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Astron. Astrophys. 351, 1066-1074 (1999)

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6. The methanol impact on the overall spectrum

The interactions described above have led us to observe RAFGL7009S from the ground. The measurements of the methanol combination modes allows to precisely determine its column density (3.3-3.8[FORMULA]1018 cm-2, Dartois et al. 1999). As a moderately complex molecule, methanol possesses numerous transitions (Fig. 7). We can then predict and compare the relative importance of the other modes in the ISO-SWS01 spectrum.

[FIGURE] Fig. 7. Pure methanol ice spectrum deposited at 10 K and transmittance spectrum extracted from the source RAFGL7009S. The laboratory spectrum is normalised to the abundance of methanol derived from ground based observations. The astronomical spectrum region highlighted by the rectangle correspond to the one covered by the observations performed at UKIRT (Dartois et al. 1999)

In particular, we address now the question of the assignment of the 6.85 µm band. We can evaluate methanol contribution to this band. To do this we invert the equation generally used to infer the molecules column densities in a given line of sight:


where [FORMULA] is the wavenumber, [FORMULA], A is the integrated absorption cross section (cm/molecule) of the considered transition and N is the column density (cm-2). The methanol CH3 deformation and OH bending modes (6.85 µm) have an integrated absorption cross section comprised between 1.1 and 1.2[FORMULA]10-17 cm molec-1 in the case of the ice mixtures used for the comparison at short wavelength (Dartois et al. 1999). We then obtain for the broad 6.85 µm band a predicted integrated absorbance given by:


which leads to a value of 36-46 cm-1. The measurement of the 6.85 µm band integrated absorbance on the astrophysical spectrum gives 95[FORMULA]15 cm-1. Methanol can then provide 32% to 56% of this band (the uncertainty coming from both the continuum evaluation and integrated absorption cross section), and part of it could be carried by another ice constituent. Up to 10% could be accounted for by the OCN- counterion, i.e. [FORMULA], (Demyk et al. 1998) but still 30% to 60% of the band remains unaccounted for.

The 2[FORMULA] methanol overtone falls at 2040 cm-1 (4.9 µm), at the place of a band attributed to OCS (Geballe et al. 1985, Palumbo et al. 1995). The integrated absorbance of this line in the astrophysical spectrum is 4[FORMULA]1 cm-1. We can now evaluate the integrated absorbance ratio between the 6.85 µm and 4.9 µm bands in the laboratory spectrum of pure methanol deposited at 10 K, which leads to 0.025. The integrated absorption cross section for this overtone is then 2.7-3.0[FORMULA]10-19 cm molec- 1. We use this value and the column density toward RAFGL7009S, derived from the short wavelength modes of methanol to evaluate the contribution of this overtone to the 4.9 µm mode: 0.9-1.2 cm-1. Comparing this to the ISO observations, one fourth to one third of the band can be attributed to methanol and the rest to another molecule such as OCS. The bandwidth of the 2[FORMULA] methanol feature explains rather well the peculiar shape of the interstellar 4.9 µm band whose base is wide and center relatively narrower. The base is certainly due to the methanol overtone. Palumbo et al. (1995, 1997) did obtain a better fit to their spectrum in this wavelength region with mixtures containing both methanol and carbonyl sulfide (OCS).

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© European Southern Observatory (ESO) 1999

Online publication: November 16, 1999