The origin of short-lived radionuclides in the solar system

doi:10.1016/j.newar.2006.06.053

New Astronomy Reviews

Volume 50, Issues 7–8, October 2006, Pages 596-599

https://doi.org/10.1016/j.newar.2006.06.053 Get rights and content

Abstract

Establishing the origin of short-lived radionuclides (SRs) with half-lives ∼Ma has important consequences for the astrophysical context of our sun’s birth place. I discuss here the different explanations that can account for the roster of SRs present in the solar accretion disk 4.57 Ga ago, emphasizing on their dependence on the poorly known initial value of SRs.

Introduction

Short-lived radionuclides (SRs) are radioactive elements whose half-life (≦100 Ma) is small compared to the age of our solar system. Although they have now decayed, their presence in the solar accretion disk is established from excesses of their daughter isotopes in primitive meteoritic components such as calcium-, aluminium-rich inclusions (CAIs) and chondrules. The abundance of some SRs in the solar accretion disk is compatible with the expectations of continuous galactic nucleosynthesis, while some require a last minute origin, such as local production via irradiation in the solar accretion disk, or external stellar nucleosynthesis followed by injection. The origin of SRs has many implications regarding the astrophysical context of our sun’s birth, early solar system chronology, stellar nucleosynthesis models or irradiation processes around young stellar objects. Since many detailed reviews have been recently published on the topic (e.g. Goswami et al., 2005, Chaussidon and Gounelle, 2006, Wadhwa et al., in press), I will concentrate here on the different models’ difficulties, and especially highlight their dependence on the often poorly determined initial value of SRs in the solar accretion disk.

Section snippets

Short-lived radionuclides in the early solar system

To constrain the origin of short-lived radionuclides, it is key to identify their initial value in the accretion disk. For many years, the initial value of SRs was identified with that of CAIs, because they have a mineralogy broadly compatible with that of an equilibrium condensation sequence, and an old Pb–Pb age (e.g. Wood, 2004). Precise Pb–Pb age of chondrules was unreachable by state-of-the-art techniques for many years. However, assuming an homogeneous distribution of ²⁶Al in the solar

Steady-state abundance of short-lived radionuclides in the interstellar medium

On-going nucleosynthesis by a diversity of stars (supernovae, novae, AGB stars, etc.) in the Galaxy continuously replenishes the interstellar medium with freshly made SRs. At a given time in the history of the Galaxy, the background abundance of a given short-lived radionuclide R relative to its stable reference isotope S will depend on a diversity of parameters such as the number and nature of nucleosynthetic events responsible for the production of R over the last few half-lives of R, the

Last minute production of short-lived radionuclides

Desch et al. (2004) proposed that ¹⁰Be originated from the trapping of high energy Galactic Cosmic Rays in the molecular cloud core progenitor of our solar system. Neglecting the focusing influence of the magnetic fields whose effect is small, it is easy to show that the ¹⁰Be abundance depends only on the trapping time, the surface density of the core and the flux of cosmic rays. In their calculation, Desch et al. (2004) adopt a trapping time (∼10 Ma) which is considerably longer than the

The astrophysical context of our sun’s birth

Elucidating the origin of short-lived radionuclides can potentially help constrain the astrophysical context of our solar system’s birth. Caricaturally, two possibilities are offered: either a setting where the sun has formed by isolation, such as in the molecular clouds Taurus or ρ Oph, or a setting where the sun forms in a crowded region such as the Orion Nebular Cluster (e.g. Ouellette et al., 2005). In the first case, there are no high-mass stars to go supernova, while in the second case,

