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Extreme NiI/FeI abundance ratio in the coma of the interstellar comet 3I/ATLAS (2509.26053v1)
Published 30 Sep 2025 in astro-ph.EP and astro-ph.GA
Abstract: Emission lines of FeI and NiI are commonly found in the coma of solar system comets, even at large heliocentric distances. These atoms are most likely released from the surface of the comet's nucleus or from a short-lived parent. The presence of these lines in cometary spectra is unexpected because the surface blackbody equilibrium temperature is too low to allow the sublimation of refractory minerals containing these metals. These lines were also found in the interstellar comet 2I/Borisov which has a NiI/FeI abundance ratio similar to that observed in solar system comets. On average, this ratio is one order of magnitude higher than the solar Ni/Fe abundance ratio. Here, we report observations of the new interstellar comet 3I/ATLAS, which were carried out with the ESO Very Large Telescope equipped with the UVES spectrograph. Spectra were obtained at six epochs, at heliocentric distances ranging from 3.14 to 2.14 au. NiI was detected at all epochs. FeI was only detected at heliocentric distances smaller than 2.64 au. We estimated the NiI and FeI production rates by comparing the observed line intensities with those produced by a fluorescence model. Comet 3I exhibits a high production rate of NiI atoms as well as a high NiI/FeI ratio, making it exceptional when compared to solar system comets and 2I/Borisov. Additionally, we found that the NiI/FeI ratio decreases rapidly with decreasing heliocentric distance, suggesting that comet 3I could soon become indistinguishable from solar system comets in this respect. We interpreted these observations assuming that the NiI and FeI atoms were released through the sublimation of Ni(CO)$_4$ and Fe(CO)$_5$ carbonyls, which supports the presence of these species in the cometary material.
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Explaining “Extreme NiI/FeI abundance ratio in the coma of the interstellar comet 3I/ATLAS”
Overview: What is this paper about?
This paper studies an interstellar comet called 3I/ATLAS. The researchers looked at the gas around the comet (called the coma) and found lots of individual nickel and iron atoms there. Strangely, they found far more nickel than iron—much more than you’d expect if the comet had the same mix of elements as the Sun. They also discovered that this nickel-to-iron ratio changes quickly as the comet gets closer to the Sun.
Key questions the researchers asked
- Are nickel (Ni) and iron (Fe) atoms present in the coma of 3I/ATLAS?
- How many nickel atoms are there compared to iron atoms?
- Does this nickel-to-iron ratio change as the comet moves toward the Sun?
- What could be releasing these metal atoms into space when the comet is still very cold?
- Is 3I/ATLAS similar to or different from comets in our own solar system?
How did they paper the comet?
The team used one of the world’s most powerful telescopes, the Very Large Telescope (VLT) in Chile, with a super-precise instrument called UVES. They observed the comet six times as it traveled from about 3.14 to 2.14 astronomical units (au) from the Sun. For reference, 1 au is the distance from Earth to the Sun.
Here’s what they did, in simple terms:
- They split the comet’s light into a rainbow (a spectrum) and looked for “fingerprints” of different atoms called emission lines. Think of these lines like the bright colors in a neon sign—each atom glows at its own special set of colors when sunlight energizes it.
- They carefully removed any sunlight reflected by dust so they could isolate the comet’s own glow.
- Using a “fluorescence model,” they converted the brightness of each nickel and iron line into the number of atoms being released per second. Fluorescence here means sunlight makes the atoms light up; the model is like a calculator that turns brightness into “how many atoms are there.”
- They also measured other gases (like OH from water, CN, and C2) to understand the comet’s overall activity and chemistry.
A few helpful terms:
- Coma: the fuzzy cloud of gas and dust surrounding a comet’s solid core (nucleus).
- Emission line: a bright, narrow color in the spectrum that reveals a specific atom or molecule.
- Production rate: how many atoms or molecules are released per second.
- Heliocentric distance (): how far the comet is from the Sun, in au.
- Temperature rule of thumb: the comet’s surface temperature drops with distance, roughly kelvin. Far from the Sun, it’s too cold for most rocky minerals to vaporize.
What did they find?
- Nickel atoms (NiI) were detected every time they observed the comet—even when it was still far from the Sun (earlier work saw Ni as far as 3.88 au).
