Clases de valores

solar neighborhood are old (10-12 Gyr); and that no significant scatter in age is seen for them (Nordstr¨om et al. 2004; Pont & Eyer 2004; Bensby et al. 2007; Sch¨onrich & Binney 2009). These stars could be connected with halo stars, which would put our result in a consistent picture respect to these results of age-metallicity relations for the Milky Way.

6.4.3

Blue metal-poor stars

There are some stars in Fig. 6.7 at high metallicities that are bluer than the MSTO traced with the red line. These stars are also seen in our [Fe/H] - Teff diagrams from SDSS.

In the special case of the G blue sample, where we have a large contamination with blue horizontal branch stars due to selection effects, we see a considerable number of stars at hotter temperatures than the MSTO (see e.g. Fig. 6.2). We discussed in Chap. 5 how blue horizontal branch stars contaminate the hotter part of the [Fe/H] - Teff diagram and how

some of the color target selections can exclude them. The F G sample (Table 5.1) had a very clear envelope traced by the MSTO, but it was biased at low metallicities due to the lack of metal-poor turn-off stars. Concerning the metal-rich side, the [Fe/H] - Teff diagram

of the F G sample (see the right panels of Fig. 5.8) contains hotter stars, even after the drastic exclusion of blue horizontal branch stars.

These stars have also been discussed by Schuster et al. (2006) and Unavane et al. (1996) in their [Fe/H] - Teff diagrams. They have been interpreted as blue stragglers

(Preston & Sneden 2000; Carney et al. 2005). Another explanation for them is that they are younger, probably formed in an external galaxy and then accreted on to the Milky Way halo (Preston et al. 1994; Unavane et al. 1996). This explanation fits well with the hier- archical formation of the Milky Way, which has gained popularity over the years thanks to the large amount of observational evidence (Ibata et al. 1994; Belokurov et al. 2006; Helmi 2008). Further studies on the kinematical properties of these stars in the domi- nating halo population would contribute to this picture. If these blue metal-poor stars belong to younger populations that come from mergers, they then must have different kinematics to the Galactic halo formed from the collapse. This has been already observed by Preston et al. (1994) and it would be interesting to analyze the SDSS data with respect to this scenario. Another possible way to reveal the origin of these stars is by detailed chemical abundance analyses, because the chemical abundance patterns of the local dwarf galaxies is different to that of the Milky Way (Tolstoy et al. 2003; Helmi 2008). Again we stress here the comment made by Schuster et al. (2006): it would be interesting to perform a follow-up high resolution spectroscopic analysis on these blue metal-poor stars.

6.5

Discussion and conclusion

The ages of metal-poor stars contribute to current debates concerning stellar evolution theory. In addition, they contribute to the understanding of the structure and formation of the Galactic halo. In this chapter we have discussed some of these topics by determining

the age of a sample halo field stars. Using the main-sequence turn-off of a halo sample from SEGUE/SDSS, we looked for the isochrones according to this turn-off temperature. This approach to find ages has been employed by other authors (Schuster & Nissen 1989; Schuster et al. 1996; Unavane et al. 1996; Schuster et al. 2006; Allende Prieto et al. 2006), where the values of the absolute ages have droped more than 4 Gyr between the studies in 1996 and in 2006.

In the first part of this Chapter, we focused on this difference of absolute ages given by the literature. For that we introduced the process of gravitational settling of heavy elements, especially helium, in the interior of stars. We explained the effect that this process has on the age determination of metal-poor low-mass stars. Stellar evolutionary models including diffusion have a tendency of predicting lower turn-off temperatures and luminosities. The determination of ages of halo field stars with this method is strongly dependent on the temperature of the turn-off, therefore on atomic diffusion. As a first result we found a difference of 4 Gyr in the absolute ages obtained by using isochrones with and without diffusion, when GARSTEC stellar evolutionary models were used. Although we were aware that this value can change when using other stellar evolutionary codes, our results have shown to be consistent with the literature. Concerning the current debate about the efficiency with which atomic diffusion acts in stellar interiors, our results are a strong argument in favor of using diffusive isochrones for the age determination of field population II stars, because canonical isochrones yield ages larger than the age of the Universe. Based on our diffusive isochrones, we determined an age of 11-12 Gyr for the Galactic halo dominating population.

