July 2014


EDITOR'S NOTE: This month Astromolecule of the Month will begin to feature molecules that have been searched for but not yet detected in the interstellar or circumstellar media. These will be indicated with the "Non-Detection" graphic in the upper right corner, which includes a link to the new page about non-detections. There are two types of non-detections: failed searches and disconfirmations of claimed detections.

Pyrimidine (C4H4N2) is a heterocyclic compound where two of the CH elements of benzene have been replaced with N atoms. It has been sought in space because three of the nucleobases (cytosine, thymine, and uracil) are derived from it. Various studies have shown that nucleobases and other prebiotic molecules will form in irradiated ices, such as the study by Materese et al. that found that thymine was formed in ices containing pyrimidine and other species that were irradiated with UV light.

To date, three unsuccessful searches have been attempted for pyrimidine. The first effort, by Simon & Simon, was reported in 1973. They looked for one rotational line in Sgr B2 using the 36-foot telescope at Kitt Peak. In 2003, Kuan et al. reported a much more extensive search for a number of rotational lines in Sgr B2(N) and two other hot molecular cores, Orion KL and W51 e1/e2, using the James Clerk Maxwell Telescope and concluded that pyrimidine was present at no greater than a column density of about 1014 cm-2. Most recently, Charnley et al. looked for two lines in two other sources, CRL 618 and IRC +10216, using various telescopes and were again not able to find evidence for it.

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