- Iron atoms (FeI) only showed up once the comet got closer than about 2.64 au.
- At first, the comet was releasing about 20 times more nickel atoms than iron atoms. Two weeks later, that dropped to about 4 times more nickel than iron. In other words, the nickel-to-iron ratio started extremely high and then fell quickly as the comet warmed.
- The brightness pattern of the nickel and iron lines faded with distance from the nucleus in a way that suggests the atoms are released very close to the surface, or from a short-lived parent molecule that breaks apart quickly.
- Compared with comets in our solar system and another interstellar comet (2I/Borisov), 3I/ATLAS is extreme: it releases both a lot of nickel and has a very high nickel-to-iron ratio—especially when it’s far from the Sun.
- The metal atoms seem to start being released before water ice becomes active, likely when more volatile gases like CO and CO2 are driving the activity. This fits with independent results showing 3I/ATLAS is unusually rich in CO2 compared to water.
What could explain this?
You’d expect solid, metal-containing minerals (like silicates) to stay solid at these cold temperatures—they shouldn’t evaporate so far from the Sun. So where do the nickel and iron atoms come from?
The authors suggest they may come from special, very easy-to-evaporate compounds called metal carbonyls:
- Nickel tetracarbonyl: Ni(CO)4
- Iron pentacarbonyl: Fe(CO)5
These are molecules where a nickel or iron atom is “carried” by carbon monoxide (CO). They can vaporize at much lower temperatures than rocks do. Even better, nickel carbonyl evaporates more easily than iron carbonyl. That naturally explains why:
- Far from the Sun (cold): you see nickel but little iron → high Ni/Fe ratio.
- Closer in (warmer): iron starts to show up → Ni/Fe ratio drops.
The team showed that how much nickel they saw and how the Ni/Fe ratio changed with distance match what you’d expect if these carbonyls are present and evaporating. They also note that local “hot spots” on the comet’s surface (caused by increased activity and dust) can speed up this process as the comet approaches the Sun.
Why does this matter?
- It suggests interstellar comet 3I/ATLAS contains unusual chemicals—metal carbonyls—that can release metals at low temperatures. That’s a big clue about how and where the comet formed in its original star system.
- The nickel-to-iron ratio might act like a thermometer for comet surfaces: if Ni(CO)4 evaporates first, a high Ni/Fe ratio could mean a colder surface, and a falling ratio could signal warming.
- This work connects the release of metals to carbon-rich gases like CO and CO2. If a comet formed in a place rich in these gases, it might “lock up” more metals in carbonyls, changing how it behaves when it visits the Sun.
- As 3I/ATLAS gets closer to the Sun, it may start to look more “normal,” like many solar-system comets, at least in its Ni/Fe ratio. But its very high nickel output still makes it stand out.
In short: 3I/ATLAS looks chemically unusual, probably carries metal carbonyls, and teaches us that not all comets—especially interstellar ones—follow the same rules. This helps scientists piece together how comets form in different planetary systems and how they evolve as they travel through space.
Knowledge Gaps
Below is a focused list of the paper’s unresolved knowledge gaps, limitations, and open questions. Each point is phrased to be concrete and actionable for follow-up research.
- The carbonyl-origin hypothesis is unconfirmed: there is no direct detection of Ni(CO)4 or Fe(CO)5 in the coma; targeted searches for diagnostic vibrational/rotational features (e.g., mid-IR CO stretch for carbonyls, mm/sub-mm rovibrational lines) are needed.
- Formation pathways and stability of metal carbonyls in cometary ices are unknown: laboratory experiments at 20–150 K with CO/CO2-rich matrices and Ni/Fe sources are required to test whether Ni(CO)4/Fe(CO)5 can form, be trapped, and survive irradiation and thermal cycling.
- Thermal state of the nucleus and near-surface layers is not constrained: there are no contemporaneous thermal IR measurements to validate the proposed temperature-driven evolution of NiI/FeI; coordinated thermophysical observations and modeling are needed.
- The proposed use of NiI/FeI as a temperature proxy is uncalibrated: empirical calibration against independent temperature measurements across multiple comets is required to assess degeneracies with composition.
- Lack of quantitative thermo-chemical modeling coupling dust-driven superheating, carbonyl sublimation, and photodissociation: develop time-dependent models that reproduce the observed rapid NiI/FeI evolution in 3I.