The MSTO traced by the hotter envelope of the [Fe/H] - Teff diagram was used

also to discuss our relative ages as a function of metallicity. The absence of a trend was interpreted as a rapid stellar formation scenario during the collapse of the inner halo. This theory was firstly suggested by Eggen et al. (1962) and has been followed by many works on globular clusters (e.g. Salaris & Weiss 2002; De Angeli et al. 2005) and field stars (e.g. Schuster et al. 2006; Carollo et al. 2007), which is combined with the theory of Searle & Zinn (1978) to explain the evidence of accreted material into the halo. We commented on the presence of stars that are hotter than the turn-off of the dominant halo population, which were also noted by Unavane et al. (1996) and Schuster et al. (2006) from their [Fe/H] - Teff diagrams. An interesting interpretation of these stars is that they are

a young population originated in small galaxies and were later on accreted by the Milky Way. Further studies of kinematics and detailed chemical abundances are necessary to consolidate these suggestions.

We combined our sample with that of Schuster et al. (2006, and references therein). For that we transformed our MSTO temperatures to Str¨omgren colors and superposed this new MSTO-color to the photometric sample. We could reproduce perfectly the blue envelope of the Schuster et al. sample, which implies that the signatures of a dominating coeval inner halo field population is independent of the data set used. A final remark concerning this result is that we have analyzed not only a completely independent data set, we also have developed a novel method to analyze the data. It is encouraging to see the consistency of our results with respect to those of Schuster et al. (2006), given the originality of our

6.5 Discussion and conclusion 101 methodology.

Chapter 7

Globular Clusters

In the Galactic halo, globular clusters are stellar systems that are relatively well studied because they constitute mainly on stars of the same age, metallicity and distance. They are laboratories of stellar evolution and reveal key information about the structure and formation of the Milky Way. In this chapter we used a sample of 11 globular clusters observed by SDSS to consolidate our results obtained in Chap. 6. We discuss the color distributions as a function of metallicity for field and cluster stars as observational evidence of a common nature between clusters and field stars. Finally we compared the results for the ages with those of the clusters obtained using different methods in the literature, where we could find an agreement.

7.1

Motivation

Approximately 150 globular clusters (GC’s) and about 20 satellite galaxies are found con- nected to the Milky Way (Harris 1996; Helmi 2008). GC’s constitute an homogeneous sample of stars with the same age, metallicity and distance1_{. They have been largely}

used as tracers of the Galactic halo because they span a wide range of distances, ages and metallicities and their properties can be estimated with better accuracy than for individual field stars. Searle & Zinn (1978) used globular clusters to suggest Galactic halo underwent a long formation period, in which it accreted external galaxies. This theory is combined with that of Eggen et al. (1962), which suggests that the halo was formed via gravitational collapse. It is now believed that the inner halo formed through the collapse while the outer halo through accretion. Current age-metallicity relations of halo globular clusters agree with this scenario, although the spread and trend of the relations is still under debate (Sarajedini et al. 1997; Salaris & Weiss 2002; De Angeli et al. 2005). In any case, there is an agreement that most of them are old, with ages of 10-14 Gyrs (Salaris & Weiss 2002; Dotter et al. 2010). The kinematics of disrupted tidal features of the clusters may also play an important role in revealing that the Galactic halo may have been formed from the accretion of dwarf galaxies (Mart´ınez Delgado et al. 2004; Law & Majewski 2010). In addition to the clusters, stellar streams have recently been observed with large-scale sur- veys of field stars. The SDSS, for instance, has shown several complex substructures of the Galactic halo (Belokurov et al. 2006; Newberg et al. 2007; Yanny et al. 2009a). Thus, the ongoing dissolution of globular clusters implies that some fraction of halo stars were initially formed in GC’s (Odenkirchen & Grebel 2004; Jordi & Grebel 2010; Chun et al. 2010).

These evidence makes us to believe that the stars of the Milky Way halo have a common history, where field and globular cluster stars are of the same nature. But there are still some pieces missing in this puzzle that do not allow to confirm this scenario. For example, there is already a significant number of extremely metal-poor field stars in the Galactic halo (Beers et al. 1992; Christlieb et al. 2002), but there are no globular clusters with [Fe/H] . ₋2.4. In addition to that, the discovery of different chemical patterns in stars of the same globular clusters suggest that they have at least a first and second population (Gratton et al. 2004). Carretta et al. (2010) propose that the primordial population of GC’s might be the main building block of the halo. A study of halo field stars coming from first and second generation of GC is performed by analyzing the chemical patterns. These observations do not lead to a census of how much the clusters have contributed to the field

1

We mention the several interesting works over the last years discussing the different chemical pat- terns observed in many globular clusters as indicators of multiple populations (Gratton et al. 2004; Baumgardt et al. 2008; Carretta et al. 2010, and references therein). We are totally in agreement that today it has become inaccurate to affirm that a GC has stars of same age and chemical composition. The age difference between the two main generations is usually of the order of the errors obtained for the age estimates (∼1 Gyr, Salaris & Weiss 2002). In our particular work, we are dealing also with metallicities that have errors of 0.25 dex, meaning that for the purpose of this work we can still approximate a globular

7.2 Globular clusters sample 105