(Proposal ID) S15B-088

(PI) Shinnaka, Yoshiharu

(Proposal Title)
Early solar system environment from the ^14N/^15N ratio in cometary ammonia

(Abstract)
Comet is a part of remnants of icy planetesimals formed in the solar nebula 
4.6 Gyrs ago. The icy materials in comets are definitely the closest key for 
us to understanding the origin and evolution of our solar system. One of 
primordial properties in comet is the isotopic ratio in volatiles. In general, 
isotopic ratios of volatiles are sensitive to their formation temperatures and 
essentially important to reveal the formation condition of volatiles since 
they are clues to temperatures where the volatiles formed. Based on our recent 
reports about the ^{14}N/^{15}N ratio in cometary ammonia, they show a large 
^{15}N fractionation (^{15}N is enriched in ammonia compared to the solar 
elemental abundance). However, {this high ^{15}N fractionation of solid-phase 
ammonia cannot be reproduced by current chemical evolution models}. Because we 
have only a small unbar of samples for ^{14}N/^{15}N ratios in cometary 
ammonia, we should measure the statistical properties of the ^{14}N/^{15}N 
ratio in cometary ammonia to give constraint the formation environment of 
cometary ammonia. We propose {ordinary observations of C/2013 US10 (Catalina)} 
and {ToO observations of bright comets with the HDS} and we request 4 half 
nights as total.