The Age-metallicity Relation Of Stars In Different Stellar Populations

The age-metallicity relation (AMR) of stars in the Galactic disk is a fundamental issue to study the formation and evolution of the Galaxy. However, up to now, whether there is a possible relation between the stellar ages and metallicities of the Galactic stars is still in debated. So that it is necessary to further study the AMR. We selected two groups of samples to discuss the AMR in detail: one group consists of four large sample works in which the stellar metallicity is determined by photometric index; the other consists of 13 works with relative fewer star samples, in which the stellar metallicity is derived from high-resolution spectroscoipic analysis.Our work mainly includes the following two parts:The first part is the statistical analysis of large samples. Our sample stars were selected from four large photometric samples which include 552, 5828, 1658, and 14000 stars respectively. First, the ages, metallicities and the AMRs of the four samples were compared with each other. Then, we make two new subsamples A and B from the common stars between the three larger sample works by selecting stars with consistent ages. Subsamples A and B includes 4007 and 1042 stars respectively. The stellar population membership of each star is determined by the pure kinematics criterions. Lastly, we discussed the photometric age-metallicity relation of thin and thick disk stars.The second part is an analysis of multi-samples. Our sample stars were selected from thirteen works with relative fewer star samples. The stellar metallicities of those works are derived from high-resolution stellar spectra. After eliminating the giants and some stars those ages can not be determined using isochrones, we obtained a sample consisting of 641 stars from 13 observational works. This sample consists of 434 thin disk stars, 157 thick disk stars and 50 halo stars. Since the stellar ages presented by different works are often different, to avoid the system errors of stellar ages among the different works, we use the Y2-isochrones recalculated the stellar ages of 641 stars. Using the spectroscoipic metallicities, we obtained the spectroscoipic age-metallicity relation for the thin disk, thick disk and halo stars.Following are our main results and conclusions: (1) For thin disk stars, two results of AMR seems to be conflicting, a weak photometric age-metallicity relation derived from large samples analysis, and a clearer spectroscoipic age-metallicity relation derived from multi-samples analysis. Further studies are needed to explore which one is more accurate. But we think that the clearer spectroscoipic AMR maybe due to the section effects of the multi-samples. For thick disk stars, there is an obvious age-metallicity relation whether derived from photometric metallicities or spectroscoipic metallicities. On the average, the stellar age increases 1 Gyr with a decrease of 0.1 dex of [Fe/H]. There is no obvious age-metallicity relation in the halo stars.(2) The scatter in [Fe/H] at all ages along the AMR gratetly exceeds the observational errors 0.1 dex. The scatter of thin disk stars has an increasing trend with the age increasing, while the scatter of thick disk stars has a decreasing trend with the age increasing. The scatter of halo stars is significant larger than that in thin and thick disk stars.(3) The relation between abundance ratio [Mg/Fe] and stellar ages is different for different stellar populations. For thin disk stars, there is a weak trend that the stellar ages increase with the increase of [Mg/Fe], and the scatter of [Mg/Fe] is increasing with the increse of stellar ages. For the thick disk stars, the mean of [Mg/Fe] is nearly constant at all ages. For the halo stars, [Mg/Fe] shows larger scatter and no correlation with the stellar ages.(4) Our resuts show that the average ages of thin disk stars, thick disk stars, and halo stars are about 4 Gyr, 9 Gyr, and 13 Gyr, respectively; and the mean values of [Fe/H] are about -0.18, -0.36, and -1.28 corresponding to the thin disk, thick disk, and halo stars.Our results show that the stellar AMR is affected by many factors, such as the selection effect of sample, the different methods of determining the stellar ages and metallicities, as well as the effect of stellar population.