Barium stars

Abstract. [La/Eu] and [Ba/Eu] for a sample of Barium stars were determined in order to evaluate the ratio of abundances of s- and r-elements. The results have been compared to previous work dealing with normal red giants and dwarfs with metallicities in the range -3 < [Fe/H] < +0.3. 

Barium stars were recognized as a distinct group of peculiar stars by [1], The objects initially included in this group were red giants of spectral type G and K, which showed strong lines of s-process elements, particularly Ba II and Sr II, as well as enhanced CH, CN and C2 bands. The discovery that HR 107, a dwarf star, shows a composition similar to that of a mild Barium giant by [6] has pushed the search for new Barium dwarfs. 

The relation between s- and r-process for a sample of Barium stars are showed, using europium as representative of r-process, since it is a nearly pure r-process element, and La and Ba as representatives of s-process. 

Ba (La) and Eu abundances have a linear relation for stars whose metallicities are below -2.75. At higher metallicities the relation is still linear, but the fit shows different slope. Since Barium stars are rich in s-elements, that relation is different relative to normal stars with similar metallicities. 

According to the Fig. 1, Ba is more overabundant than La in Barium stars relative to normal stars with similar metallicities. No dependence on luminosity classes was found in the s/r behavior among those Barium stars. 

Figure 2 - S-process enrichment vs. carbon enrichment in peculiar warm giants. The expected locus of the subgiant CH stars following the first dredge-up is indicated by open triangles. The barium stars, mild barium stars, subgiant CH stars, and CH stars all seem to follow a common relationship.

The CH stars and the CH-like stars are included in Table 1 for completeness, but we don t know much about them. The CH stars are metal-deficient, Population II giants enriched in carbon and s-process elements. They may be enriched in (Lambert 1985). They appear to be binaries (McClure 1985). The CH stars are probably Pop II barium stars, and to accept their designation as Pop II barium stars. Not much is known about the CH-like stars of Yamashita (1972, 1975), which are Pop I, carbon-rich giants. At low spectral resolution the 14554 line of Ba II is enhanced. These stars may be related to the barium or barium-carbon stars, but we need more information about their compositions, and especially about their binary status. 

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]. 

As a nuclear reaction, the s process is relatively well understood, but the problem lies in identifying an astrophysical site for it and determining the relevant physical parameters, such as neutron flux, mean time separating two neutron captures, and temperature. It has been shown that the most propitious temperatures are those of helium fusion. Added to the fact that the surfaces of certain red giants are rich in s isotopes, such as radioactive technetium and barium, this observation confirms the idea that the s process may be related to helium fusion regions in stars. 

The heavy elements show varying trends but, on the whole, strontium follows iron, whilst barium is underabundant below 0.01 Z . It is observed that, in every case, marked differences occur between stars with the same iron content. 

Fig. 8.6. Relative abundances of heavy nuclei in a halo star. Abundances are normalised to the barium abundance. The continuous line represents r abundances (from Kappeler). The excellent agreement suggests that previous nucleosynthesis was dominated by the r process and that the star CS 22892-052 formed from the debris of a type 11 supernova. (From Sneden 2001.)...

Inside the plastic cone at the head of the rocket might be a payload of small green stars, each about the size of a pea, based on a composition containing barium nitrate (36%), potassium chlorate (48%), shellac (13%) and dextrin (3%). 

The chemistry that governs barium salts, as used in green stars, is somewhat analogous to that of the strontium salts employed in red stars since both elements are found in the same group of the periodic table. While the use of compounds such as strontium chloride, strontium chlorate or strontium perchlorate might be considered to be appropriate... 

The second German edition of Ruggieri s book (we have not seen the first) contains a Nachtrag or supplement which lists nine compositions,10 of which four contain Kali oxym. or potassium chlorate. These are (1) for red fire, strontium nitrate 24 parts, sulfur 3, fine charcoal 1, and potassium chlorate 5 (2) for green fire, barium carbonate 20 parts, sulfur 5, and potassium chlorate 8 parts (3) for green stars, barium carbonate 20 parts, sulfur 5, and potassium chlorate 9 parts and (4) for red lances, strontium carbonate 24 parts, sulfur 4, charcoal 1, and potassium chlorate 4 parts. Ruggieri says ... 

The picric acid is to be dissolved in boiling water, the strontium or barium nitrate added, the mixture stirred until cold, and the solid matter collected and dried. The same author26 gives picric acid compositions for stars, not suitable for shells, as follows ... 

Green Stars. Thirty parts of chlorate of barium, 10 of flowers of sulphur, and 1 of mastic. 

The most abundant product of the evolution of massive stars is oxygen, in particular—the third most abundant isotope in the Universe and the most abundant metal. Massive stars are also the main source of most heavy elements up to atomic mass number A 80, of some of the rare proton-rich nuclei, and of the r-process nuclei from barium to uranium. In the following, we will briefly review the burning stages and nuclear processes that characterize the evolution of massive stars and the resulting core collapse supemovae. 

The evidence for this is provided by an increase in the ratio of barium (the abundance of which in galactic matter is dominated by s-process contributions) to europium (almost exclusively an r-process product). The ratio [Ba/Eu] is shown in Figure 9 as a function of [Fe/H] for a large sample of halo and disk stars. Note that at the lowest metallicities the [Ba/Eu] ratio clusters around the pure r-process value ([Ba/Eu]r-process —0-9) at a metallicity [Ee/H]—2.5, the Ba/Eu ratio... 

---