- Alternative release mechanisms (e.g., superheating of Ni-rich nanograins, sputtering, photodesorption) are not ruled out or parameterized; multi-mechanism fits to the data are needed to quantify each contribution.
- The steep decline of NiI/FeI with decreasing heliocentric distance in 3I does not follow the simple carbonyl-sublimation curve: investigate additional processes (localized hot spots, changing dust opacity, compositional heterogeneity) with 3D nucleus/coma models.
- NiI production implies an effective emitting area of ~1.3 m² under the carbonyl model, which lacks a physical explanation: identify plausible source geometries (small vents, exposed patches, grain surface area) via high-resolution imaging or modeling.
- Time coverage is limited to ~1 month pre-perihelion: post-perihelion and longer-term monitoring are missing to confirm whether 3I’s NiI/FeI converges to solar-system values and to detect possible hysteresis or seasonal effects.
- Rotational modulation and viewing-geometry effects are not considered: obtain rotationally resolved time series to separate intrinsic evolution from rotational variability.
- The FeI non-detections at larger heliocentric distances may be sensitivity-limited: deeper, higher-throughput spectroscopy at UV/blue wavelengths is necessary to tighten FeI upper limits and test early FeI onset.
- Neutral-only analysis omits ionization balance: measure NiII/FeII (if feasible) and model ionization/recombination to ensure NiI/FeI reflects total metal production rather than differential ionization.
- Background contamination near FeI (e.g., [O II] 3727/3729 Å) could bias FeI fluxes: implement sky-fiber/nodding strategies and quantify systematic uncertainty from sky subtraction.
- Spatial origin and parent lifetimes are inferred via SB ∝ p⁻¹ but not directly constrained: acquire spatially resolved profiles and inversion modeling to recover parent lifetimes and release distances.
- Production rates assume fixed gas velocities (0.85 r_h⁻⁰·⁵ km s⁻¹) and Haser scalelengths: measure outflow velocities (e.g., line widths) and adopt vectorial/photochemical models to reduce known Haser artifacts, especially for CN and C₂.
- The observed correlation with CO/H₂O uses mismatched epochs (CO/H₂O at r_h≈3.32 au vs NiI+FeI at r_h≈3.14 au): remeasure simultaneous CO, CO₂, H₂O, and metal production rates to avoid cross-epoch biases.
- CO₂ is implicated in 3I’s composition, but metal correlations are tested only versus CO/H₂O: incorporate time-matched CO₂ production to test whether Ni/Fe release tracks CO₂ more strongly than CO or H₂O.
- Atomic fluorescence modeling uncertainties (oscillator strengths, g-factors, solar spectrum variability) are not quantified: update atomic data, propagate systematic errors, and perform sensitivity analyses on derived production rates.
- The C₂/CN correlation with NiI/FeI relies on Haser-derived Q(C₂), which is known to be model-dependent: reanalyze with vectorial/kinetic models to verify the correlation’s robustness.
- Sample comparisons pool heterogeneous datasets and methods across comets: undertake a standardized re-reduction and modeling of archival spectra to ensure consistent NiI/FeI and volatile production rates.
- Dust composition is not constrained (e.g., Ni-rich sulfides, nanograin prevalence): obtain dust spectroscopic or polarimetric diagnostics to test the nanograin/superheating mechanism.
- Survival of metal carbonyls during interstellar transit and solar irradiation is unquantified: model photodissociation/shielding and matrix trapping to assess their plausibility as parent species over Gyr timescales.
- Isotopic measurements (e.g., 58Ni/60Ni, 54Fe/56Fe) are absent: attempt isotopic ratio constraints to distinguish primordial composition differences from release-process biases.
- Instrumental/observational variations (slit widths, seeing, dichroic settings) across epochs are not fully accounted for: quantify their impact on flux calibration, spatial profiles, and production rates with rigorous error budgets.
Practical Applications
Immediate Applications
Below are applications that can be deployed now based on the paper’s findings, methods, and workflows.
- Rapid spectroscopic thermometer for comet nuclei
- Sector: Astronomy (observatories, academia)
- Use case: Infer approximate nucleus or localized surface temperatures by using the observed NiI/FeI abundance ratio as a diagnostic, especially in early activity phases when carbonyl-driven release dominates.
