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FeI and NiI emission lines have recently been found in the spectra of 17Solar System comets observed at heliocentric distances between 0.68 and 3.25 auand in the interstellar comet 2I/Borisov. The blackbody equilibrium temperatureat the nucleus surface is too low to vaporize the refractory dust grains thatcontain metals, making the presence of iron and nickel atoms in cometaryatmospheres a puzzling observation. Moreover, the measured NiI/FeI abundanceratio is on average one order of magnitude larger than the solar photospherevalue. We report new measurements of FeI and NiI production rates and abundanceratios for the Jupiter-family comet (JFC) 46P/Wirtanen in its 2018 apparitionand from archival data of the Oort-cloud comet (OCC) C/1996 B2 (Hyakutake). Thecomets were at geocentric distances of 0.09 au and 0.11 au, respectively. Theemission line surface brightness was found to be inversely proportional to theprojected distance to the nucleus, confirming that FeI and NiI atoms areejected from the surface of the nucleus or originate from a short-lived parent.Considering the full sample of 20 comets, we find that the range of NiI/FeIabundance ratios is significantly larger in JFCs than in OCCs. We also unveilsignificant correlations between NiI/FeI and C$_2$/CN, C$_2$H$_6$/H$_2$O, andNH/CN. Carbon-chain- and NH-depleted comets show the highest NiI/FeI ratios.The existence of such relations suggests that the diversity of NiI/FeIabundance ratios in comets could be related to the cometary formation ratherthan to subsequent processes~in~the~coma.

astronomy and astrophysics

FeI and NiI in cometary atmospheres. Connections between the NiI/FeI  abundance ratio and chemical characteristics of Jupiter-family and Oort-cloud  comets