Formation and photostability of N-heterocycles in space

I. The effect of nitrogen on the photostability of small aromatic molecules

¹ Astrobiology Laboratory, Leiden Institute of Chemistry, PO Box 9502, 2300 RA Leiden, The Netherlands e-mail: peeters@strw.leidenuniv.nl
² International Space Science Institute, Hallerstrasse 6, 3012 Bern, Switzerland
³ Space Science Division, NASA Ames Research Center, MS 245-3, Moffett Field, CA 94035, USA
⁴ Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland
⁵ Department of Earth Sciences, National Taiwan Normal University, 88 Sec. 4 Ting-Chou Road, Taipei 116, Taiwan
⁶ Institute of Astronomy and Astrophysics, Academia Sinica, PO box 23-141, Taipei 106, Taiwan

Received: 26 November 2004
Accepted: 14 December 2004

Abstract

Nitrogen-containing cyclic organic molecules (N-heterocycles) play important roles in terrestrial biology, for example as the nucleobases in genetic material. It has previously been shown that nucleobases are unlikely to form and survive in interstellar and circumstellar environments. Also, they were found to be unstable against ultraviolet (UV) radiation. However, nucleobases were detected in carbonaceous meteorites, suggesting their formation and survival is possible outside the Earth. In this study, the nucleobase precursor pyrimidine and the related N-heterocycles pyridine and s-triazine were tested for UV stability. All three N-heterocycles were found to photolyse rapidly and their stability decreased with an increasing number of nitrogen atoms in the ring. The laboratory results were extrapolated to astronomically relevant environments. In the diffuse interstellar medium (ISM) these N-heterocycles in the gas phase would be destroyed in 10–100 years, while in the Solar System at 1 AU distance from the Sun their lifetime would not extend beyond several hours. The only environment where small N-heterocycles could survive, is in dense clouds. Pyridine and pyrimidine, but not s-triazine, could survive the average lifetime of such a cloud. The regions of circumstellar envelopes where dust attenuates the UV flux, may provide a source for the detection of N-heterocycles. We conclude that these results have important consequences for the detectability of N-heterocycles in astronomical environments.