Halo stars

Fig. 1. Left panel Demonstration that SNela have contributed to the chemical enrichment for stars that are kinematically selected to trace the thick disk (i.e. knee in the trend for [O/Fe]). Figure taken from [4], Thick disk stars are represented by filled symbols (circles from [4] and triangles from [16]), thin disk by open symbols (circles from [4] and triangles from [16]), and halo stars by (from [16]). Right panel Shows the kF-velocity as a function of [Fe/H] for stars kinematically selected to be thin (open circles) or thick (filled circles) disk stars. Data taken from [3] and Bensby et al. submitted. This plot establishes that metal-rich stars can also have high velocities perpendicular to the plane (sometimes taken as an evidence for the star to belong to the thick disk)...

Among the sample of outer halo stars being investigated are Keck I HIRES observations of four globular clusters with [Fe/H] -2. Information regarding these clusters and comparison objects is presented in Table 1. 

So after more than 20 years of searching for an answer, we have much better data on the nature of star-to-star variations in the abundances of the light elements in GCs, but still no definitive understanding of the physical mechanism(s) responsible, nor of why metal poor field halo stars do not show these phenomena. 

A weak but useful carbon line [Cl] 8727.13 A disappears in halo dwarfs with metallicities below —1. To measure carbon abundance in halo stars one can use four Cl high excitation lines near 9100 A and the CH band at 4300 A. The Cl lines at 9100 A together with the OI triplet at 7771 A have been used by Tomkin et al. (1992) and Akerman et al. (2004) to study the behaviour of C/O versus metallicity. However, Cl and OI lines employed in these papers are sensitive to a non-LTE effects and one has to bare in mind that this sensitivity is different for C and O. The CH band at 3145 A used by Israelian et al. (1999) is almost saturated in disk stars and several blends makes the abundance analysis less accurate. To ensure a homogeneous analysis of the C/O and N/O ratio from NH,CH and OH lines in the near-UV, we used the same model atmospheres and tools as in our previous studies. The oxygen abundances were compiled from Israelian et al. (1998, 2001) and Boesgaard et al. (1999). 

The first study on sulphur in halo stars is from [4]. And since then, the metallicity of the stars in which we could measure sulphur lines has decreased. 

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. 

The difficulties in inferring the primordial Li abundance from halo star observations can be separated into two broad categories ... 

We conclude that the Li-poor stars definitely have different evolutionary histories to Li-normal plateau stars. A mass-transfer mechanism may explain the origin of these objects, but irrespective of whether this is the correct explanation, the Li-poor objects cannot be included in studies of Li depletion mechanisms that affect normal single stars. Consequently we conclude that the small (zero ) intrinsic spread in plateau Li abundances inferred by [25] is representative of normal halo stars, and thus signifies at most a small depletion in Li, <0.1 dex by the models of [17]. 

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. 

Fig. 2. The abundance ratio of Ba from the Shetrone et al. (1998), squares, Fulbright et al. (2004), circle, and Shetrone et al. (2005), triangles shown plotted against their derived metallicities. The upper limit from Fulbright et al. (2004) is more than an order of magnitude lower than that of the slightly more metal rich Draco dSph stars or comparable metallicity MW halo stars, small symbols.

That is, the straightforward interpretation of abundance data for Galactic field stars in terms of stellar populations is feasible only because the Galaxy apparently acquired its gas early, or at a rate which was well-matched to the star formation rate across the whole volume now sampled by local halo stars, and kept this gas well-mixed and because the stellar IMF is (close to) invariant over time and metallicity. Neither deduction was obvious, nor is the underlying physics understood. However, these two deductions apply so well they have become assumed authors use any violation to rule out some possible Galaxy merger histories, as in the Venn et al. analysis from which Figure 1 is taken. 

Fig. 4. Nitrogen in DLAs, circles, and halo stars from Spite et al (2004), lozenge, and from Israelian et al (2004), squares.

A New Class of Type la Supernovae Inferred from Abundance Patterns of Halo Stars and High-z Galaxies... 

One of the major cosmological developments of the 1980s was the discovery of the Li plateau in low metallicity halo stars (Spite and Spite 1982, see Fig. 1 top left panel). The unique behaviour of that element, i.e. the constancy of the Li/H ratio with metallicity, strongly suggests a primordial origin. The observed... 

The recent report of a plateau for 6Li/H in halo stars (Asplund et al. this meeting) gives a new twist to the LiBeB saga. The detected 6Li/H (and corresponding 7Li/6Li ) value at [Fe/H]=-2.8 is much larger than expected if GCR are the only source of the observed Be/H and 6Li/H in that star (see Fig. 1). A few explanations have been proposed for such a high early amount of 6Li ... 

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

The resulting dependences of [Th/Eu] on stellar age are shown in Fig. 10.5, together with a selection of observational data from del Peloso et al. for disk stars Westin et al. (2000) for two of the r-process enriched ultra-metal-poor (UMP) halo stars and Cowan et al. (2002) for a third one. The range among disk stars is little more than can be expected from uncertainties in the determination, while the UMP stars show more or less the expected deficiency. For BD +17 0 3248, Cowan et al. used theoretical production ratios for Th relative to Eu, Ir and Pt, based on certain nuclear models, to deduce a Galactic-model-independent age of 13.8 4 Gyr, which happens to fit our two production-ratio-independent model curves simple inflow and modFowler T = 15 quite nicely, but the error bars are large so that what we have is more a test of consistency than an independent chronometer. 

Figure 11.5 shows the a-element to iron ratios in the LMC and in the anomalous halo stars of Nissen and Schuster (1997). The trend with metallicity is much reduced compared to the disk and the majority of halo stars in the Galaxy, which is attributed to the longer timescale for star formation in the Clouds see Fig. 8.7. However, below [Fe/H] = -1.3, there are similar plateaux to those in the Milky Way,...

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