We present a high-resolution ( R=75,000 , S/N \sim 500 ) spectroscopic analysis of the bright ( V=11.7 ) , extreme halo giant CS 31082-001 ( [ Fe/H ] = - 2.9 ) , obtained in an ESO-VLT Large Programme dedicated to very metal-poor stars .
We find CS 31082-001 to be extremely rich in r -process elements , comparable in this respect only to the similarly metal-poor , but carbon-enriched , giant CS 22892-052 .
As a result of the extreme overabundance of the heaviest r -process elements , and negligible blending from CH and CN molecular lines , a reliable measurement is obtained of the U II line at 386 nm , for the first time in a halo star , along with numerous lines of Th II , as well as lines of 25 other r -process elements .
Abundance estimates for a total of 43 elements ( 44 counting Hydrogen ) are reported in CS 31082-001 , almost half of the entire periodic table . The main atmospheric parameters of CS 31082-001 are as follows : T _ { eff } = 4825 ~ { } \pm 50 K , \log g = 1.5 \pm 0.3 ( cgs ) , [ Fe/H ] = - 2.9 \pm 0.1 ( in LTE ) , and microturbulence 1.8 \pm 0.2 km/s .
Carbon and nitrogen are not significantly enhanced relative to iron .
As usual in giant stars , Li is depleted by dilution ( \log ( Li/H ) =0.85 ) .
The \alpha -elements show the usual enhancements with respect to iron , with [ O/Fe ] = 0.6 \pm 0.2 ( from [ O I ] 6300 Å ) , [ Mg/Fe ] = 0.45 \pm 0.16 , [ Si/Fe ] = 0.24 \pm 0.1 , and [ Ca/Fe ] = 0.41 \pm 0.08 , while [ Al/Fe ] is near - 0.5 .
The r -process elements show unusual patterns : among the lightest elements ( Z \sim 40 ) , Sr and Zr follow the Solar r -element distribution , but Ag is down by 0.8 dex .
All elements with 56 \leq Z \leq 72 follow the Solar r -element pattern , reduced by about 1.25 dex .
Accordingly , the [ r /Fe ] enhancement is about +1.7 dex ( a factor of 50 ) , very similar to that of CS 22892-052 .
Pb , in contrast , seems to be below the shifted Solar r -process distribution , possibly indicating an error in the latter , while thorium is more enhanced than the lighter nuclides .
In CS 31082-001 , log ( Th/Eu ) is -0.22 \pm 0.07 , higher than in the Solar System ( - 0.46 ) or in CS 22892-052 ( - 0.66 ) .
If CS 31082-001 and CS 22892-052 have similar ages , as expected for two extreme halo stars , this implies that the production ratios were different by about 0.4 dex for the two objects .
Conversely , if the Th/Eu production ratio were universal , an age of 15 Gyr for CS 22892-052 would imply a negative age for CS 31082-001 .
Thus , while a universal production ratio for the r -process elements seems to hold in the interval 56 \leq Z \leq 72 , it breaks down in the actinide region . When available , the U/Th is thus preferable to Th/Eu for radioactive dating , for two reasons : ( i ) because of its faster decay rate and smaller sensitivity to observational errors , and ( ii ) because the inital production ratio of the neighboring nuclides ^ { 238 } U and ^ { 232 } Th is more robustly predicted than the ^ { 151 } Eu/ ^ { 232 } Th ratio .
Our current best estimate for the age of CS 31082-001 is 14.0 \pm 2.4 Gyr .
However , the computed actinide production ratios should be verified by observations of daughter elements such as Pb and Bi in the same star , which are independent of the subsequent history of star formation and nucelosynthesis in the Galaxy .