The anomalous 14N/15N ratio in comets 122P/1995 S1 (de Vico) and 153P/2002 C1 (Ikeya-Zhang)

High-resolution (R similar to 60,000) spectra of the CN B(2)Sigma(+)-X(2)Sigma(+)(0, 0) band (near 3880 Angstrom) in the Halley-type comet 122P/1995 S1 de Vico (with a period of 74 yr) and the intermediate-period comet 153P/2002 C1 Ikeya-Zhang (P similar to 370 yr) were obtained with the 2dcoude spectrograph at the 2.7 m Harlan J. Smith telescope of the Observatory. The comets were within 1 AU from the Sun (0.66 and 0.92 AU, respectively) at the time of the observations. While the measured C-12/C-13 isotope ratios of both comets (90 +/- 10 and 90 +/- 25, respectively) are in very good agreement with the solar system value, the N-14/N-15 ratios (140 +/- 20 and 170 +/- 50, respectively) are approximately half the value in Earth's atmosphere. The similarity is striking between these 50 ratios and those obtained recently for two other long-period Oort Cloud comets, C/1995 O1 (Hale-Bopp) and C/2000 WM1 (LINEAR). While these optical determinations of N-14/N-15 are consistent with each other, they disagree with those obtained in comet Hale-Bopp from submillimeter measurements of HCN, generally believed to be the main parent of CN. This puzzling difference points toward the existence of ( an) other unknown parent(s) of CN, with an even higher N-15 excess. Organic compounds like those found in interplanetary dust particles are good candidates.