Ethyl Alcohol in Space
During 1991 and 1992 a 330-360 GHz spectral survey of the hot molecular core associated with the ultracompact HII region G34.3+0.15 was carried out at the JCMT. This HII region is a prototypical example of the cometary morphology and has been interpreted in terms of the bow-shock interaction between the ambient molecular cloud and the wind from an energetic young star moving through the cloud. NH3(3,3) observations with the VLA by Heaton et al. (1989), showed that the highly compact molecular cloud appears to be wrapped around the head of the cometary structure.
The survey (Macdonald et al. 1995) resulted in the detection of around 350 lines down to a one sigma noise level of around 0.16 K, roughly 11 lines per GHz to a detection limit of about 0.5 K in Ta*. The identification of these lines proved very time-consuming due to the relative lack of measured laboratory lines at such high frequencies. This is particularly the case for large molecules such as methyl formate, HCOOCH3, ethyl alcohol, C2H5OH, dimethyl ether, CH3OCH3, and ethyl cyanide, C2H5CN. Currently, we have identified 32 distinct chemical species (some 40 per cent of all presently identified interstellar molecules) plus 18 isotopomers, with possible detections of 8 further species. Around 70 lines, many detected in the spectral surveys of Orion, remain unidentified.
Based on new laboratory measurements at Ohio State University, we were able to identify 14 lines with ethyl alcohol (Millar et al. 1995). A rotation diagram analysis showed two surprises: first, that the ethanol was hot, with a rotation temperature of 125 K; second, that it was abundant — with a column density of 2.0 x 10(15) cm-2 averaged over the 14 arcsec beam. This corresponds to a fractional abundance of about 5 x 10(-9) with respect to H2, about one million times larger than predicted by pure gas-phase ion-molecule chemistry and perhaps indicates that ethyl alcohol is formed on dust grain mantles and subsequently evaporated into the gas-phase due to the interaction of the star and the molecular cloud.
Figure 1. Double-sideband spectra including the 55,2 - 54,1 and 65,1 - 54,2 lines of ethanol at 340.189 GHz. Frequency scales shown correspond to lower sideband (top) and upper sideband (bottom) components. The local oscillator frequency for the lower spectrum was increased by 10 MHz for sideband identification. A velocity of 58 kms-1 relative to the local standard of rest has been assumed.
Ethyl alcohol was first detected in the interstellar medium in 1975 by a team led by Ben Zuckerman, but proved elusive in subsequent searches. Before this detection, only Sgr B2 and W51M showed conclusive proof (!) of the presence of ethyl alcohol. Subsequently, Ohishi and collaborators at Nobeyama have detected it toward a number of hot core sources in regions of massive star formation. In a recent article, they show that in Orion the spatial distribution of ethyl alcohol is similar to that of other, large oxygen-bearing molecules such as methanol, methyl formate and dimethyl ether and is associated with a hot core called the Compact Ridge cloud.
The detection of such large amounts of ethyl alcohol, and previous detections of even larger amounts of methanol, in hot cores suggests that grain surface chemistry may be very efficient in producing alcohols. Recently, Charnley et al. (1995) have discussed the chemistry following the evaporation of various alcohols, including methanol, ethanol, propanol and butanol from grains in hot cores. Using the limited laboratory data available, they have shown that even larger molecules can be formed in the gas-phase from these evaporated molecules. The most abundant are predicted to be methyl ethyl ether (CH3OC2H5) and diethyl ether (C2H5OC2H5) and could be detectable in hot cores. Unfortunately, the submillimetre spectra of these are as yet unknown. Searches at low frequencies (< 20 GHz) have been made but do not constrain the models. Charnley et al. discuss various mechanisms by which alcohols might be formed in dust mantles including H and O atom attack on species containing carbon double and triple bonds, radical-radical surface
reactions and diffusion-controlled reactions. There are problems associated with all of these mechanisms; work is currently underway to examine these quantitatively.
Finally, it is worth pointing out how much ethyl alcohol is present in G34.3. Geoff Macdonald, who has a keen interest in such matters, calculated that there is enough for 300,000 pints of beer for every person on Earth every day for the next billion years, and created quite a media stir as a result. Of course, he didn't say that the proof is low, less than one per cent (alcohol-free beer !), nor that condensing the gas directly would give a brew guaranteed to leave a headache next morning — the cloud contains a lot of HCN. Of course, those of us Mancunions (and others) addicted to Boddingtons beer know that a headache is part of the pleasure and not something which necessarily implies a bad pint.
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Macdonald, G.H., Gibb, A.G., Habing, R.J. & Millar, T.J. 1995, Astron. Astrophys. Suppl., submitted.
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Ohishi, M., Ishikawa, S-I., Yamamoto, S., Saito, S. & Amano, T. 1995, Astrophys. J. Lett., 446, L43.
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T J Millar,
Department of Physics, UMIST