Some selected comments on cosmic radioactivities

Radionuclides with half-lives ranging from some years to billions of years presumably synthesized outside the solar system are now recorded in 'live' or 'fossil' form in various types of materials, like meteorites or the galactic cosmic rays. They bring specific astrophysical messages, the deciphering of which is briefly reviewed here, with special emphasis on the contribution of Jerry Wasserbug First, the virtues of the long-lived (half-lives t(1/2) close to, or in excess of 10(9) yr) radionuclides as galactic chronometers are discussed in the light of recent observational and theoretical works. It is concluded that the trans-actinide clocks based on the solar system abundances of Th-232, U-235, and U-238, or on the Th-232 surface content of some old stars are still unable to meaningfully complement galactic age estimates derived from other independent astrophysical methods. In this respect, there is reasonable hope that the Re-187-Os-187 chronometric pair could offer better prospects. The special case of Lu-176, which is a pure s-process product, is also reviewed. It is generally considered today that this radionuclide cannot be viewed as a reliable s-process chronometer. Second, we comment on the astrophysical messages that could be brought by short-lived (10(5) less than or similar to t(1/2) less than or similar to 10(8) yr) radionuclides that have been present in live or in fossil form in the early solar system. From an astrophysical point of view, the demonstrated early existence of live short-lived radionuclides is generally considered to provide the most sensitive radiometric probe concerning discrete nucleosynthetic events that presumably contaminated the solar system at times between about 10(5) and 10(8) yr prior to the isolation of the solar material from the general galactic material. Of course, this assumes implicitly that the radionuclides of interest have not been synthesized in the solar system itself. This is still a matter of debate, as we briefly stress. If indeed the short-lived radionuclides that have been present live in the early solar system are not of local origin, the external contaminating agents that have been envisioned are supernovae, evolved stars of the Asymptotic Giant Branch (AGB) type, or massive mass-losing stars of the Wolf-Rayet (WR) type. We comment on some aspects of the AGE or WR contamination. In the latter case, we discuss more specifically the role of rotation and of binarity on the predicted yields of Al-26, a radionuclide of special cosmochemistry and astrophysics interest. Some comments are also devoted to Sm-146 and Pb-205. The former one is a short-lived p-process radionuclide that has most probably been in live form in the solar system, while the latter one is of s-process origin. It is shown to raise interesting nuclear physics and astrophysics questions, and to deserve further cosmochemical studies in order to evaluate its probability of existence in live form in the early solar system. Third, the case of extinct short-lived radioactivities carried by pre-solar grains is shortly mentioned, and some comments are made about the possible origin of these grains. Finally, a brief mention is mode to gamma-ray line astrophysics, which provides interesting information on live short-lived mdionuclides in the present interstellar medium, and thus complements in a very important way, the study of extinct radionuclides in meteorites. This is illustrated in the case of Al-26. (C) 2000 Elsevier Science B.V. Ail rights reserved.