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

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).