[Paper Review] Are all metal-poor stars of second-generation?

Irene Vanni, Stefania Salvadori+1 more|arXiv (Cornell University)|May 3, 2023

Stellar, planetary, and galactic studies4 citations

TL;DR

This study uses a parametric model of stellar enrichment to show that not all metal-poor halo stars are pure second-generation descendants of Population III stars. By comparing observed carbon and iron abundances in 132 metal-poor stars, it finds that only the most carbon-enhanced, iron-poor stars (with [C/Fe] ≥ +2.5) are predominantly enriched by massive, low-energy Pop III supernovae, while most C-normal and less C-enhanced stars show significant contributions from later Pop II stars.

ABSTRACT

Hydrodynamical cosmological simulations predict that the metal-free Population III (Pop III) stars were likely very massive and, therefore, short-lived. However, they left their chemical imprint on their descendants, which can also have masses $ < 0.8 \mathrm {M_{\odot}}$ and still be alive today. The Milky Way stellar halo is one of the oldest and most metal-poor component of the Local Group and a peculiar class of stars, the so-called Carbon-Enhanced Metal-Poor (CEMP-no) stars, seem to be directly related to Pop III stars. We aim at revealing if all metal-poor halo stars are true second-generation stars or if they have also been enriched by the subsequent generations of normal (Pop II) stars. For this purpose, we compare the measured carbon and iron abundances of the metal-poor halo stars with the ones predicted by our simple parametric model, varying the pollution level from Pop III and normal stars. We find that only the most C-enhanced and Fe-poor stars enclose in their photospheres the pure imprint of Pop III stars, while, as the [C/Fe] decreases, the probability of being also polluted by normal Pop II stars increases.

Motivation & Objective

• To determine whether all metal-poor halo stars are true second-generation stars formed from gas enriched solely by Population III stars.
• To quantify the relative contribution of Population III versus normal Population II stars to the chemical enrichment of metal-poor stellar birth clouds.
• To develop a diagnostic framework using carbon and iron abundances to distinguish stars primarily enriched by Pop III stars from those significantly polluted by later Pop II stars.
• To assess the limitations of using only C and Fe abundances in disentangling Pop III supernova explosion models and identify the need for broader elemental analysis.
• To provide a general parametric model applicable to both Galactic halo stars and high-redshift interstellar medium observations to trace the chemical imprint of first stars.

Proposed method

• A simple parametric model is used to simulate the chemical enrichment of interstellar medium (ISM) by Population III and normal Population II stars, varying the pollution level from each source.
• The model computes expected [C/Fe] ratios for stars formed in ISM enriched by different combinations of Pop III and Pop II supernovae, including varying explosion energies (e.g., low-energy, hypernovae).
• Observed high-resolution abundance data from 132 metal-poor halo stars (including CEMP-no and C-normal stars with [Fe/H] ≤ -2) are compared against model predictions.
• The model accounts for different Pop III SN types, including low-energy (E_SN ≤ 1.2×10⁵¹ erg) and high-energy (E_SN ≥ 3×10⁵¹ erg) explosions, to assess their distinct chemical imprints.
• Statistical comparison is performed between observed abundance scatter (especially in [C/Fe]) and model predictions to infer the dominant enrichment source.
• The analysis includes stars with [Fe/H] down to -7.1, focusing on ultra-metal-poor and extremely metal-poor regimes to isolate primordial signatures.

Experimental results

Research questions

• RQ1 Are all metal-poor halo stars exclusively descendants of Population III stars, or have they been significantly polluted by later generations of normal Population II stars?
• RQ2 What level of Pop III enrichment is required to reproduce the observed [C/Fe] ratios in CEMP-no and C-normal metal-poor stars?
• RQ3 How does the inclusion of Pop II star pollution affect the observed scatter in [C/Fe] among metal-poor stars, and can this help distinguish between different Pop III SN explosion models?
• RQ4 Can the observed abundance patterns in C-normal stars with [Fe/H] < -2 be explained by Pop III-only enrichment, or is a significant Pop II contribution required?
• RQ5 To what extent do the chemical abundance patterns of the most metal-poor stars reflect the properties of the first supernovae, and what diagnostic power do C and Fe abundances offer in identifying first-star descendants?

Key findings

• CEMP-no stars with [C/Fe] ≥ +2.5 are predominantly enriched by massive Population III stars that exploded with low to normal energy (E_SN ≤ 1.2×10⁵¹ erg), indicating they are pure second-generation stars.
• CEMP-no stars with [C/Fe] < +2.5 likely experienced a non-negligible contribution (~10–30%) from normal Population II stars, indicating mixed enrichment from both Pop III and Pop II sources.
• The majority of C-normal metal-poor stars have been enriched by normal Population II stars at a level of ≥50%, meaning they do not retain strong chemical features from Pop III stars.
• The scatter in [C/Fe] is significantly larger (~5 dex) for stars purely enriched by Pop III stars, but reduces to ~2 dex when Pop III and Pop II contributions are comparable, matching observed data.
• High-energy Pop III supernovae (E_SN ≥ 3×10⁵¹ erg) can produce [C/Fe] ≲ 0, implying that rare C-normal stars with low carbon-to-iron ratios may be direct descendants of such hypernovae.
• The model predicts that a C-excess in high-redshift gaseous systems (z > 4) is a strong indicator of chemical enrichment by Population III stars, and abundance scatter is a key diagnostic for the dominance of Pop III pollution.