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Distribution of Europium in The Milky Way Disk; Its Connection to Planetary Habitability and The Source of The R-Process

Evan M. Carrasco, Matthew Shetrone, Francis Nimmo, Enrico Ramirez-Ruiz, Joel Primack, Natalie M. Batalha

TL;DR

This study establishes a tight intrinsic dispersion of $[Eu/H]$ in the local Galactic disk by detrending Eu abundances against $T_{eff}$ and $[\alpha/H]$, yielding $\sigma_{Intrinsic} \approx 0.025$ dex. The observed anticorrelation between $[Eu/\alpha]$ and $[\alpha/H]$ constrains the dominant r-process enrichment sites, favoring either metallicity-dependent core-collapse supernovae or NS-NS mergers with nonstandard delay-time distributions. By treating Eu as a proxy for the radiogenic actinides U and Th, the authors link Galactic chemical evolution to planetary geodynamics, predicting that near-solar metallicity stars are more likely to host Earth-like planets with persistent dynamos, while low-$[\alpha/H]$ environments are prone to extended dynamo failures. Overall, the work connects stellar nucleosynthesis, Galactic chemical evolution, and exoplanet habitability, offering a metallicity-informed view of where robust planetary dynamos are most likely to persist.

Abstract

The energy provided in the radioactive decay of thorium (Th) and uranium (U) isotopes, embedded in planetary mantles, sustains geodynamics important for surface habitability such as the generation of a planetary magnetic dynamo. In order to better understand the thermal evolution of nearby exoplanets, stellar photospheric abundances can be used to infer the material composition of orbiting planets. Here we constrain the intrinsic dispersion of the r-process element europium (Eu) (measured in relative abundance [Eu/H]) as a proxy for Th and U in local F, G, and K type dwarf stars. Adopting stellar-chemical data from two high quality spectroscopic surveys, we have determined a small intrinsic scatter of 0.025 dex in [Eu/H] within the disk. We further investigate the stellar anti-correlation in [Eu/$α$] vs [$α$/H] at late metallicities to probe in what regimes planetary radiogenic heating may lead to periods of extended dynamo collapse. We find that only near-solar metallicity stars in the disk have Eu inventories supportive of a persistent dynamo in attendant planets, supporting the notion of a ``metallicity Goldilocks zone'' in the galactic disk. The observed anti-correlation further provides novel evidence regarding the nature of r-processes injection by substantiating $α$ element production is decoupled from Eu injection. This suggests either a metallicity-dependent r-process in massive core-collapse supernovae, or that neutron-star merger events dominate r-process production in the recent universe.

Distribution of Europium in The Milky Way Disk; Its Connection to Planetary Habitability and The Source of The R-Process

TL;DR

This study establishes a tight intrinsic dispersion of in the local Galactic disk by detrending Eu abundances against and , yielding dex. The observed anticorrelation between and constrains the dominant r-process enrichment sites, favoring either metallicity-dependent core-collapse supernovae or NS-NS mergers with nonstandard delay-time distributions. By treating Eu as a proxy for the radiogenic actinides U and Th, the authors link Galactic chemical evolution to planetary geodynamics, predicting that near-solar metallicity stars are more likely to host Earth-like planets with persistent dynamos, while low- environments are prone to extended dynamo failures. Overall, the work connects stellar nucleosynthesis, Galactic chemical evolution, and exoplanet habitability, offering a metallicity-informed view of where robust planetary dynamos are most likely to persist.

Abstract

The energy provided in the radioactive decay of thorium (Th) and uranium (U) isotopes, embedded in planetary mantles, sustains geodynamics important for surface habitability such as the generation of a planetary magnetic dynamo. In order to better understand the thermal evolution of nearby exoplanets, stellar photospheric abundances can be used to infer the material composition of orbiting planets. Here we constrain the intrinsic dispersion of the r-process element europium (Eu) (measured in relative abundance [Eu/H]) as a proxy for Th and U in local F, G, and K type dwarf stars. Adopting stellar-chemical data from two high quality spectroscopic surveys, we have determined a small intrinsic scatter of 0.025 dex in [Eu/H] within the disk. We further investigate the stellar anti-correlation in [Eu/] vs [/H] at late metallicities to probe in what regimes planetary radiogenic heating may lead to periods of extended dynamo collapse. We find that only near-solar metallicity stars in the disk have Eu inventories supportive of a persistent dynamo in attendant planets, supporting the notion of a ``metallicity Goldilocks zone'' in the galactic disk. The observed anti-correlation further provides novel evidence regarding the nature of r-processes injection by substantiating element production is decoupled from Eu injection. This suggests either a metallicity-dependent r-process in massive core-collapse supernovae, or that neutron-star merger events dominate r-process production in the recent universe.

Paper Structure

This paper contains 15 sections, 5 equations, 2 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Methodology
  3. Systematic Biases in Metallicity - $\alpha$ Elements
  4. Systematic Biases in $T_{\rm eff}$
  5. Detrended Contour Fitting
  6. Results and Analysis
  7. Combining Data Sets
  8. Intrinsic Dispersion
  9. Discussion
  10. Implications for r-Process Production
  11. Implications Regarding Dynamo Failure Duration
  12. Conclusion
  13. Appendix
  14. Solar Twins Analysis
  15. Potential outliers

Figures (2)

  • Figure 1: (a) Top panel shows [Eu/$\alpha$] independent of the temperature bias as a function of [$\alpha$/H]. Small orange triangles represent data from DM and small green triangles represent data from BB. The cross matched sample of 68 stars described in Section \ref{['sec:combining data sets']} is shown in big red squares. Stars from the solar twins analysis by Spina2018, Bedell2018 are shown as big blue circles. A black star is used to represent the solar abundance value. The red dashed line at [Eu/$\alpha$] = 0.06 represents the critical value for an extended dynamo failure given by the model from Nimmo2020. Outliers in [Eu/H] abundances discussed in Section \ref{['sec:Outliers']} are outlined in black.(b) The bottom panel shows [Eu/Fe] independent of the temperature bias as a function of [Fe/H]. The legend is the same as in (a).
  • Figure 2: Dynamo failure duration for an Earth-like planet as a function of terrestrial radiogenic element concentrations (U and Th) relative to $\alpha$ elements. A value of [0] indicates an Earth-like ratio.