Astronomy. The cradle of the solar system.

Department of Physics and Astronomy, Arizona State University, Tempe, AZ 85287-1504, USA.
Science (Impact Factor: 31.48). 06/2004; 304(5674):1116-7. DOI: 10.1126/science.1096808
Source: PubMed

ABSTRACT The recent discovery of decay products of 60Fe in meteorites challenges
conventional wisdom about the environment in which the Sun and planets
formed. Rather than a region like the well-studied Taurus-Auriga
molecular cloud, the solar system must have formed instead in a region
more like the Eagle nebula--a region that contained one or more massive
stars that went supernova, injecting newly synthesized radionuclides
into the nascent solar system. In their Perspective, Hester et al.
discuss a scenario by which the solar system--and other low-mass stars
like the Sun--could have formed. Radiant energy from massive, luminous
stars first compresses surrounding interstellar gas, triggering the
formation of Sun-like stars, then quickly disperses this material,
exposing newborn stars and their protoplanetary disks to harsh radiation
from the massive stars. When the massive stars go supernova, they pelt
surrounding protoplanetary disks with ejecta laden with the products of
stellar nucleosynthesis that are required to explain the isotopic
composition we see today.

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    ABSTRACT: The long-lived176Lu-to-176Hf decay system is a powerful tool to understand ancient chemical fractionation events associated with planetary differentiation. Detrital Hadean zircons (>3.8 Gyr) from the Jack Hills metasedimentary belt of Western Australia record extremely enriched Hf-isotope signals suggesting early extraction of a continental crust (>4.5 Gyr) but fail to identify a prevalent complementary depleted mantle reservoir, suggesting that crust formation processes in the early Earth were fundamentally distinct from today. However, this conclusion assumes that the Hf-isotope composition of bulk chondrite meteorites can be used to estimate the composition of Earth prior to its differentiation into major silicate reservoirs, namely the bulk silicate Earth (BSE). We report a176Lu-176Hf internal mineral isochron age of 4869 ± 34 Myr for the pristine SAH99555 angrite meteorite. This age is ˜300 Myr older than the age of the Solar System, confirming the existence of an energetic process yielding excess 176Hf in affected early formed Solar System objects through the production of the 176Lu isomer (t1/2 ˜3.9 hours). Thus, chondrite meteorites contain excess 176Hf and their present-day composition cannot be used to infer the Lu-Hf parameters of BSE. Using a revised BSE estimate based on the SAH99555 isochron, we show that Earth's oldest zircons preserve a record of coexisting enriched and depleted hafnium reservoirs as early as ˜4.3 Gyr in Earth's history, with little evidence for the existence of continental crust prior to ˜4.4 Gyr. This new view suggests continuous juvenile crustal growth and recycling throughout the Hadean and Archean eras, perhaps analogous to modern plate tectonics.
    Geochemistry Geophysics Geosystems 03/2012; 13(3):3002-. DOI:10.1029/2011GC004003 · 3.05 Impact Factor
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    ABSTRACT: This chapter concerns the long-term dynamical evolution of planetary systems from both theoretical and observational perspectives. We begin by discussing the planet-planet interactions that take place within our own Solar System. We then describe such interactions in more tightly-packed planetary systems. As planet-planet interactions build up, some systems become dynamically unstable, leading to strong encounters and ultimately either ejections or collisions of planets. After discussing the basic physical processes involved, we consider how these interactions apply to extrasolar planetary systems and explore the constraints provided by observed systems. The presence of a residual planetesimal disc can lead to planetary migration and hence cause instabilities induced by resonance crossing; however, such discs can also stabilise planetary systems. The crowded birth environment of a planetary system can have a significant impact: close encounters and binary companions can act to destabilise systems, or sculpt their properties. In the case of binaries, the Kozai mechanism can place planets on extremely eccentric orbits which may later circularise to produce hot Jupiters.
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    ABSTRACT: In agreement with previous work, we show that the presence of the short-lived radionuclide (SLR) 26Al in the early solar system was unlikely (less than 2% a priori probability) to be the result of direct introduction of supernova (SN) ejecta into the gaseous disk during the Class II stage of protosolar evolution. We also show that Bondi-Hoyle accretion of any contaminated residual gas from the Sun's natal star cluster contributed negligible 26Al to the primordial solar system. Our calculations are consistent with the absence of the oxygen isotopic signature expected with any late introduction of SN ejecta into the protoplanetary disk. Instead, the presence of 26Al in the oldest solar system solids (calcium-aluminum-rich inclusions (CAIs)) and its apparent uniform distribution with the inferred canonical 26Al/27Al ratio of (4.5-5) × 10–5 support the inheritance of 26Al from the Sun's parent giant molecular cloud. We propose that this radionuclide originated in a prior generation of massive stars that formed in the same molecular cloud and contaminated that cloud by Wolf-Rayet winds. We calculated the Galactic distribution of 26Al/27Al ratios that arise from such contamination using the established embedded cluster mass and stellar initial mass functions, published nucleosynthetic yields from the winds of massive stars, and by assuming rapid and uniform mixing into the cloud. Although our model predicts that the majority of stellar systems contain no 26Al from massive stars, and that the a priori probability that the 26Al/27Al ratio will reach or exceed the canonical solar system value is only ~6%, the maximum in the distribution of nonzero values is close to the canonical 26Al/27Al ratio. We find that the Sun most likely formed 4-5 million years (Myr) after the massive stars that were the source of 26Al. Furthermore, our model can explain the initial solar system abundance of a second, co-occurring SLR, 41Ca, if ~5 × 105 yr elapsed between ejection of the radionuclides and the formation of CAIs. The presence of a third radionuclide, 60Fe, can be quantitatively explained if (1) the Sun formed immediately after the first SNe from the earlier generation of stars; (2) only 5% of SN ejecta was incorporated into the molecular cloud, or (3) the radionuclide originated in an even earlier generation of stars whose contributions to other radionuclides with a shorter half-life had completely decayed.
    The Astrophysical Journal 04/2009; 696(2):1854. DOI:10.1088/0004-637X/696/2/1854 · 6.28 Impact Factor


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