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

5. Discussion

5.1. CH₄ abundance

The ISO-SWS observations toward W 33A and C 7538 : IRS9 indicate that the abundance of interstellar CH₄ is low: . This value is determined with respect to the integrated hydrogen column density derived from the depth of the silicate bands (Table 2). However, it is an average along the line of sight, and strong abundance variations may occur locally. Notably, the rather high temperature ( K) of the gas phase CH₄ toward C 7538 : IRS9 indicates that it is not associated with the large amount of cold CO gas along this line of sight (Mitchell et al. 1990). Contrary, the kinetic temperature of the warm CO gas ( K) is significantly higher than the CH₄ gas temperature, and the gaseous CH₄ toward C 7538 : IRS9 then must reside in a separate volume. However, if we assume a two temperature model for the CH₄ gas, at least 70% of the CH₄ could have a temperature of 200 K (Sect. 4). Then, assuming the warm CH₄ and CO gas are in the same volume, we find that , which is an order of magnitude larger than the average along the line of sight (using a conversion factor ; Lacy et al. 1994).

The temperature of the warm CO component toward W 33A ( K; Mitchell et al. 1990) is comparable to the CH₄ gas temperature, and these molecules may be present in the same volume. In this line of sight, the warm and cold CO gas components are equally abundant, and the local gas phase CH₄ abundance probably does not vary significantly from the average along the line of sight (Table 2).

5.2. Chemistry of interstellar CH₄

Toward both W 33A and C 7538 : IRS9, the CH₄ ice is embedded in a matrix of polar molecules, and the CH₄ gas is warm,  K. The combined CH₄ gas and ice abundance is . The CH₄ gas/solid state abundance ratio () is very low compared to CO, but higher than for H₂O. These results allow us to constrain the models for the origin of interstellar CH₄.

CH₄ may have been formed through grain surface reactions involving accreted atomic C and H, similar to the reaction that converts atomic O into H₂O ice (e.g. Brown et al. 1988). This reaction is very efficient. When starting from an atomic gas, the abundance will be , which is two orders of magnitude larger than the observed abundance (Table 2). Hence, if the observed CH₄ was formed on grain surfaces, the initial atomic C abundance must have been low, i.e. CH₄ was formed during a cold, dense cloud core phase. At the same time, most of the atomic C is locked up in CO, which will, at sufficiently low temperatures, stick to the grains as well. However, CH₄ is absent in non-polar ices (Boogert et al. 1996). This could imply that during CH₄ formation, CO accreted at a high atomic H abundance, and is efficiently hydrogenated to CH₃OH. Using the model calculations on CH₃OH formation by Charnley et al. (1997), we find that this conclusion is supported by the low CO/CH₃OH ratio in the polar solid phase ( toward C 7538 : IRS9; Allamandola et al. 1992; Tielens et al. 1991).

The low observed gas/solid state ratio for CH₄, contrary to CO, is a natural consequence of surface chemistry models. At low temperatures, no CH₄ is expected in the gas phase. The high derived temperature for gaseous CH₄ for both W 33A and C 7538 : IRS9 supports the location of the CH₄ in a hot core region near the protostar. Although pure CH₄ ice sublimates at  K at low interstellar pressures, it sublimates at temperatures up to 90 K in polar ices, depending on the relative amount of CH₄ in the ice (see Sandford & Allamandola (1988) for a discussion on sublimation of H₂O:CO ices). The width and peak position of the interstellar CH₄ ice band indicate that the ratio of CH₄ ice with respect to polar molecules in the ice (H₂O and CH₃OH) is at most 10% (Boogert et al. 1996). Thus the CH₄ sublimation temperature is probably close to 90 K. The observed CH₄ excitation temperature is 70 and 110 K for C 7538 : IRS9 and W 33A respectively, and is expected to be close to the gas kinetic temperature. At the high densities in the hot core, the gas and dust temperatures are closely coupled (Ceccarelli et al. 1996), and therefore the observed warm gas phase CH₄ may indeed result from out-gassing of H₂O-rich ices. The somewhat higher gas/solid CH₄ ratio toward W 33A is then probably related to the larger abundance of warm CO gas in this line of sight. Note that the CH₄ gas/solid ratio is significantly higher than the H₂O gas/solid ratio (Table 2), perhaps indicating that the CH₄ molecules diffuse out off the H₂O ice matrix at temperatures less than 90 K, well before the H₂O ice itself sublimates. We conclude that the presence of CH₄ in a polar ice and the low gas/solid ratio are naturally explained in the grain surface models of CH₄. These models are restricted to have a high initial CO/C ratio, and an efficient CH₃OH formation at a high atomic H abundance.

