Evolutionary models for two hyper-iron-poor low-mass stars

ABSTRACT

The possible origin of two low-mass hyper-iron-poor (HIP) stars, HE0107-5240 and HE1327-2326 are investigated. The three scenarios tested are as follows. (i) The star forms metal free (Population III) and accretes metals throughout its lifetime. (ii) A Population III binary star system forms, and the lower mass star accretes metals from the higher mass star during its asymptotic giant branch phase. (iii) An HIP Population II star is formed and evolves with no further pollution. Using the ng-elms code, stellar evolution models of these three scenarios are computed and fitted to the observed colours. Models are computed at 0.8 and  0.7 M_⊙ , and for several metal accretion rates.

It is not possible to unambiguously identify the origin, evolutionary stage or mass for either star with the present observational data. Our evolutionary models indicate that HE0107−5240 is either a red giant or a subgiant close to the base of the red giant branch. In line with the study of Aoki et al., HE1327−2326 is found to be either a main-sequence or a subgiant star. If HE1327−2326 is a main-sequence star the fits indicate that it must have a mass less than  0.8 M_⊙ ; if the star is assumed to have an age of around 12.5 Gyr it will have a mass closer to  0.7 M_⊙ .

Main-sequence low-mass model Population III stars which have been polluted with metals are found to closely resemble Population II stars. However, the case is different for subgiants. The polluted and unpolluted Population III subgiants have luminosity up to twice that of a comparable Population II model. If HE1327−2326 is a subgiant, its distance is measured and if its mass can be reasonably well constrained, it should be possible to determine if it is a Population II or III star.