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).
