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The Age of the Universe from Nuclear Chronometers Proc. Natl. Acad. Sci. USA Vol. 95, pp. 18–21, January 1998 Colloquium Paper This paper was presented at a colloquium entitled ‘‘The Age of the Universe, Dark Matter, and Structure Formation,’’ organized by David N. Schramm, held March 21–23, 1997, sponsored by the National Academy of Sciences at the Beckman Center in Irvine, CA. The age of the universe from nuclear chronometers J. W. TRURAN Department of Astronomy and Astrophysics, Enrico Fermi Institute, The University of Chicago, 5640 S. Ellis Avenue, Chicago, IL 60637 ABSTRACT An overview is presented of the current sit- constrained by our lack of an adequate knowledge of their uation regarding radioactive dating of the matter of which our nucleosynthesis histories. The long lifetime of 187Re makes the Galaxy is comprised. A firm lower bound on the age from 187Re–187Os chronometer pair an attractive choice for dating nuclear chronometers of '9–10 Gyr is entirely consistent with galactic nucleosynthesis (11). A major difficulty here, however, age determinations from globular clusters and white dwarf is the fact that 187Os is also produced in the s-process. The cooling histories. The reasonable assumption of an approxi- uncertainties introduced by the subtraction of the s-process mately uniform nucleosynthesis rate yields an age for the contribution to isolate the cosmoradiogenic component are Galaxy of 12.8 6 3 Gyr, which again is consistent with current significant. Further complications are associated with the fact determinations from other methods. that the b decay rate of 187Re in stellar environments is sensitive to temperature. Estimates of the age of the Galaxy, and thereby limits on the Abundance clues to r-process history and the identification age of the Universe, can be obtained by three independent of the astrophysical site of r-process synthesis are reviewed in means: (i) the age of the elements by radioactive dating section 1. Critical input to these chronological studies is (nucleocosmochronology); (ii) the ages of the globular clusters identified and discussed in section 2. The equations of nucleo- (the oldest stars in the halo of our Galaxy); and (iii) the ages cosmochronology are presented in section 3, together with a of white dwarfs from cooling calculations (the age of the model-independent age determination. In section 4, we obtain Galactic disk?). This paper will focus on radioactive dating, an lower bounds on the time scale of galactic nucleosynthesis, with approach that has played a particularly important role histor- the assumption of an early ‘‘single event’’ nucleosynthesis ically. epoch. Observational evidence for a uniform rate of nucleo- The presence of naturally occurring radioactive nuclei in synthesis, and its implications for the age of the Galaxy, is Galactic matter testifies to the fact that the age of the elements presented in section 5. The use of the thorium abundance in an is finite. To the extent that the long-lived nuclear species of extremely metal-deficient halo star for dating purposes is interest are the products of nuclear transformations proceed- considered in section 6. Discussion and conclusions follow. ing in stars and supernovae over the course of our own 1. Abundance Clues to r-Process History. Significant con- Galaxy’s history, they can be used to provide a measure of the straints on the site of r-process nucleosynthesis are provided by duration of star formation activity and concomitant nucleo- observations of the heavy element patterns in halo stars. Early synthesis in the Galaxy. The use of long-lived radioactivities as abundance studies of metal poor stars [see, e.g., the reviews by a mechanism for the determination of a lower limit on the age Wheeler, Sneden, and Truran (12) and McWilliam (13)] of the Galaxy has a history that spans much of the 20th century. showed that abundances of nuclei normally attributable to the An early paper by Rutherford (1) outlined the essential s-process were systematically depleted relative to r-process nuclei. The recognition that the heavy element abundance features of this science. Subsequently, the defining works in y '2 nucleosynthesis theory by Burbidge et al. (2) and Cameron (3) patterns in extremely metal-deficient stars ([Fe H] 3) involve exclusively r-process products (14) is now strongly established the nature of the astrophysical r-process of neutron supported by spectroscopic studies of an increasing number of capture, by which the critical long -lived chronometers 187Re, such stars. This includes the recent study of the star CS 232Th, 235U, and 238U are synthesized. The task, since then, has 22892–052 ([FeyH] 523.2) by Sneden et al. (15), in which been to identify the astrophysical site for the operation of this thorium also was detected. HST observations (16) have sub- nucleosynthesis process and to calculate the appropriate rates stantiated further this behavior; with the first detection of of production