Fingerprints of the Protosolar Cloud Collapse in the Solar System. I. Distribution of Presolar Short-lived 26 Al

Abstract

The short-lived radionuclide 26 Al is widely used to determine the relative ages of chondrite components and timescales of physical and thermal events that attended the formation of the solar system. However, an important assumption for using 26 Al as a chronometer is its homogeneous distribution in the disk. Yet, the oldest components in chondrites, the Ca–Al-rich inclusions (CAIs), which are usually considered as time anchors for this chronometer, show evidence of 26 Al/ 27 Al variations independent of radioactive decay. Since their formation epoch may have been contemporaneous with the collapse of the parent cloud that formed the disk, this suggests that 26 Al was heterogeneously distributed in the cloud. We model the collapse of such a heterogeneous cloud, using two different 26 Al distributions (monotonic and nonmonotonic), and follow its redistribution in the first condensates and bulk dust that populate the forming disk. We find that CAIs inherit the 26 Al/ 27 Al ratio of the matter infalling at the time of their formation, so that variations of 26 Al/ 27 Al among primordial CAIs can be accounted for, independently of radioactive decay. The prevalence of a canonical ratio among them and its necessity for the differentiation of the first planetesimals suggest a (monotonic) scenario where 26 Al sharply rose relatively close to the center of the protosolar cloud and essentially remained at a high level outward (rather than decreased since). As the 26 Al abundance would be relatively homogeneous after cessation of infall, this would warrant the use of the Al–Mg chronometer from the formation of “regular” CAIs onward, to chondrules and chondrite accretion.