- Tools/products/workflows: Fluorescence modeling of NiI and FeI lines; line-intensity ratio pipeline; simple “Ni/Fe comet thermometer” calculator tied to K and carbonyl sublimation curves.
- Assumptions/dependencies: Assumes Ni(CO)/Fe(CO) sublimation is a dominant source of NiI/FeI and that photodissociation rates are comparable; requires high-S/N spectra and robust line identification; temperatures inferred are approximate and can be affected by dust-induced local heating.
- Early-activity trigger for follow-up observations
- Sector: Astronomy (telescope operations), Policy (resource allocation for interstellar object TOOs)
- Use case: Prioritize and schedule rapid-response observations when NiI is detected before FeI and CN, indicating carbonyl-driven activity at larger heliocentric distances.
- Tools/products/workflows: UVES-like spectral settings (e.g., 346/348/390/437 nm blue arms); automated detection of NiI lines and alerting; slit/seeing-aware spatial-profile check (surface brightness ∝ ).
- Assumptions/dependencies: Requires availability of medium/high-resolution spectrographs and real-time data processing; NiI preeminence as an early marker depends on comet composition.
- Observation planning and instrument configuration guidance
- Sector: Astronomy (observatories)
- Use case: Optimize echelle settings, slit widths, and spectral windows to capture FeI/NiI, OH, CN, and C bands simultaneously for efficient compositional diagnostics.
- Tools/products/workflows: Recommended UVES configurations (346/348/390/437 nm blue + 580/860 nm red); planning templates that avoid strong sky [OII] contamination near 3727–3729 Å; exposure-time calculators tuned for FeI/NiI detection thresholds.
- Assumptions/dependencies: Instrument-specific constraints (resolving power ≈ 35,000, S/N targets); sky-background management is essential for weak lines.
- Robust data-reduction and QA pipeline for faint line spectroscopy
- Sector: Software/data (instrument pipelines), Academia
- Use case: Deploy a reproducible workflow for faint emission-line extraction and quality control in comet spectra.
- Tools/products/workflows: Python
lacosmicfor cosmic-ray removal; UVES pipeline for calibration; custom 1D extraction integrated with solar-scattered light removal; fluorescence model comparison to validate FeI/NiI column densities; spatial profile checks (SB ∝ ). - Assumptions/dependencies: Access to calibrated instrument pipelines; accurate solar/twilight subtraction; model validation on known comets.
- Comet taxonomy enhancement using coupled NiI/FeI and C/CN metrics
- Sector: Academia (planetary science, astrochemistry)
- Use case: Classify and track compositional families (e.g., C-depleted comets) and identify unusual interstellar objects by combining metal-atom ratios with carbon-bearing species ratios.
- Tools/products/workflows: Multi-band production-rate dashboard; re-analysis of archival echelle spectra to populate comparative plots like Q(NiI)/Q(FeI) versus Q(C)/Q(CN).
- Assumptions/dependencies: Haser-model scalelengths and parent/daughter velocities introduce systematic effects; standardization across instruments needed.
- Comet activity forecasting for operations
- Sector: Astronomy operations, Policy (planetary defense coordination)
- Use case: Use correlation Q(FeI+NiI)/Q(HO+CO) versus CO/HO to anticipate non-water-driven activity and plan observation cadence well before perihelion.
- Tools/products/workflows: Pre-perihelion monitoring playbook integrating CO/CO measurements with NiI/FeI trends; prioritization matrix for TOOs.
- Assumptions/dependencies: Availability of CO/CO rates (e.g., from IR/sub-mm observations); correlation reflects current sample but may not generalize to all comets.
- Safety flagging for sample curation and mission concept reviews
- Sector: Space missions (sample return), Policy/Industry (lab safety)
- Use case: Incorporate potential presence of highly toxic Ni(CO) and Fe(CO) into preliminary hazard analyses and curation protocols for any cometary/interstellar sample missions.
- Tools/products/workflows: Risk registers, ventilation and monitoring requirements, PPE guidance, and containment procedures referencing organometallic carbonyl hazards.
- Assumptions/dependencies: Carbonyl presence is inferred, not directly detected; practices should be proportionate and adaptive to future in-situ/in-lab measurements.
