This element was discovered by E. A. Demarcay in Paris in 1901. The name comes from the continent, which in turn alludes to a Greek mythological figure.

Ionization energies
EuI 5.7 eV, EuII 11.2 eV, EuIII 24.9 eV, EuIV 42.6 eV.

Eu is the rare earth that has been most extensively studied in stars. Whereas Eu I lines are very weak, Eu II lines are more easily detectable.

Absorption lines of EuI
In the sun EuI 4627(1) has W = 0.004. According to Gopka et al. (1990), W(5765) = 0.013 in both K5III and M0III.

Absorption lines of EuI

Table 1: Equivalent widths of EuII

  4205(1)   6645(8)    

Group V Ib V III Ib

F0 0.010        
F5 0.010 0.22      
G0 0.120       0.083
S blend with VI     0.004    
G2     0.004   0.063
G5 0.176        
K0       0.031  
K2       0.033 0.098
K3       0.045 0.089
K5         0.098

EuII lines (for instance those at 4205 and 6645) appear at mid-F-type and grow in intensity toward cooler stars. A positive luminosity effect exists.

Behavior in non-normal stars
EuII lines are very strong in Ap stars of the Cr-Eu-Sr subgroup. Typically W(4205) = 0.20 (Sadakane 1976). Attention should be paid to the fact that many lines are widened by hyperfine structure (Cowley 1980) and/or by Zeeman broadening. Abundances should thus be corrected for these effects (Hartoog et al. 1974). If these corrections are applied, then the abundance of Eu becomes comparable to that of other rare earths. However, Cowley (1984) calls attention to the fact that there exists at least one star in which Eu is the dominant rare earth.

EuII is also strong in Am stars. A typical value for early type Am stars is W(4205) = 0.010, and, for late Am stars, W(4205) = 0.300.

Figure 17

EuII is also strong in delta Del stars. For instance W(4205) > 0.150 (Kuntz 1976).

EuI and EuII lines are enhanced in some Ba stars to varying degrees. This leads to overabundances which are, however, smaller than those of other rare earths (Lambert 1985). As a general comment Cowley and Dempsey (1984) observe that, whereas Eu is often very strong in Ap stars, the enhancement in Ba stars is less pronounced.

Eu II lines are present in S-type stars (Merrill 1947).

Eu II lines have been observed in G- and K-type metal-weak dwarfs by Gilroy et al. (1988). These authors find Eu overabundant with respect to Fe. However, Sneden et al. (1988) call attention to the large scatter in the Eu/Fe ratio in different stars. Magain (1989) confirmed the overabundance with respect to iron and indicated an overabundance of 1.0 dex. (See also Part Two, section 2.2.) Gilroy et al. (1988) pointed out a close correlation between the abundances of Ba and Eu, which seems also to be valid for globular cluster stars of our galaxy (Francois 1991). Spite et al. (1991) find the same correlation in one cluster of the Small Magellanic Cloud.

Eu has two stable isotopes, Eu151 and Eu153. In the solar system they represent 48% and 52% respectively of all Eu. There exist 19 unstable isotopes and isomers.

Kato (1987) found from the 4130 line in one F 5 star that the ratio Eu153 /Eu 151 agrees with the solar ratio.

Eu151 and Eu153 are produced by either the r or the s process.

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