The origin of short-lived radionuclides and the astrophysical environment of solar system formation

Based on early solar system abundances of short-lived radionuclides (SRs), such as $^{26}$Al (T$_{1/2} = 0.74$ Myr) and $^{60}$Fe (T$_{1/2} = 1.5$ Myr), it is often asserted that the Sun was born in a large stellar cluster, where a massive star contaminated the protoplanetary disk with freshly nucleosynthesized isotopes from its supernova (SN) explosion. To account for the inferred initial solar system abundances of short-lived radionuclides, this supernova had to be close ($\sim$ 0.3 pc) to the young ($\leqslant$ 1 Myr) protoplanetary disk. Here we show that massive star evolution timescales are too long, compared to typical timescales of star formation in embedded clusters, for them to explode as supernovae within the lifetimes of nearby disks. This is especially true in an Orion Nebular Cluster (ONC)-type of setting, where the most massive star will explode as a supernova $\sim$ 5 Myr after the onset of star formation, when nearby disks will have already suffered substantial photoevaporation and/or formed large planetesimals. We quantify the probability for {\it any} protoplanetary disk to receive SRs from a nearby supernova at the level observed in the early solar system. Key constraints on our estimate are: (1) SRs have to be injected into a newly formed ($\leqslant$ 1 Myr) disk, (2) the disk has to survive UV photoevaporation, and (3) the protoplanetary disk must be situated in an enrichment zone permitting SR injection at the solar system level without disk disruption. The probability of protoplanetary disk contamination by a supernova ejecta is, in the most favorable case, 3 $\times$ 10$^{-3}$

Silicon isotope variations in the Earth and meteorites

A fluorhydric acid-free sample preparation method derived from Georg et al. [1] has been used to measure the natural variations of silicon isotope compositions in terrestrial (including 12 geological standard materials) and meteoritic bulk-rock samples. All measurements were done using a Neptune MC-ICPMS in medium resolution mode (m/Δm = 7000, peak-edge definition). Magnesium was used as internal standard for mass-bias drift correction. The δ30Si values are expressed relative to the NBS-28 silica standard. IRMM-17 reference material yields a δ30Si of -1.4‰ ± 0.05‰ (2SD, n=11) in agreement with previous data [2-3]. Long-term reproducibilities were obtained for BHVO-2 (δ30Si = -0.27‰ ± 0.08‰ (2SD, n=30)) and a in-house Si standard (δ30Si = -0.01‰ ± 0.07‰ (2SD, n=20)) on a 7 months time scale. Total variation of δ30Si in natural samples ranges from - 0.5‰ to -0.1‰. Comparison with δ29Si values shows that this isotopic fractionation is mass-dependent. A 0.2‰ isotopic variation occurs among terrestrial samples suggesting an enrichment in the heavier silicon isotopes as a function of magma differentiation, as initially hinted by Douthitt [4]. Terrestrial samples mean value (δ30SiEarth= -0.23‰) is heavier by about 0.24‰ in δ30Si compared to chondrites. This may be explained by silicon isotope fractionation during planetary accretion and/or differentiation