NiI/FeI Abundance Ratio in Astrophysics

Updated 3 October 2025

NiI/FeI abundance ratio is the relative measure of neutral nickel to neutral iron atoms detected via optical spectroscopy, crucial for understanding nucleosynthetic pathways.
Observations across comets, stars, and galaxy clusters show elevated ratios compared to solar values, indicating unique sublimation and release mechanisms.
Advanced spectroscopic techniques and plasma modeling provide actionable insights into the chemical evolution and metallicity of diverse cosmic environments.

The NiI/FeI abundance ratio is an astrophysical diagnostic quantifying the relative abundance of neutral nickel (Ni I) to neutral iron (Fe I), primarily derived from emission line spectroscopy in various cosmic environments. This ratio is pivotal in tracing nucleosynthetic processes, chemical evolution pathways, and the conditions governing the release and incorporation of iron-peak elements in planetary systems, stellar populations, the interstellar medium, and intracluster plasma. Recent high-resolution spectroscopic campaigns in comets—both solar system and interstellar—as well as galactic and cluster studies, have elucidated surprising systematics and diversity in NiI/FeI ratios, revealing complex underlying mechanisms of elemental release and enrichment.

1. Physical and Observational Definition

The NiI/FeI abundance ratio refers specifically to the number ratio of neutral nickel to neutral iron atoms, typically measured in emission in tenuous astrophysical plasmas such as cometary comae. In cometary contexts, this is expressed via production rates, $Q(\mathrm{NiI})$ and $Q(\mathrm{FeI})$ , with the abundance ratio often quoted as $\log_{10}\left[\frac{Q(\mathrm{NiI})}{Q(\mathrm{FeI})}\right]$ (Hutsemékers et al., 2021, Opitom et al., 2021, Hutsemékers et al., 30 Sep 2025).

For the solar photosphere, the elemental Ni/Fe abundance ratio (irrespective of ionization state) is $\log_{10}(\mathrm{Ni}/\mathrm{Fe})_\odot \approx -1.25 \pm 0.04$ (Hutsemékers et al., 2021, Hutsemékers et al., 30 Sep 2025). In most cometary comae, measured values of $\log_{10}\left[\frac{Q(\mathrm{NiI})}{Q(\mathrm{FeI})}\right]$ cluster around $-0.06 \pm 0.31$ , i.e., up to an order of magnitude higher than solar, highlighting an unexpected enrichment or release mechanism (Hutsemékers et al., 2021). Recent observations of interstellar comets such as 2I/Borisov and 3I/ATLAS have extended these studies beyond the solar system context (Opitom et al., 2021, Hutsemékers et al., 30 Sep 2025).

Table 1: Typical log(NiI/FeI) Abundance Ratios

Environment	log(NiI/FeI)	Reference
Solar photosphere	$-1.25 \pm 0.04$	(Hutsemékers et al., 2021 Hutsemékers et al., 30 Sep 2025)
Solar system comets	$-0.06 \pm 0.31$	(Hutsemékers et al., 2021)
2I/Borisov (interstellar)	$0.21 \pm 0.18$	(Opitom et al., 2021)
3I/ATLAS (interstellar; max.)	$1.27$ (at $r_h=2.64$ au)	(Hutsemékers et al., 30 Sep 2025)

2. Measurement Techniques and Methodological Considerations

The NiI/FeI ratio is determined by high-resolution optical spectroscopy targeting specific atomic lines. For comets, the methodology involves:

Acquisition of spectra covering Ni I and Fe I lines (e.g., 3028, 3414, 4421 Å for Ni I; 3720, 3860, 3729 Å for Fe I) at multiple heliocentric distances (Hutsemékers et al., 30 Sep 2025, Hutsemékers et al., 2021).
Measurement of line intensities and application of fluorescence models accounting for solar irradiance, excitation mechanisms, and relevant atomic data to derive column densities and production rates (Hutsemékers et al., 30 Sep 2025, Opitom et al., 2021).
Empirical scaling of spatial surface brightness profiles (typically declining as $p^{-1}$ with projected nucleocentric distance) as evidence for near-nucleus origin of the emitting atoms (Hutsemékers et al., 30 Sep 2025, Hutsemékers et al., 2021).
Conversion of observed flux ratios to abundance ratios using theoretical or laboratory-derived calibration factors.
In X-ray studies of intracluster media (ICM), the Ni/Fe ratio is derived from analysis of blends such as the 7.8 keV Ni K $\alpha$ feature relative to the Fe K $\alpha$ line, modeled via APEC or MEKAL plasma codes (Matsushita et al., 2012).

Systematic uncertainties in plasma codes, excitation models, and atomic data can affect the derived ratios at the 10% level or greater, particularly in regions of the spectrum subject to instrumental calibration limits or line blending (Matsushita et al., 2012).

3. Key Results Across Astrophysical Contexts

3.1 Comets: Solar System and Interstellar

Cometary comae display systematically elevated NiI/FeI ratios compared to solar values. For 20 comets considered (including both Jupiter-family and Oort-cloud objects), the mean ratio is $\log_{10}(\mathrm{NiI}/\mathrm{FeI}) = -0.06 \pm 0.31$ (Hutsemékers et al., 2021). For the interstellar comet 2I/Borisov, a similar ratio of $0.21 \pm 0.18$ is observed (Opitom et al., 2021). However, 3I/ATLAS exhibits an extreme ratio, peaking at $\log_{10}(\mathrm{NiI}/\mathrm{FeI}) \approx 1.27$ at $r_h = 2.64$ au, subsequently dropping to near-solar system comet values as perihelion approached (Hutsemékers et al., 30 Sep 2025).

A key observational signature is that NiI lines can be detected at larger heliocentric distances than FeI, with FeI emerging only at reduced $r_h$ , indicating a difference in release/sublimation thresholds (Hutsemékers et al., 30 Sep 2025). The variance in ratio is notably higher for Jupiter-family comets than Oort-cloud comets, pointing to greater heterogeneity in the source reservoirs (Hutsemékers et al., 2021).

3.2 Galactic and Cluster Environments

In both the Milky Way and the intracluster medium of massive galaxy clusters (e.g., the Coma cluster), the Ni/Fe abundance ratio (in all ionization states) is generally within a factor of unity of the solar value, typically $[{\rm Ni/Fe}] \approx 0$ in Galactic stars and $0.6$–$1.5$ in solar units in the Coma ICM (Matsushita et al., 2012, Eitner et al., 2022). In the Coma cluster core, the ratio is tightly constrained to this range and does not exhibit a strong radial gradient.

In Galactic stellar populations, state-of-the-art non-LTE analyses of [Ni/Fe] reveal only a mild anti-correlation with metallicity, suggestive of a dominant role for sub-Chandrasekhar-mass SNe Ia in recent Galactic enrichment (Eitner et al., 2022).

4. Physical Mechanisms and Astrophysical Implications

4.1 Origin and Ejection of Nickel and Iron

The spatial and temporal profiles of NiI/FeI emission in comets suggest that both elements are released from very near the nucleus surface, or from the rapid photo-destruction of highly volatile parents (Hutsemékers et al., 2021, Hutsemékers et al., 30 Sep 2025). The canonical equilibrium surface temperature at large $r_h$ is insufficient to sublimate refractory silicates; thus, alternative mechanisms are invoked:

Sublimation of organometallic carbonyls (Ni(CO)₄ and Fe(CO)₅), which have low binding energies and can release Ni and Fe at relatively low temperatures (Hutsemékers et al., 30 Sep 2025).
Superheating of Ni-rich sulfides, or photolytic/photodestructive release from complex dust aggregates (Hutsemékers et al., 2021).

Because Ni(CO)₄ volatilizes at lower temperatures than Fe(CO)₅, NiI can be released at larger distances, yielding a high NiI/FeI ratio early in activity phases (as in 3I/ATLAS). As temperatures increase closer to the Sun, FeI is released more efficiently, suppressing the ratio over time and bringing it into the range found in solar system comets (Hutsemékers et al., 30 Sep 2025).

