HE 1327-2326, an unevolved star with [Fe/H] < -5.0 III. Does Its Atmosphere Reflect Its Natal Composition?

TL;DR

This study investigates the stellar parameters and surface composition of the hyper-metal-poor star HE 1327-2326, examining whether its atmosphere reflects its original formation conditions or has been altered by internal processes.

Contribution

It combines spectroscopic analysis and stellar evolution modeling to assess the star's evolutionary stage and the impact of atomic diffusion on its surface abundances.

Findings

HE 1327-2326 is likely a subgiant, not a turnoff star.

Atomic diffusion can significantly alter surface abundances over time.

Lithium abundance remains inconsistent with primordial levels despite diffusion corrections.

Abstract

Based on spectroscopic constraints derived from NLTE line formation, we explore the likely range of stellar parameters (T_eff and log g) for the hyper-metal-poor (HMP) star HE 1327-2326. Combining the constraints from Balmer line profiles and the Ca I/II ionization equilibrium, a subgiant stage of evolution is indicated. This result is further supported by spectrophotometric observations of the Balmer jump. If a higher T_eff value was used (as favoured by some photometric calibrations), the spectroscopic analysis would indicate a turnoff-point stage of evolution. Using a stellar-structure code that treats the effects of atomic diffusion throughout the star in detail, we evolve a low-mass model star to reach the HR-diagram position of HE 1327-2326 after roughly 13 Gyr. While the surface abundances are modified significantly (by more than 1 dex for the case of uninhibited diffusion), such corrections can not resolve the discrepancy between the abundance inferred from the non-detection of the Li I resonance line at 6707 Angstroem and the WMAP-based primordial lithium abundance. As there are numerous processes that can destroy lithium, any cosmological interpretation of a lower-than-expected lithium abundance at the lowest metallicities will have to await sample sizes of unevolved hyper-metal-poor stars that are one order of magnitude larger. The situation remains equally inconclusive concerning atomic-diffusion corrections. Here, attempts have to be made to better constrain internal mixing processes, both observationally and by means of sophisticated modelling. With constraints on additional mixing processes taken from a recent globular-cluster study, the likeliest scenario is that HE 1327-2326's surface abundances have undergone mild depletion (of order 0.2 dex).