The Galactic-halo stars with [Fe/H] ≤ −5 — including SMSS J031300.36-670839.3, in which no iron lines are detected — define the observational frontier of stellar archaeology. Komiya, Suda & Fujimoto (2015) proposed that these most iron-deficient stars are low-mass Population III stars whose surfaces were polluted after birth by accretion of interstellar matter, and identified the unresolved issues this scenario faces: the unknown mass loss and winds of metal-free low-mass stars, which control whether accretion proceeds; the artificial overabundance that homogeneous mixing of supernova ejecta produces in the model metallicity distribution function near [Fe/H] ≈ −5 to −4; the unmodelled origin of the non-iron abundance anomalies (C, N, O, Na, Mg, Al, Sr) via asymptotic-giant-branch nucleosynthesis and binary mass transfer; the prediction and search for un-polluted Population III survivors dynamically expelled from their birth halos; and the fine tuning of iron yields demanded by the competing faint-supernova enrichment scenario. A decade of follow-up work has strengthened the second-generation interpretation and challenged the interstellar-accretion mechanism (wind blocking; interstellar-object accretion as an alternative channel), but no pristine Population III star has been found and no decisive test has discriminated the scenarios. The problem of establishing the physical origin of the most iron-deficient stars, together with the existence and detectability of pristine Population III survivors, remains open.

Background

Population III (Pop III) stars are the first generation of stars, formed from primordial, metal-free gas before any heavy elements existed. Stellar metallicity is conventionally expressed as [Fe/H], the logarithm of the iron-to-hydrogen ratio relative to the Sun; the Galactic halo contains extremely metal-poor (EMP) stars with [Fe/H] < −3, ultra metal-poor (UMP) stars with −5 < [Fe/H] ≤ −4.5, and hyper metal-poor (HMP) stars with [Fe/H] ≤ −5. As of 2015 only a handful of HMP stars were known — HE 0107-5240 ([Fe/H] = −5.4), HE 1327-2326 ([Fe/H] = −5.7), and the record holder SMSS J031300.36-670839.3, in which no iron lines are detected ([Fe/H] < −7.5) and only calcium could be measured. Because subsolar-mass stars live longer than the current age of the Universe, any low-mass Pop III star formed in the first minihalos should still exist today; none has ever been found. The metallicity distribution function (MDF), the number of halo stars per unit [Fe/H], is the key statistical diagnostic of early chemical enrichment.

Two origins have been proposed for the HMP stars. In the now-standard picture they are second-generation stars, born from gas enriched by one or a few faint Pop III supernovae with mixing and fallback — explosions in which most iron-group material falls back onto the compact remnant, so that the ejecta are carbon-rich but iron-poor. In the surface-pollution picture advocated by Komiya, Suda & Fujimoto (2015), the HMP stars are genuinely Pop III stars, metal-free at birth, whose surfaces were contaminated after birth by Bondi-Hoyle accretion — the classical formula for the rate at which a star moving through ambient gas gravitationally captures it — of metal-enriched interstellar matter during the hierarchical merging of the protogalaxy. Komiya et al. computed this with a merger-tree chemical-evolution model (a semi-analytic representation of dark-matter halo growth by repeated mergings) and reported that the observed halo MDF, including its lowest-metallicity tail, is reproduced if the HMP population consists of such polluted survivors, with the model MDF of polluted Pop III giants peaking near [Fe/H] ≈ −5. The large non-iron enhancements of HMP/UMP stars — most are carbon-enhanced (CEMP), with elevated N, O, Na, Mg, Al, and Sr but weak neutron-capture elements (the CEMP-no subclass) — are in this picture attributed to mass transfer from an asymptotic-giant-branch (AGB) binary companion, the late evolutionary phase in which low- and intermediate-mass stars synthesize carbon and s-process elements (heavy nuclei built by slow neutron captures) and dredge them to the surface.

Komiya et al. themselves list what their scenario leaves unresolved. Mass loss from metal-free low-mass stars — with no metals and possibly no magnetic fields to drive solar-type winds — is essentially unknown, yet it decides whether interstellar accretion is blocked or proceeds. The model's assumption of homogeneous mixing of supernova ejecta within massive halos artificially overpopulates the MDF near [Fe/H] ≈ −5 to −4. The detailed AGB nucleosynthesis and internal mixing that would explain the observed non-iron anomalies were deferred to later work. The scenario predicts that un-polluted Pop III survivors should exist — dynamically ejected from their birth halos before pollution — and must be searched for, with their expected spatial and kinematic distributions still to be worked out. Finally, the rival faint-supernova scenario requires iron ejection suppressed by a factor of order 10^4 relative to ordinary supernovae, a fine tuning the authors regard as implausible, but quantifying which scenario actually produced each known object was left open. A decade of subsequent work has deepened rather than settled this confrontation, which is what makes the problem stated below.

Problem Statement

Determine the physical origin of the iron-group content of the Galactic-halo stars with [Fe/H] ≤ −5 — the hyper metal-poor stars, including SMSS J031300.36-670839.3, in which no iron lines are detected — and, inseparably, establish whether genuinely metal-free low-mass Population III stars survive in the Milky Way and its satellites.

The following sub-questions are posed jointly by this origin question:

(i) which post-birth pollution channels actually operate on a surviving low-mass Population III star — Bondi-Hoyle-type accretion of interstellar gas, and accretion of interstellar objects (free-floating planetesimals such as 'Oumuamua, the first interstellar object discovered passing through the Solar System), both modulated by the star's own winds and magnetosphere — and what surface metallicity each produces as a function of stellar mass, orbit, and epoch, given that mass loss from metal-free (and possibly magnetically weak) low-mass stars is essentially unconstrained;

(ii) whether the resulting surface-pollution history, embedded in a hierarchical chemical-evolution model with realistic (inhomogeneous) mixing of supernova ejecta, reproduces the observed halo metallicity distribution function down to its lowest-metallicity tail and the full abundance patterns of the known hyper and ultra metal-poor stars — including their carbon, nitrogen, oxygen, sodium, magnesium, aluminium, and strontium anomalies — and does so without the extreme per-object fine tuning of iron yields that the competing faint-supernova (mixing-and-fallback) second-generation scenario is argued to demand; and

(iii) the expected number, spatial distribution, and kinematics of un-polluted Population III survivors (pristine, or [Fe/H] ≲ −7) expelled from their birth halos, together with the observational status of their existence.

The question that ties these sub-questions together is which of the two competing origin scenarios for the most iron-deficient stars holds: post-birth surface pollution of low-mass Population III survivors, or formation from gas enriched by faint Population III supernovae. None of the sub-questions settles this on its own: the wind physics, or the asymptotic-giant-branch origin of a single abundance anomaly, does not by itself decide the origin question.

References

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