4.2 Correlations with Cometary and Cosmic Chemistry

The NiI/FeI ratio in comets is correlated with volatile and organic proxies such as C $_2$ /CN, C $_2$ H $_6$ /H $_2$ O, and NH/CN (Hutsemékers et al., 2021). Carbon-chain– and NH-depleted comets exhibit higher ratios, suggesting a primordial origin linked to the formation environment rather than to secondary processing in the coma. This correlation positions the NiI/FeI ratio as a novel diagnostic of the chemical and thermal conditions in the protoplanetary disk regions from which comets originate.

In Galactic stars and the ICM, the ratio traces the integrated yields of iron-peak elements from core-collapse and Type Ia SNe. In particular, the observed [Ni/Fe] trend (and its anti-correlation with [Fe/H] in NLTE) supports a scenario in which sub-Chandrasekhar-mass SNe Ia (producing less Ni per unit Fe) dominate iron enrichment at later times (Eitner et al., 2022, Matsushita et al., 2012).

5. Evolution, Diversity, and Outstanding Questions

The time evolution of NiI/FeI in comet 3I/ATLAS—initially extremely high, then decreasing toward expected values—demonstrates that surface composition, temperature-dependent sublimation physics, and perhaps dust-driven insulation or thermal gradients play major roles in shaping observed abundance ratios (Hutsemékers et al., 30 Sep 2025). The broader spread in the ratio among Jupiter-family comets, mirroring other compositional tracers, suggests that the Kuiper belt and related reservoirs retained more diversity in their building blocks, whereas Oort cloud comets are more chemically homogeneous (Hutsemékers et al., 2021).

The universality of elevated NiI/FeI in comets (relative to the solar elemental ratio) remains an open question and may point toward widespread presence of Ni-rich carbonyls or other non-refractory Ni-rich compounds in soluble cometary materials. The origin of this chemical state—inheritance from molecular clouds, nebular chemistry, or alteration by early irradiation—remains debated.

In the context of clusters and galaxies, the absence of strong anomalies in Ni/Fe compared to the solar benchmark and the consistency across mass scales reinforce the view that iron-peak element enrichment is dictated by well-mixed contributions from both SNe Ia and SNe II, and that large-scale mixing and feedback preserve a universal enrichment pattern (Matsushita et al., 2012).

6. Research Directions and Methodological Developments

Future progress on the NiI/FeI ratio as a cosmochemical probe entails:

Continued spectroscopic surveys of comets—targeting both interstellar and diverse dynamical classes—at multiple heliocentric distances and activity phases (Hutsemékers et al., 30 Sep 2025).
Laboratory simulations of Ni(CO)₄ and Fe(CO)₅ sublimation under nucleus-like thermal and radiative environments, to calibrate sublimation rates and model temperature sensitivity (Hutsemékers et al., 30 Sep 2025).
Detailed modeling incorporating thermal gradients, dust blanketing, and non-uniform sublimation to explain rapid changes in observed ratios and nucleus heterogeneity.
Cross-comparison of NiI/FeI with additional chemical indicators (e.g., CO/H $_2$ O, C $_2$ /CN) and isotopic measurements to constrain formation scenarios (Hutsemékers et al., 2021).
In galactic and cluster settings, continued refinement of NLTE abundance analyses and joint chemo-dynamical modeling of element enrichment, with a focus on SN Ia channel fractions and their variable nucleosynthetic yield patterns (Eitner et al., 2022).

A plausible implication is that further paper of NiI/FeI diversity—spanning comets, stars, and diffuse plasmas—will illuminate both local and cosmological processes of metallicity evolution and allow robust discrimination among competing models of solar system and galactic assembly.

References:

(Matsushita et al., 2012, Opitom et al., 2021, Hutsemékers et al., 2021, Eitner et al., 2022, Hutsemékers et al., 30 Sep 2025)