7 Conclusions and future work

The apparent brightness, very low [Fe/H], and enormous r-process enhancement, in combination with the lack of molecular blanketing, makes CS 31082-001 a uniquely favorable object for study of the r-process nucleosynthesis history in the early Galaxy. The most conspicuous result of these characteristics is the breakthrough in the measurement of the abundance of uranium in this very old star, facilitated by our excellent VLT/UVES spectra, which enable the determination of accurate abundances for 41 (plus 2 upper limits - N and Pb - and 3 detections which could not be translated into accurate determinations - Ho, Lu and Yb - due to a lack of input atomic physics) other elements as well. But the importance of CS 31082-001 extends further, as it has provided the first solid evidence that variations in progenitor mass, explosion energy, distance to dense interstellar clouds, and/or other intrinsic and environmental factors preceding the formation of the extreme halo stars, may produce significantly different r-process abundance patterns from star to star in the actinide region ($Z \geq 90$).

A striking consequence of these variations is the complete failure of the conventional Th/Eu chronometer in CS 31082-001, assuming an initial production ratio for the pair as in CS 22892-052, or as in the r-process elements of the Solar System. No such problem is seen for the pair U/Th, which leads to an age of $14.0\pm 2.4$ Gyr (not including systematic errors in the initial production ratio). This suggests that the closely similar masses and nuclear structures of ²³⁸U and ²³²Th lead to a more stable production ratio between these two nuclides than, e.g., that of ²³²Th and ^151-153Eu. However, further studies are needed of the robustness of the U/Th ratio produced by exposing iron nuclei to neutron exposures of various strengths, as well as of the abundances of the daughter elements of their decay, Pb and Bi.

Observations in CS 31082-001 itself of the Pb and Bi created in the same neutron exposure event as U and Th, and by their subsequent decay, will be particularly crucial as constraints on the predicted production ratios of all r-process elements. Of equal importance is the continuation of searches for new r-process-enhanced metal-poor stars, so that better measures of the star-to-star variation in the observed patterns of the r-process elements, in particular those of the third-peak and the actinides, may be obtained.

Acknowledgements

This research has made use of the Simbad database, operated at CDS, Strasbourg, France. We are grateful to Pr. Johansson and his group for their timely response to our request for accurate lifetime measurements of uranium and thorium electronic levels. BN and JA thank the Carlsberg Foundation and the Swedish and Danish Natural Science Research Councils for financial support for this work. T.C.B acknowledges partial support from grants AST 00-98549 and AST 00-98508 from the U.S. National Science Foundation.