Metal poor dwarfs

Various spectral features can be used to derive the nitrogen abundance in dwarfs. Unfortunately weak high excitation (x=10.34 eV) near-infrared NI lines at 7468.31, 8216.34, 8683.4, 8703.25 and 8718.83 A disappear at metallicities [Fe/H] < -1 and for the analysis of N in metal-poor stars we are left with the CN and NH molecular bands at 3883 and 3360 A, respectively. It must be mentioned... 

Abstract. We examine outstanding issues in the analysis and interpretation of the halo Li plateau. We show that the majority of very Li-poor halo Li-plateau stars (5 out of 8) have high projected rotation velocities usim between 4.7 and 10.4 km s 1. Such stars have very different evolutionary histories to Li-normal plateau stars, and hence cannot be included in studies of Li depletion by normal halo dwarfs. Uncertainties in the effective temperature scale for metal-poor stars continue to challenge the analysis of Li. 

It is well-known that dSphs and dlrrs differ in their metallicity-luminosity relations. This finding is based on stellar metallicities in gas-poor dwarfs and nebular... 

Abstract. We present metallicities for 487 red giants in the Carina dwarf spheroidal (dSph) galaxy that were obtained from FLAMES low-resolution Ca triplet (CaT) spectroscopy. We find a mean [Fe/H] of —1.91dex with an intrinsic dispersion of 0.25 dex, whereas the full spread in metallicities is at least one dex. The analysis of the radial distribution of metallicities reveals that an excess of metal poor stars resides in a region of larger axis distances. These results can constrain evolutionary models and are discussed in the context of chemical evolution in the Carina dSph. 

The results of the differential study are presented in table 1. Similarly as found by [3] for dwarf and subgiant stars, 3D model atmospheres of metal-poor red giants appear to be significantly cooler at the surface than their ID counterparts. Consequently, at a given abundance, the populations of neutral atoms and molecules (e.g. Fe I, Ca I, OH) in these upper layers tend to be enhanced in 3D models comparing to ID, leading to negative abundance corrections. 

Fig. 10.4. Above spectrum of the solar-type G-dwarf star HR 509 showing features of Th ii and Nd n near k 4019 A, after Butcher (1987). Th ii is blended with a strong feature due to Fe and Ni, as well as weaker features. The tracing around the zero level shows 10 x the difference between the observed spectrum (dots) and the fitted synthetic spectrum (continuous curve). Reproduced with permission from Macmillan Magazines Ltd. Below spectrum of the same region in the very metal-poor giant star CS 22892—052 ([Fe/H] —3) with a large relative excess of r-process elements ([r/Fe] = 1.7), adapted from Sneden el al. (1996).

Figure 6. The abundance ratio [Y/Ba] versus [Fe/H] for metal-poor giants and dwarfs. Plotted data are tkaen from a variety of sources - please see Lambert Allende Prieto (2002) for the key to the symbols. The solid lines sketch possible boundaries to the data.

There are other classes of carbon-rich stars including the cool and warm R-type stars, the 13C-rich J-type stars, CH-stars, and dwarf carbon stars [73]. Barium stars also show enrichments in carbon and heavy elements, although they have C/O < 1 in general. Possibly 20% of all very metal-poor with [Fe/H] < —2 are also carbon rich, with [C/Fe] 2 in some cases [20]. Stars in these other classes are not on the AGB and are not responsible for producing their own carbon enrichments. Some of them, such as the barium and CH-type stars, are all known binaries [87,88] and thus presumably obtained their carbon from a former AGB companion. These stars are also devoid of Tc enrichments [89,90]. The warm R-type stars are all single stars [91], and may all result from some type of binary-star merger event (see Izzard, Jeffery Lattanzio [92] and references therein). The rare J-type stars, with very low 12C/13C ratios, are still a mystery [93,94]. 

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