Interferometric imaging of Titan's HC${}_3$N, H${}^{13}$CCCN and HCCC${}^{15}$N

Abstract

We present the first maps of cyanoacetylene isotopologues in Titan's atmosphere, including H${}^{13}$CCCN and HCCC${}^{15}$N, detected in the 0.9 mm band using the Atacama Large Millimeter/submillimeter array (ALMA) around the time of Titan's (southern winter) solstice in May 2017. The first high-resolution map of HC${}_3$N in its $v_7=1$ vibrationally excited state is also presented, revealing a unique snapshot of the global HC${}_3$N distribution, free from the strong optical depth effects that adversely impact the ground-state ($v=0$) map. The HC${}_3$N emission is found to be strongly enhanced over Titan's south pole (by a factor of 5.7 compared to the north pole), consistent with rapid photochemical loss of HC${}_3$N from the summer hemisphere combined with production and transport to the winter pole since the April 2015 ALMA observations. The H${}^{13}$CCCN/HCCC${}^{15}$N flux ratio is derived at the southern HC${}_3$N peak, and implies an HC${}_3$N/HCCC${}^{15}$N ratio of $67\pm 14$. This represents a significant enrichment in ${}^{15}$N compared with Titan's main molecular nitrogen reservoir, which has a ${}^{14}$N/${}^{15}$N ratio of 167, and confirms the importance of photochemistry in determining the nitrogen isotopic ratio in Titan's organic inventory.

Accepted for publication in ApJL, May 2018