as a function of time over the course of galactic platinum, osmium, and lead in a metal-poor halo star, they evolution. The early developments of the use of the uranium– have shown that nuclei in the r-process peak at mass A ' 195 thorium chronometers by Fowler and Hoyle (4) and Cameron also are formed in solar r-process proportions. In general, (5) were based necessarily on rather simple prescriptions for excellent agreement with the solar r-process heavy element the history of galactic nucleosynthesis. As our understanding abundance pattern is obtained, in metal-poor stars, over the of the processes of stellar and supernova nucleosynthesis entire range of elements from barium to osmium. The level of improved, it became possible to address the problem of nuclear abundance of thorium in the star CS 22892–052 again confirms chronology in the context of increasingly realistic models of the that the r-process occurring in the earliest stages of evolution chemical evolution of the Galaxy (6–10). of our Galaxy was generally consistent with that which formed In this paper, we will be concerned with the determination the bulk of the r-process heavy elements in solar system matter; of realistic age constraints from nucleocosmochronology. We the relative abundance levels in the barium region and imme- will restrict ourselves to the 232Th–238U and 235U–238U chro- ' t . 9 diately beyond, in the mass region A 195, and in the actinide nometer pairs. The use of such other long-lived ( 1y2 10 40 87 138 147 176 region are compatible with the corresponding solar ratios. We years) radioactivities as K, Rb, La, Sm, and Lu is note that this is true despite the fact that the r-processyFe ratio in the star CS 22892–052 is '10–50 times the solar system 232 © 1998 by The National Academy of Sciences 0027-8424y98y9518-4$2.00y0 value. This does not, however, guarantee that the Th PNAS is available online at http:yywww.pnas.org. abundance in this star can provide a good age estimate. 18 Downloaded by guest on September 23, 2021 Colloquium Paper: Truran Proc. Natl. Acad. Sci. USA 95 (1998) 19 The straightforward conclusion to be drawn from these Pi 2l ~ ! 5 ~ 2 it! observational behaviors is that r-process nucleosynthesis, and Ni t l 1 e the associated production of the critical actinide nuclear i chronometers we have identified, first occurs during the very The astrophysical input to these equations involves the rate of earliest stages of galactic evolution and, therefore, most likely formation of the range of stellar masses within which r-process is associated with the environments provided by the evolution nucleosynthesis occurs. Although calculations of r-process '. of massive stars (M 10 MJ) and type II supernovae. This nucleosynthesis have been carried out for a variety of plausible supports the viewpoint that the nucleosynthesis history we are astrophysical sites [see, e.g., the reviews by Hillebrandt (20) probing with the actinide radioactive isotopes is indeed the and Meyer (21)], a firm identification of the appropriate entire history of the Galaxy. The production history of the environment has become possible only recently. Observations 232Thy238U and 235Uy238U chronometers produced by the of heavy element abundance patterns in metal-deficient halo r-process should trace the rate of star formation activity in the stars point strongly to the identification of r-process nucleo- Galaxy. This implies that 232Thy238U and 235Uy238U chronom- synthesis with the environments provided by the evolution of eter dating should therefore provide an excellent measure of massive stars and supernovae of Type II. In this context, the the age of the Galaxy. most promising mechanism of r-process synthesis would ap- 2. Age Determinations with r-Process Chronometers. The pear to be that associated with the neutrino-heated ‘‘hot critical input astrophysical quantities required for dating the bubble’’ supernova ejecta (22–23), although an r-process as- epoch of galactic nucleosynthesis include: (i) the abundance sociated with the decompression of cold neutron matter from ratios characterizing the matter that condensed into meteorites neutron star mergers (24) provides a viable alternative. An when the solar system formed and (ii) the production ratios of important consequence of the identification of the r-process the isotopes of uranium and thorium in the relevant (r-process) with such massive stars (M .'10 MJ) of short lifetimes (t , nucleosynthesis site. '108 years) is that we can reasonably expect that the age we Abundance determinations for the thorium and uranium determine from r-process chronometer studies is indeed rep- isotopes of interest are provided by analyses of meteoritic resentative of the age of the Galaxy itself. material. The situation for the 232Th–238U pair is complicated The effects of galactic chemical evolution introduce signif- by the chemical differences between the two elements. Anders icant complications for age determinations.
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