- Education and outreach modules on interstellar objects and spectroscopy
- Sector: Education
- Use case: Teach spectroscopy, fluorescence, and comet chemistry using the paper’s figures (line evolution, production-rate plots) and workflows.
- Tools/products/workflows: Classroom labs using public spectra; simple Python notebooks reproducing line-ratio analyses; concept demos of “spectroscopic thermometer.”
- Assumptions/dependencies: Access to open data or simulated spectra; appropriate scaffolding for non-experts.
Long-Term Applications
These opportunities require further research, scaling, technology development, or confirmation of hypotheses.
- In-situ detection of metal carbonyls on comets/interstellar objects
- Sector: Space missions (payload design), Robotics
- Use case: Fly or rendezvous missions equipped to unambiguously detect Ni(CO) and Fe(CO) (and products) to validate the carbonyl sublimation hypothesis.
- Tools/products/workflows: Tuned mass spectrometers/GC-MS, UV/IR spectrometers targeting parent/daughter signatures, contamination-controlled sampling systems.
- Assumptions/dependencies: Mission opportunity and funding; sensitivity to fleeting, photolabile species; high-vacuum/low-temperature handling.
- Laboratory astrochemistry of organometallic formation in ices
- Sector: Academia (chemistry, materials science)
- Use case: Experimentally test formation pathways, stability, and sublimation kinetics of Ni(CO)/Fe(CO) in CO/CO-rich ice analogs at comet-like temperatures and UV fields.
- Tools/products/workflows: Cryogenic chambers, UV sources replicating solar spectrum, surface-analytical techniques (RAIRS, TOF-MS); kinetic modeling.
- Assumptions/dependencies: Replicability of interstellar/cometary conditions; safe handling protocols for toxic organometallics.
- Next-generation remote sensors for trace metal carbonyls
- Sector: Industry (environmental monitoring, process safety), Software/Photonics
- Use case: Explore whether atomic-line fluorescence diagnostics used here can inspire remote detection systems for metal carbonyls in industrial settings.
- Tools/products/workflows: Narrowband laser-induced fluorescence or passive spectrometers tuned to relevant atomic/fragment lines; real-time signal processing.
- Assumptions/dependencies: Translating astrophysical line detection to terrestrial environments is non-trivial; species are rare but highly toxic—cost/benefit must be clear.
- Advanced thermal-compositional models for comet nuclei
- Sector: Software/Academia
- Use case: Incorporate dust-driven local heating, carbonyl sublimation kinetics, and photodissociation pathways to predict spatiotemporal NiI/FeI release and activity evolution.
- Tools/products/workflows: Open-source multiphysics codes; data assimilation from multi-epoch spectra; uncertainty quantification for mission/observatory planning.
- Assumptions/dependencies: Requires validation against more comets; sensitivity to surface heterogeneity and porosity.
- AI-enabled global rapid-response observation networks
- Sector: Software/Policy/Astronomy operations
- Use case: Coordinate telescopes worldwide using machine-learning triggers (e.g., NiI pre-detection) to optimize coverage of interstellar objects and early-activity phases.
- Tools/products/workflows: Event brokers ingesting spectral alerts; scheduling optimizers; standardized data formats for echelle spectra.
- Assumptions/dependencies: Community buy-in; interoperability across facilities; robust false-positive control.
- Refined planet formation and chemical inheritance models
- Sector: Academia (planet formation, astrochemistry)
- Use case: Update models to include metal sequestration in carbonyls under high CO/CO conditions and its later release, linking to anomalous NiI/FeI and CO/HO ratios.
- Tools/products/workflows: Reaction-network simulations across protoplanetary disk regimes; constraints from interstellar object statistics; integration with dust growth/transport models.
- Assumptions/dependencies: Requires larger sample of interstellar comets; cross-validation with meteoritic/IDP data.
- Sample-return curation robotics and containment systems
- Sector: Robotics/Space missions/Policy
- Use case: Design automated, inert-atmosphere containment and handling systems for toxic, volatile organometallics that may be present in comet samples.
- Tools/products/workflows: Micro-robotic manipulators; gas monitoring and scrubbing; modular glovebox systems; emergency response protocols.
