HE 1327-2326, An Unevolved Star with [Fe/H] < -5.0. III. Does its Atmosphere Reflect its Natal Composition?

Date

2009

Authors

Korn, A J
Richard, O.
Mashonkina, L
Bessell, Michael
Frebel, Anna
Aoki, Wako

Journal Title

Journal ISSN

Volume Title

Publisher

IOP Publishing

Abstract

Based on spectroscopic constraints derived from nonlocal thermodynamic equilibrium line formation, we explore the likely range of stellar parameters (Teff 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 Teff value was used (as favored 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 Hertzsprung-Russell-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 cannot resolve the discrepancy between the abundance inferred from the nondetection of the LiI resonance line at 6707 and the Wilkinson Microwave Anisotropy Probe 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 HMP stars that are 1 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 modeling. 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).

Description

Keywords

Keywords: Diffusion; Stars: abundances; Stars: evolution; Stars: fundamental parameters; Stars: individual (HE 1327-2326); Stars: Population II

Citation

Source

Astrophysical Journal, The

Type

Journal article

Book Title

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DOI

10.1088/0004-637X/698/1/410

Restricted until

2037-12-31