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Astron. Astrophys. 336, 352-358 (1998)

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6. Conclusions

ISO-SWS observations show ro-vibrational lines of gaseous CH4 toward the massive protostars W 33A and C 7538 : IRS9. From the rotational diagrams we conclude that the absorbing gas is warm toward both sources ([FORMULA] and 70 K respectively). The gas/solid CH4 ratio is low, 0.7 and 0.3, which contrasts strongly with CO. Also in contrast with CO is the non-detection of cold CH4 gas. Using velocity broadenings from rotational emission lines, we find that [FORMULA]. A significant amount of cold CH4 gas could be hidden in the data if the velocity broadening b is much less than 2 km s-1. High resolution (Fabry-Perot) infrared observations are needed to settle this issue.

We discuss models for the formation of interstellar CH4. Gas phase models can explain the observed abundance ([FORMULA]), and the low CH4 gas/solid ratio, but the observations impose strong restrictions. First, the presence of CH4 in a polar ice is unexplained by these models, unless an alternative way (i.e. grain surface chemistry) of H2O formation is invoked. Second, to inhibit at the same time grain surface formation of CH4 the CO/C ratio must have been high during accretion. On the other hand, a low initial CO/C is required to produce the observed abundance of interstellar CH4 in the gas phase. Finally, the formation of CH4 and subsequent accretion on the grains is limited to a narrow time interval ([FORMULA] yr), which decreases for (probably more realistic) lower initial CO/C ratio's. A more likely explanation is the formation of CH4 from atomic C through grain surface reactions, similar to H2O formation from atomic O. The presence of CH4 in a polar ice mantle and the low gas/solid ratio are natural consequences of this model. Since this reaction is very efficient, a high CO/C ratio would be needed to explain the low observed CH4 abundance, i.e. interstellar CH4 is formed at the high densities deep inside the molecular cloud. To inhibit the inclusion of CH4 in a non-polar ice, the accreted CO must have reacted on the grain surface with abundantly present atomic H to form CH3OH, which is in agreement with the low CO/CH3OH ratio in the polar solid phase. The detected warm CH4 gas probably has sublimated from the grains at the high temperatures in the vicinity of the protostar. The gas/solid ratio of CH4 is high compared with H2O and may indicate a grain temperature [FORMULA] K, when the CH4 molecules diffuse out off the H2O ice matrix, before the H2O itself sublimates. That would be consistent with the derived CH4 rotation temperature for C 7538 : IRS9, but seems unlikely at the higher derived temperature toward W 33A. Further laboratory work on the outgassing behaviour of CH4 containing ices is needed, as well as a systematic determination of gas/solid ratio's in lines of sight tracing different physical conditions.

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© European Southern Observatory (ESO) 1998

Online publication: July 7, 1998