- Assumptions/dependencies: Depends on future sample missions and confirmed chemical risks.
- Operational risk models for spacecraft near comets
- Sector: Aerospace operations
- Use case: Integrate early volatile release (carbonyls, CO/CO) and dust heating effects into spacecraft proximity planning and contamination control.
- Tools/products/workflows: Outgassing risk simulators; plume interaction models; instrument contamination limits based on volatile chemistry.
- Assumptions/dependencies: Requires coupling to evolving activity models; data from multiple targets.
- Generalization of “spectroscopic thermometer” to other low-temperature plasmas
- Sector: Materials processing (CVD/PVD), Plasma diagnostics
- Use case: Investigate whether metal-atom line ratios can serve as temperature or precursor-decomposition diagnostics in low-temperature deposition environments.
- Tools/products/workflows: In-situ optical emission spectroscopy with calibration to precursor chemistry; control loops for process optimization.
- Assumptions/dependencies: Industrial chemistries differ markedly from cometary contexts; feasibility depends on accessible line strengths and safe precursors.
- Community-standard data and workflow packages for comet echelle spectroscopy
- Sector: Software/Data infrastructure
- Use case: Standardize pipelines and data products for FeI/NiI analyses, enabling cross-comet meta-studies and reproducibility.
- Tools/products/workflows: Versioned processing scripts (cosmic-ray removal, continuum subtraction, fluorescence fitting); shared calibration libraries; FAIR-compliant repositories.
- Assumptions/dependencies: Requires governance and maintenance; alignment across instruments beyond UVES.
Notes on overarching assumptions and dependencies:
- The central chemical interpretation—sublimation of Ni(CO) and Fe(CO) as parents of NiI/FeI—is plausible and quantitatively consistent in key regimes but not yet directly confirmed; many applications hinge on this being at least partly true.
- Accurate applications depend on high-quality spectroscopy (resolution, S/N), careful sky-subtraction, and validated fluorescence models.
- Trends and correlations (e.g., with CO/HO or C/CN) derive from a limited sample and may evolve as more interstellar objects are observed.
- Safety-related applications should be proportionate: they anticipate potential risks for future sample missions without implying immediate terrestrial hazard.
Glossary
- au (astronomical unit): Standard unit of astronomical distance equal to the average Earth–Sun distance. "heliocentric distances ranging from 3.14 to 2.14 au."
- blackbody equilibrium temperature: The temperature an object attains when absorbed radiation balances emitted blackbody radiation. "the surface blackbody equilibrium temperature is too low to allow the sublimation of refractory minerals containing these metals."
- carbonyls: Organometallic complexes where a metal is bonded to carbon monoxide; here Ni(CO) and Fe(CO). "We interpreted these observations assuming that the NiI and FeI atoms were released through the sublimation of Ni(CO) and Fe(CO) carbonyls, which supports the presence of these species in the cometary material."
- CN(0-0) band: A specific molecular emission band of cyanogen used to estimate CN production rates. "the CN(0-0) band at 3870~\AA"
- column density: The number of particles per unit area along the line of sight. "to derive FeI and NiI column densities."
- coma: The diffuse atmosphere of gas and dust surrounding a comet’s nucleus. "Extreme NiI/FeI abundance ratio in the coma of the interstellar comet 3I/ATLAS"
- continuum-subtracted spectra: Spectra processed to remove the continuum, highlighting emission/absorption features. "Continuum-subtracted spectra of comet 3I obtained on August 28 and September 3+4."
- dichroic: An optical element that splits light into separate wavelength bands. "346+580 (dichroic 1), 390+580 (dichroic 1), and 437+860 (dichroic 2) were used."
- echelle spectrograph: A high-resolution spectrograph using an echelle grating to disperse light. "equipped with the UV-Visual Echelle Spectrograph(UVES\footnote{UVES User Manual, VLT-MAN-ESO-13200-1825, \ https://www.eso.org/sci/facilities/paranal/instruments/instruments.html})."
- ESO Very Large Telescope (VLT): A major optical observatory operated by the European Southern Observatory. "Observations were carried out with the Very Large Telescope (VLT) at the European Southern Observatory (ESO), equipped with the UV-Visual Echelle Spectrograph(UVES..."
