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 gala ctic cosmic rays. They bring specific astrophysical messages, the decipheri ng of which is briefly reviewed here, with special emphasis on the contribu tion of Jerry Wasserbug First, the virtues of the long-lived (half-lives t(1/2) close to, or in exc ess of 10(9) yr) radionuclides as galactic chronometers are discussed in th e 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 st ill unable to meaningfully complement galactic age estimates derived from o ther independent astrophysical methods. In this respect, there is reasonabl e hope that the Re-187-Os-187 chronometric pair could offer better prospect s. The special case of Lu-176, which is a pure s-process product, is also r eviewed. 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 s hort-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 t he early solar system. From an astrophysical point of view, the demonstrate d early existence of live short-lived radionuclides is generally considered to provide the most sensitive radiometric probe concerning discrete nucleo synthetic events that presumably contaminated the solar system at times bet ween 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 syste m 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 sol ar system are not of local origin, the external contaminating agents that h ave been envisioned are supernovae, evolved stars of the Asymptotic Giant B ranch (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 c ase, we discuss more specifically the role of rotation and of binarity on t he 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 proba bly been in live form in the solar system, while the latter one is of s-pro cess origin. It is shown to raise interesting nuclear physics and astrophys ics questions, and to deserve further cosmochemical studies in order to eva luate 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 prov ides interesting information on live short-lived mdionuclides in the presen t interstellar medium, and thus complements in a very important way, the st udy of extinct radionuclides in meteorites. This is illustrated in the case of Al-26. (C) 2000 Elsevier Science B.V. Ail rights reserved.