SIMBAD references

Interstellar CH₄ may consume a fair amount of the carbon budget in dense molecular clouds, but probably less than CO, CH₃OH, and CO₂. However, it can only be observed at wavelength regions in the infrared that are heavily affected by the earth atmosphere. With new space and airborne missions (e.g. ISO, SOFIA) in mind we have studied the near infrared absorption spectra of solid and gaseous CH₄. We obtained laboratory spectra of the ν₄ deformation mode (1302cm^–1, 7.68µm) of solid CH₄ in astrophysically relevant mixtures. We found that the peak position and width of this absorption band vary strongly as a function of molecular environment, compared to temperature and particle shape effects. Hence, observations of this feature will provide a powerful probe of the molecular composition of interstellar ices. Also the gas phase CH₄ ro-vibrational spectrum of the same band has been calculated. Using observed physical conditions around the protostar W 33A, we show that unresolved gaseous CH₄ lines are detectable (at the 2-5% level) at a resolution R>1000, when the column density N≥10¹⁶ cm^–2. An astrophysically relevant molecule with a very strong transition in the same wavelength regime, is SO₂. We studied the ν ₃ asymmetric stretching mode (1319 cm^–1, 7.58 µm) of solid SO₂ in several mixtures, revealing that the peak position, width and detailed profile of this band are very sensitive to the molecular environment. Besides probing the composition of ice mantles, observations of solid SO₂ will provide important information on the sulfur budget locked up in grain mantles, which is currently poorly known. We compare the laboratory and calculated spectra of CH₄ and SO₂ with previously published ground based spectra and new airborne observations of young stellar objects in the 7-8µm region. W 33A, NGC 7538 : IRS1 and IRS9 show a feature near 7.68µm that is consistent with absorption by solid CH₄ or the Q-branch of gaseous CH₄. The column density of solid CH₄ would be 0.3-4% of solid H₂O, indicating that solid CH₄ consumes 0.5±0.3% of the cosmic carbon abundance. A gaseous origin would imply a column density of at least this amount, being highly dependent on the assumed temperature of the absorbing gas. A second absorption feature is detected toward W 33A and NGC 7538 : IRS1 at 7.58 µm. The peak position and width of this feature are consistent with the ν₃ mode of solid SO₂ in a matrix of solid CH₃OH or pure SO₂. The derived column density is 0.1-1% of solid H₂O, indicating that solid SO₂ locks up 0.6-6% of the cosmic sulfur abundance. This study shows that 7-8µm spectroscopy of dense molecular clouds, using new airborne and space-based platforms, will provide valuable information on the composition of icy grain mantles and molecular cloud chemistry.