The Anomalous 14 N/ 15 N Ratio in Comets 122P/1995 S1 (de Vico) and 153P/2002 C1 (Ikeya-Zhang)

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Abstract
High-resolution (R ~ 60,000) spectra of the CN B 2Σ+-X 2Σ+ (0, 0) band (near 3880 Å) 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 ~ 370 yr) were obtained with the 2dcoudé spectrograph at the 2.7 m Harlan J. Smith telescope of the McDonald 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 12C/13C isotope ratios of both comets (90 ± 10 and 90 ± 25, respectively) are in very good agreement with the solar system value, the 14N/15N ratios (140 ± 20 and 170 ± 50, respectively) are approximately half the value in Earth's atmosphere. The similarity is striking between these 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 14N/15N 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 15N excess. Organic compounds like those found in interplanetary dust particles are good candidates.