavelengths, supplemented by careful and detailed modeling, is a powerful tool to investigate the physical and chemical conditions of astrophysical objects. Green [6] has computed these rates for the system SO2-He for temperatures between 25-125 K for the first 50 rotational levels of this molecule (i.e., for levels with energies below 90 K) These rates are useful for clouds with temperatures around 25-40 K. For warm molecular clouds in which lines involve energy levels of several hundred K, or in O-rich evolved stars which show lines with levels of more than one thousand Kelvin, adequate radiative transfer models can not be carried out due to the lack of collisional rates at these high temperatures. Calculations for high temperatures and for a large number of rotational levels is under progress In this contribution we discuss on the scope of the new collisional rates and we predict the expected emission of several lines of SO2 in dark clouds. We show that the line 111-202 could be in absorption against the cosmic background and that the 404-313 line could be a strong maser in dark clouds

SO2 line absorption and maser emission in dark clouds

Observation of absorption of SO2 against the cosmic background

SO2 rotational lines in the millimeter domain

Conclusions