Rather than grain surface chemistry, an origin of solid CH₄ in UV photolysis of CH₃OH containing ices has sometimes been suggested as well (Allamandola et al. 1988; Gerakines et al. 1996). Both W 33A and C 7538 : IRS9 contain about 5% solid state CH₃OH (Allamandola et al. 1992). However, judging from the strength of the 4.62 µm XCN feature, which is often ascribed to FUV photolysis of interstellar ices, FUV photolysis has been much more important towards W 33A than towards C 7538 : IRS9. Yet, the abundances of solid CH₄ are very similar and the solid state CH₄/H₂O ratio is much less toward W 33A than toward C 7538 : IRS9. Therefore, it seems unlikely that photo-processing of CH₃OH-rich ices is an important production mechanism of interstellar CH₄.

Alternatively, CH₄ may have been formed by low temperature gas phase chemistry (e.g. Millar & Nejad 1985; Helmich 1996) and preserved through accretion in ice mantles. In these models, the observed average abundance along the line of sight is produced in a narrow time interval early in the collapsing phase. The high local abundance of toward C 7538 : IRS9, if it were located in the same volume as the warm CO gas (Sect. 5.1), can never be reproduced by gas phase models. The length of the interval with large CH₄ abundances depends strongly on the assumed initial atomic C abundance, i.e.  yr when starting from an atomic gas, and  yr when starting from CO/C=10 (translucent clouds; Helmich 1996). The models with large initial CO/C ratio's, and thus a lower CH₄ production, are probably more realistic. This is because the free fall time to form molecular clouds from diffuse, atomic clouds ( yr; e.g. Elmegreen 1987) is much larger than the time assumed in gas phase models to form hot cores ( yr). Thus, at the start of the collapse, the gas is no longer atomic. After this short peak in the gas phase CH₄ abundance, the CH₄ `burns' to CO, and consequently the observed CH₄ ice must originate from accretion during a very narrow time interval at  yr. Furthermore, at this (or any) stage of the collapse, little H₂O is present (), and pure gas phase models cannot explain the formation of polar ice mantles. Grain surface formation of H₂O is needed to explain the presence of interstellar CH₄ in a polar ice mantle (Boogert et al. 1996). Other molecules that, in this model, would be formed in the gas phase (CO, CH₄) will then co-condense with the atomic O and H and are trapped in the H₂O-rich ice. However, inevitably any accreted atomic C will react rapidly to CH₄ on the grain surface as well. Thus, if the observed CH₄ ice originates primarily from the gas, the CO/C ratio must have been very high during accretion. This contradicts the low CO/C ratio required to explain the observed CH₄ abundance. Finally, at later times ( yr) these gas phase models predict the absence of (cold) gas phase CH₄, since it is easily converted to CO (e.g. Helmich 1996). Since star formation has occurred in the cores of C 7538 : IRS9 and W 33A, they are probably older than 0.5 million years, and indeed no cold CH₄ gas was detected toward these sources. The observed hot CH₄ gas is most likely located in a hot core near the protostar, where the gas phase composition reflects evaporated ice mantles. The species released from the grains in the hot core survive on a time scale of  yr (Brown et al. 1988).

Concluding, the observations impose strict conditions to the models of gas phase formation of CH₄. Contrary to the grain surface models, a low, perhaps unrealistic, initial CO/C abundance is required to explain the observed CH₄ abundance, and the time window for CH₄ production and accretion on the grains is narrow in any case. Additionally, the presence of CH₄ in a polar ice cannot be explained by pure gas phase models. Additional grain surface formation of H₂O and CH₃OH is required. We conclude that formation of CH₄ on grain surfaces is a more likely explanation.

© European Southern Observatory (ESO) 1998

Online publication: July 7, 1998
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