BaII (for instance the 4554 and 5853 lines) appears in early A-type dwarfs and increases in intensity toward F-type. From there on the lines increase asymptotically in strength. The lines have a strong positive luminosity effect.

Behavior in non-normal stars

BaII lines are weak (W(4554) = 0.07) in Ap stars of the Cr-Eu-Sr subgroup (Adelman 1973b) and are also weak in the Hg-Mn subgroup (Adelman 1992). Cowley (1976) finds Ba to behave erratically in Ap stars.

BaII lines are strengthened in general in Am stars by factors of less than two in W when compared with stars of the same temperature (Smith 1973, 1974). There exist, however, stars in which Ba lines are about normal.

BaII lines are also slightly strengthened in delta Del stars (Kurtz 1976). Ba is overabundant in blue stragglers of an old open cluster (Mathys 1991) and underabundant in HB stars (Aldelman and Philip 1992a). BaII lines are very strong in the spectra of the so-called Ba stars. These are giants (C 2-K 4) showing a very strong BaII 4554 line (Bidelman and Keenan 1951). Fernandez-Villacañas et al. (1990) provide measures of W(5853) for a sample of Ba stars, with W varying between 0.103 and 0.259. Danziger (1965) found that W(4554) may be up to 1.0. This implies overabundances of Ba of more than one order of magnitude.

The coolest Ba stars have weak carbon compounds and weak Sr lines (Sowell 1989).

BaII lines are also strengthened in the spectra of subgiant GH stars (also called hot Ba stars) (Luck and Bond 1982, Kxishnaswamy and Sneden 1985).

Ba lines are probably strengthened in R CrB stars (Cottrell and Lambert 1982).

BaII lines are very strong in late C-stars (> C 5), with W(5853) typically 1.5 Å (Fujita et al. 1966). The overabundance of Ba in some, but not all, C-stars was confirmed by Kilston (1975), who suggests that abundance of this element is correlated with that of Zr. Dominy (1985) suggests that the C-stars in which Ba is not enhanced are predominantly of type J. Utsumi (1895) finds Ba overabundant with respect to Ti in most late C-type stars.

Ba I is strengthened in S-type stars. According to Smith and Lambert (1986), typically W values are larger by a factor of two than in normal stars of the same temperature. Ba is also enhanced in SC stars (Kipper and Wallerstein 1990).

The behavior of Ba in metal-weak stars has been studied by several authors, with conclusions that do not always agree completely. Ba seems to be underabundant With respect to iron in metal-weak stars according to Luck and Bond (1985). Molaro and Bonifacio (1990) found the same result in the most extreme metal-weak stars (Fe/H = 10^-4), but in less pronounced metal-weak stars its behavior probably parallels that of Fe (Magain 1989). This behavior is also followed in globular cluster stars (Wheeler et al. 1989, Francois 1991). However, Gonzalez and Wallerstein (1992) found Ba to be overabundant with respect to iron in one globular cluster supergiant. In general Ba seems to behave in metal-weak stars in a manner parallel to that of Eu (Spite 1992). In some halo stars Ba lines are strengthened.

Ba is enhanced in the Magellanic Cloud stars (Luck and Lambert 1992).

Isotopes

There exist 25 isotopes and isomers, of which seven are stable, namely Ba 130, 132, 134, 135, 136, 137 and 138. In the solar system 72% and 11% of Ba is in the form of Ba¹³⁷ and Ba¹³⁸ respectively.

Origin

Ba is produced by a variety of processes: Ba¹³⁰ and Ba¹³² by the p process, Ba 134, 136 and 138 by the s process and Ba¹³⁵ and Ba¹³⁷ by either the s or the r process.

Published in "The Behavior of Chemical Elements in Stars", Carlos Jaschek and Mercedes Jaschek, 1995, Cambridge University Press.