NITROGEN ISOTOPIC RATIO OF COMETARY AMMONIA FROM HIGH-RESOLUTION OPTICAL SPECTROSCOPIC OBSERVATIONS OF C/2014 Q2 (LOVEJOY)

Abstract

ABSTRACT The icy materials present in comets provide clues to the origin and evolution of our solar system and planetary systems. High-resolution optical spectroscopic observations of comet C/2014 Q2 (Lovejoy) were performed on 2015 January 11 (at 1.321 au pre-perihelion) with the High Dispersion Spectrograph mounted on the Subaru Telescope on Maunakea, Hawaii. We derive the 14N/15N ratio of NH2 (126 ± 25), as well as the ortho-to-para abundance ratios (OPRs) of the H2O+ ion (2.77 ± 0.24) and NH2 (3.38 ± 0.07), which correspond to nuclear spin temperatures of >24 K (3σ lower limit) and 27 ± 2 K, respectively. We also derive the intensity ratio of the green-to-red doublet of forbidden oxygen lines (0.107 ± 0.007). The ammonia in the comet must have formed under low-temperature conditions at ∼10 K or less to reproduce the observed 14N/15N ratio in this molecule if it is assumed that the 15N-fractionation of ammonia occurred via ion–molecule chemical reactions. However, this temperature is inconsistent with the nuclear spin temperatures of water and ammonia estimated from the OPRs. The interpretation of the nuclear spin temperature as the temperature at molecular formation may therefore be incorrect. An isotope-selective photodissociation of molecular nitrogen by protosolar ultraviolet radiation might play an important role in the 15N-fractionation observed in cometary volatiles.