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We report elemental abundances and the isotopic systematics of the short-lived ²⁶Al–²⁶Mg (half-life of ∼0.73 Ma) and long-lived U–Pb radiochronometers in the ungrouped basaltic meteorite Northwest Africa (NWA) 2976. The bulk geochemical composition of NWA 2976 is clearly distinct from that of the eucrites and angrites, but shows broad similarities to that of the paired NWA 001 and 2400 ungrouped achondrites indicating that it is likely to also be paired with these two samples. The major and trace element abundances in NWA 2976 further indicate that it formed by extensive melting and magmatic fractionation processes on its parent body. The Al–Mg and Pb–Pb isotope systematics indicate that this meteorite represents the earliest stages of crust formation on a differentiated parent body in the early Solar System. The absolute Pb–Pb internal isochron age of NWA 2976, obtained from acid leaching residues of three whole-rock samples and two pyroxene separates, is 4562.89 ± 0.59 Ma (MSWD = 0.02). This Pb–Pb age is calculated using the measured ²³⁸U/²³⁵U ratio of a NWA 2976 whole-rock of 137.751 ± 0.018 (2σ) which was determined relative to the recently revised value of 137.840 ± 0.008 for the SRM 950a U isotope standard. The Al–Mg systematics reveal the presence of ²⁶Mg isotopic anomalies produced by the decay of ²⁶Al with an (²⁶Al/²⁷Al)₀ of (3.94 ± 0.16) × 10⁻⁷, and indicate a time of formation of 0.26 ± 0.18 Ma after the D’Orbigny angrite. Using the revised Pb–Pb age of 4563.36 ± 0.34 Ma for the D’Orbigny anchor (corrected for its U isotopic composition), we deduce an Al–Mg model age of 4563.10 ± 0.38 Ma for NWA 2976, which is consistent with its Pb–Pb internal isochron age.
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The supernova injection model for the origin of the short-lived radionuclides (SLRs) in the early solar system is reviewed. First, the meteoritic evidence supporting the model is discussed. Based on the presence of ⁶⁰Fe it is argued that a supernova must have been in close proximity to the nascent Solar System. Then, two models of supernova injection, the supernova trigger model and the aerogel model, are described in detail. Both these injection model provide a mechanism for incorporating SLRs into the early solar system. Following this, the mechanisms present in the disk to homogenize the freshly injected radionuclides, and the timescales associated with these mechanisms, are described. It is shown that the SLRs can be homogenized on very short timescales, from a thousand years up to ∼1 million years. Finally, the SLR ratios expected from a supernova injection are compared to the ratios measured in meteorites. A single supernova can inject enough radionuclides to explain the radionuclide abundances present in the early solar system.
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The origin of short-lived ( $T \sim 1$ Myr) radionuclides (SRs) in the early solar system is a matter of debate. Some short-lived radionuclides had abundances in the solar protoplanetary disk in excess compared to the expected galactic background ( ${}^{7}{Be}$ , ${}^{10}{Be}$ , ${}^{26}{Al}$ , ${}^{36}{Cl}$ , ${}^{41}{Ca}$ and possibly ${}^{53}{Mn}$ and ${}^{60}{Fe}$ ). These SRs thus either originated from a supernova contamination, or were produced by in situ irradiation of solar system dust or gas, or by Galactic Cosmic Ray (GCR) trapping in the molecular cloud core progenitor of our solar system (for ${}^{10}{Be}$ only). GCR trapping in the molecular cloud core seems to fail to reproduce the initial abundance of ${}^{10}{Be}$ , because trapping timescales exceed by one order of magnitude the observed core lifetimes. On the other hand, irradiation models can synthesize large quantities of ${}^{10}{Be}$ and other SRs ( ${}^{7}{Be}$ , ${}^{36}{Cl}$ , ${}^{26}{Al}$ , ${}^{41}{Ca}$ and ${}^{53}{Mn}$ ). In addition, X-ray observations of young, solar-like stars provide direct evidence for protoplanetary disk irradiation in a different energy window. The initial abundance of ${}^{60}{Fe}$ is poorly known, and its presence in the early solar system might be accounted for galactic abundance rather than by a nearby supernova.
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The origin of short-lived radionuclides in the solar system

Abstract

Introduction

Section snippets

Short-lived radionuclides in the early solar system

Steady-state abundance of short-lived radionuclides in the interstellar medium

Last minute production of short-lived radionuclides

The astrophysical context of our sun’s birth

Icarus

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Science

Nature

ApJ

ApJ

ApJ

ApJ

Nature

ApJS