Galactic halos

The growing scenario for the Bulge is pointing toward an early formation and a rapid chemical enrichment, as for the Galactic Halo but possibly at higher star formation rate. 

From observations of 11 main-sequence stars belonging to the Galactic halo, Spite Spite [27] concluded that the lithium abundance was essentially independent of metallicity for halo stars hotter than 5600 K, and inferred that the Li abundance was hardly altered from the Big Bang. Two decades of work has followed, increasing the number of stars observed and the range of metallicity that they span, in an effort to establish the primordial Li abundance more securely. 

In the central region of Sculptor we have high resolution spectra providing direct abundance measurements for numerous elements (Hill et al., in prep). In Fig. 3 we show the preliminary results for the a-elements (Ca, Mg Ti) compared with similar observations of stars in our Galaxy. It is clear that the overall distribution of [a/Fe] versus [Fe/H] in Scl does not match our Galaxy, except for a small number of the most metal poor stars in Scl which overlap with Galactic halo stars. 

It is currently very fashionable to identify new fragments of accretion events in the outer Galactic halo, each of which becomes evidence to support some concept of accretion onto the galaxy as an important process. Yet there has been strong evidence for many years, now made irrefutable by the abundance... 

Fig. 8.9. Even-numbered element iron ratios measured in ultra-metal-deficient Galactic halo stars, after Cayrel et al. (2004). 

Figure 8.19 shows an estimate of the distribution function of oxygen abundances among field stars of the Galactic halo and Fig. 8.20 shows the iron abundance... 

Models for the Galactic halo and disk Table 8.3. Some parameters of the mixing modela... 

Another view, equally consistent with the source abundances and better suited to account for the abundance of light elements like beryllium in stars of the Galactic halo (see below), is that dust particles in the supernova ejecta are the source of ions that are preferentially accelerated within the hot, tenuous gas of superbubbles surrounding regions of star formation (Lingenfelter, Ramaty Kozlovsky 1998). 

Ryan et al. (1992) noted that in models for the Galactic halo assuming homogeneous outflow (Hartwick 1976), the limiting behaviour of Eq. (9.44) sets in at a... 

The VLT, centrepiece of the ESO, promises an exceptional harvest of astronomical data. Understanding the chemical evolution of the Universe requires a coordinated study of the most remote objects, ancient stars in the galactic halo and absorbent clouds in the line of sight of quasars. To this end, the high-resolution spectrograph UVES (Ultraviolet Echelle Spectrograph) was set at one focus of Kueyen, one of the four components of the VLT, perched at the top... 

The first major observation is thus that the Sun is a rich star compared with the ancient stars in the galactic halo, placed like a crown around the Milky Way, but that its composition is close to that of the stars in the disk where it itself resides. Our daytime star therefore belongs to the wealthy fellowship of the disk. Whereas the halo is almost completely devoid of gases, the disk abounds in them. 

We thus arrive at the following composition for the ancestral cloud that spawned the Solar System in 1 gram of matter, we find 0.72 g of hydrogen, 0.26 g of helium and 0.02 g of heavier elements. Despite the superb efforts of past generations of stars, the Sun, like its nebulous father, is singularly poor in metals, since these make up a mere 2% mass fraction of its matter. This, however, is a small fortune compared with the ancient stars in the galactic halo. 

As already mentioned on several occasions, two types of star can be distinguished by their chemical composition, their motion (velocity) and their membership of the galactic halo or the disk. The first population is old and poor in metals. These are witness to the epoch when the newly bom galaxy still sought its final shape. 

In fact, the quest to find an astrophysical site for the r process is in full swing. It is being helped along by the rapid accumulation of observational data concerning surface abundances in ageing stars gravitating in the galactic halo. Such analyses aim to correlate observed abundances of typical r or s isotopes with stellar metallicities. It has thereby been discovered that the slow process comes later than the rapid process. 

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