Origin and evolution of NiI and FeI in the coma of the interstellar comet 3I/ATLAS throughout its trajectory
Year: 2026
Type: article
Abstract: We present high-resolution UVES+VLT observations of neutral nickel (NiI) and iron (FeI) atoms in the coma of the interstellar comet 3I/ATLAS taken after perihelion at heliocentric distances (r_h) ranging from 1.88 to 4.35 au. Metal emission was strong shortly after perihelion and persisted at large heliocentric distances. Spatial profiles indicate a short-scale source for both NiI and FeI. At all heliocentric distances, NiI dominates the metal budget. At r_h ∼ 2 au the total metal production rate was found to be at least an order of magnitude higher than that of typical Solar System comets. Post-perihelion production rates exhibit clear asymmetry compared to the pre-perihelion behavior: production rates are higher after perihelion and decline more gradually with r_h, with the difference being stronger for FeI. The NiI/FeI abundance ratio, initially anomalously high before perihelion, evolved toward values comparable to Solar System comets near 2 au, and shows a weaker r_h dependence after perihelion. To interpret these results, we revisited and extended the carbonyl hypothesis in which FeI and NiI are produced by the rapid photodissociation of Fe(CO)_5 and Ni(CO)_4 vaporized from the nucleus. Using sublimation laws parameterized with simple temperature profiles, T(r_h), we fitted the full pre- and post-perihelion datasets and demonstrated that the direct sublimation of carbonyls can reproduce the observed rates and the high NiI/FeI abundance ratio. This ratio is controlled by the relative volatilities and active fractions of Ni(CO)_4 and Fe(CO)_5. The temperature profiles needed to reproduce the observations were found to be shallower than the equilibrium T ∝ r_h^ -1/2 relation. The full dataset could also be reproduced using the temperature profiles extracted from a thermal model of the nucleus, but only with significant empirical modifications to the model. In particular, a significant reduction in the active areas through the perihelion was required. An additional transient heat source (T ∼ 120 K) was also necessary. This heat source, possibly related to the amorphous-crystalline ice transition, could explain the early NiI excess before perihelion. Within the framework of this modified thermal model, carbonyl sublimation occurs below the surface of the nucleus. Although carbonyls provide a self-consistent explanation for the presence of metal atoms in cometary comae at large heliocentric distances, the direct detection of carbonyls in comets remains elusive. Alternative mechanisms, such as release from superheated metallic nanoparticles, cannot be ruled out.
Source: Astronomy and Astrophysics
Language: English
Topic: Astro and Planetary Science
Subfield: Space and Planetary Science
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,865
OpenAlex ID: W7160824641