FeI and NiI in cometary atmospheres. Connections between the NiI/FeI abundance ratio and chemical characteristics of Jupiter-family and Oort-cloud comets
FeI and NiI emission lines have recently been found in the spectra of 17
Solar System comets observed at heliocentric distances between 0.68 and 3.25 au
and in the interstellar comet 2I/Borisov. The blackbody equilibrium temperature
at the nucleus surface is too low to vaporize the refractory dust grains that
contain metals, making the presence of iron and nickel atoms in cometary
atmospheres a puzzling observation. Moreover, the measured NiI/FeI abundance
ratio is on average one order of magnitude larger than the solar photosphere
value. We report new measurements of FeI and NiI production rates and abundance
ratios for the Jupiter-family comet (JFC) 46P/Wirtanen in its 2018 apparition
and from archival data of the Oort-cloud comet (OCC) C/1996 B2 (Hyakutake). The
comets were at geocentric distances of 0.09 au and 0.11 au, respectively. The
emission line surface brightness was found to be inversely proportional to the
projected distance to the nucleus, confirming that FeI and NiI atoms are
ejected 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/FeI
abundance ratios is significantly larger in JFCs than in OCCs. We also unveil
significant correlations between NiI/FeI and C2/CN, C2H6/H2O, and
NH/CN. Carbon-chain- and NH-depleted comets show the highest NiI/FeI ratios.
The existence of such relations suggests that the diversity of NiI/FeI
abundance ratios in comets could be related to the cometary formation rather
than to subsequent processes~in~the~coma.Comment: Accepted for publication in A&A Letter