- fluorescence efficiencies: Factors describing how effectively absorbed photons are re-emitted, used in production rate calculations. "using the fluorescence efficiencies and Haser scalelengths from \citet{Schleicher1988}, \citet{Cochran1993}, \citet{AHearn1995}, and \citet{Schleicher2010}"
- fluorescence model: A model that computes expected emission-line intensities from fluorescence to infer abundances. "by comparing the observed line intensities with those produced by a fluorescence model."
- FWHM (full width at half maximum): A measure of the width of a profile determined at half its maximum amplitude. "convolved with a 1.5\arcsec\ FWHM Gaussian to account for the seeing and tracking imperfections."
- geocentric distance: The distance from Earth to the observed object. "in spectra obtained for comets 103P/Hartley2 and 46P at geocentric distances of 0.17 and 0.09 au"
- Haser model: A parametric model of cometary comae used to estimate production rates via parent/daughter species. "which is an artifact produced by the oversimplified Haser model used when deriving the C production rates"
- heliocentric distance: The distance from the Sun to the observed object. "at heliocentric distances ranging from 3.14 to 2.14 au."
- Jupiter-family comets (JFCs): Short-period comets dynamically influenced by Jupiter. "Solar-system comets were classified into two broad categories: Jupiter-family comets (JFCs) and Oort-cloud comets (OCCs)."
- lacosmic: An algorithm/package for detecting and removing cosmic ray hits in astronomical images. "using the python implementation of the ``lacosmic'' package \citep{2001VanDokkum,2012VanDokkum}."
- nanoparticles: Extremely small solid grains that can reach high temperatures (superheat) relative to bulk material. "superheating of Ni-rich sulfides, possibly located in nanoparticles"
- NiI/FeI abundance ratio: The relative abundance (or production rate) of neutral nickel to neutral iron atoms. "with a NiI/FeI abundance ratio similar to the solar system comets."
- nucleocentric distance: The projected distance from the center of a comet’s nucleus. "where is the projected nucleocentric distance"
- OII forbidden emission: Emission from forbidden transitions of singly ionized oxygen, denoted by square brackets. "Broad emission features due to background [OII] emission at 3727\AA\ and 3729\AA\ are also observed."
- Oort-cloud comets (OCCs): Long-period comets originating from the distant Oort Cloud. "Solar-system comets were classified into two broad categories: Jupiter-family comets (JFCs) and Oort-cloud comets (OCCs)."
- parent and daughter velocity: Expansion speeds assigned to parent and daughter species in the Haser model. "with a parent and daughter velocity of 0.85 km s \citep{Cochran1993}."
- photocenter: The center of light of an image or profile. "The measured surface brightness (SB; normalized to one at the photocenter) is plotted..."
- photodissociation: The breaking of chemical bonds in a molecule due to absorption of photons. "\citet{Bromley2021} showed that the photodissociation of Ni(CO) and Fe(CO) into NiI and FeI occurs at very similar rates"
- production rate Q(X): The number of particles of species X produced per second in the coma. "Production rate ratio Q(FeI+NiI)/Q(HO+CO) versus Q(CO)/Q(HO) for interstellar and solar system comets."
- resolving power: A spectrograph’s ability to distinguish closely spaced wavelengths. "Most of the time, a 1.8\arcsec-wide slit was used, delivering a resolving power of about 35000."
- seeing: Atmospheric blurring affecting astronomical observations. "to account for the seeing and tracking imperfections."
- sublimation: Phase change from solid directly to gas; a key process driving cometary activity. "too low to allow the sublimation of silicate and sulfide minerals containing the metals."
- Sun-grazing comet: A comet that passes extremely close to the Sun, often leading to strong heating and vaporization. "in the coma of the Sun-grazing comet Ikeya-Seki \citep{Manfroid2021}."
- surface brightness: Brightness per unit area on the sky. "The measured surface brightness (SB; normalized to one at the photocenter) is plotted as a function of the projected nucleocentric distance in arcsec."
- UVES (UV-Visual Echelle Spectrograph): A high-resolution echelle spectrograph mounted on the VLT. "equipped with the UV-Visual Echelle Spectrograph(UVES\footnote{UVES User Manual, VLT-MAN-ESO-13200-1825, \ https://www.eso.org/sci/facilities/paranal/instruments/instruments.html})."
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