科研之友
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摘要
The first million years of the Solar System is a key period when the first solids were formed from the nebula gas. The chronology of the different processes at the origin of these solids is still largely unknown and relies strongly on the assumption made of homogeneous distribution for short-lived radioactive nuclides such as Al-26. This assumption is questioned. In this study, in situ Al-26-Mg-26 isotope systematics was studied with high precision in 12 calcium, aluminum-rich inclusions (CAIs) (1 type A, 2 type B, 5 type C, and 4 fine grained spinel-rich), 2 amoeboid olivine aggregates (AOAs), and 2 Al-rich chondrules from Efremovka and Vigarano. The (Al-26/Al-27)(i) in these early Solar System solids (the subscript %26apos;i%26apos; stands for the initial isotope ratio obtained from the mineral Al-26 isochron) range from similar to 6.5 x 10(-5) to 0.2 x 10(-5) with delta Mg-26(i)* from -0.08 to +0.37%(o). The (Al-26/Al-27)(i) and delta Mg-26(i)* of CAIs and chondrules can be explained by formation of their precursors from a homogeneous reservoir (Solar System Initial, noted hereafter SSI) with initial magnesium isotopic composition of delta Mg-26(SSI)* = -0.052 +/- 0.013%(o) and initial (Al-26/Al-27)(SSI) abundance of (5.62 +/- 0.42) x 10(-5). The high precision magnesium isotope data obtained in the present study and literature data allows identifying a few epochs of formation/reprocessing of CAIs. The time periods of these epochs correspond well with the median life times of the pre-main sequence evolution of stars of Solar mass if we anchor the (Al-26/Al-27)(SSI) = (5.62 +/- 0.42) x 10(-5) to the beginning of class I phase. This provides a natural explanation to the range of (Al-26/Al-27)(I) - (similar to 6 to 0.02) x 10(-5) seen in corundum grains, FUN (Fractionation and Unidentified Nuclear Effects) CAIs, ultrarefractory CAIs, normal CAIs, and chondrules, and suggests a possible relationship between the astrophysical conditions and the formation of these early solids. Corundum grains, FUN CAIs, ultrarefractory CAIs would have formed during the class 0 of the pre main-sequence, during the collapse of the parent molecular cloud, before homogenization was complete while normal CAIs and chondrules formed later during class I to the beginning of class III. Alternatively, if one wants to explain the present data without calling for an early homogenization of Al and Mg isotopes in the forming Solar System, at least four distinct isotopic reservoirs are required to produce by mixing the range observed in delta Mg-26(i)* and (Al-26/Al-27)(i). There is no obvious astrophysical explanation of the existence of such distinct reservoirs, which makes this hypothesis quite ad hoc and thus very unlikely.
- 出版日期2014-3-15
