The N2production rate in comet C/2016 R2 (PanSTARRS)

Abstract

Observations of comet C/2016 R2 (PanSTARRS) have revealed exceptionally bright emission bands of N$_2^+$, the strongest ever observed in a comet spectrum. Alternatively, it appears to be poor in CN compared to other comets, and remarkably depleted in H2O. Here, we quantify the N2 production rate from N$_2^+$ emission lines using the Haser model. We derived effective parent and daughter scale lengths for N2 producing N$_2^+$. This is the first direct measurement of such parameters. Using a revised fluorescence efficiency for N$_2^+$, the resulting production rate of molecular nitrogen is inferred to be Q(N2) ∼1 × 1028 molecules s-1 on average for 2018 February 11, 12, and 13, the highest for any known comet. Based on a CO production rate of Q(CO) ∼1.1 × 1029 molecules s-1, we find Q(N2)/Q(CO) ∼0.09, which is consistent with the N$_2^+$/CO+ ratio derived from the observed intensities of N$_2^+$ and CO+ emission lines. We also measure significant variations in this production rate between our three observing nights, with Q(N2) varying by plus or minus 20 per cent according to the average value.