Astrophysics > Solar and Stellar Astrophysics arxiv.org/abs/2508.18382
[Submitted on 25 Aug 2025]
Rahatgaonkar, R. Carvajal, J. P. Puzia, T. H. et al. VLT observations of interstellar comet 3I/ATLAS II. From quiescence to glow: Dramatic rise of Ni I emission and incipient CN outgassing at large heliocentric distances
No sooner do I post the last ATLAS spectrum paper do they release another. Rahatgaonkar et al. used a different telescope (VLT) in a different band (mostly optical). And what did they see at the interstellar object? In this band, most of the appearance of the comet is reflected sunlight, from dust in the coma; not much of its emission consists of spectral features in the optical. The few spectral emissions are from minor (but pedagogically significant) species, as seen at other comets.
Cyanide and cyano-radicals (CN) are found in comets; both carbon and (to a lesser extent) nitrogen are cosmically abundant, and condense at comet temperatures along with other comet volatiles. But most importantly, CN has a spectral line in the visible- green- making it accessible to our most common and available telescopes and instruments. This is not true for literal water (H2O). CN is therefore used as an indicator species, disproportionate in its importance. Specifically, Rahatgaonkar et al. followed the comet from the cold outer Solar System, to most recently the inner Main Belt, and succeeded in tracking the onset of CN as ATLAS warmed.
Another such marker element is nickel- yes, nickel. Not particularly common, yet not rare either, nickel is often found in ‘ores’ as nickel sulfide. Since sulfides are not that stable, warming a body (including ‘warming’ by radiation or micrometeorites) can cause release of ores, and of its metal cation. Rahatgaonkar et al. now describe that release- a release that’s in their telescope band.