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arXiv 1602.00966astro-ph.SRastro-ph.EP
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太阳系中~(92)Nb的初始丰度和分布

The initial abundance and distribution of 92Nb in the Solar System

Tsuyoshi Iizuka, Yi-Jen Lai, Waheed Akram, Yuri Amelin, Maria Schönbächler

摘要:

铌-92是一种已灭绝的富质子核素,衰变为92 Zr,半衰期为37 Ma。这种放射性核素可能提供一个独特的机会,以确定早期太阳系过程的时间尺度和p-核的核合成的网站,一旦它在太阳系中的初始丰度和分布得到很好的确定。在这里,我们提出了内部Nb-Zr等时线的三个玄武质无球粒陨石与已知的U-Pb年龄:angrite NWA 4590,eucrite Agoult,和ungrouped无球粒陨石Ibitira。结果表明,3块陨石的Nb-Zr等时线年龄与辉石和斜长石Pb-Pb测年结果一致,表明92 Nb在源区分布均匀。NWA 4590的Nb-Zr和Pb-Pb数据提供了最可靠和精确的参考点,可以将Nb-Zr计时器固定在绝对时间尺度上:在$4557.93 \pm 0.36$ Ma时,92 Nb/93 Nb的初始比值为$(1.4 \pm 0.5) \times 10^{-5}$,这对应于太阳系形成时的92 Nb/93 Nb比值为$(1.7 \pm 0.6) \times 10^{-5}$。在这个新的初始比率的基础上,我们证明了铌锆计时器的能力,以日期早期太阳系天体,包括陨硫铁和金红石,如铁和石铁陨石。此外,我们估计92 Nb与p核92 Mo的核合成生产比在0.0015和0.035之间。如果这些轻p核主要是通过Ia型超新星中的光致蜕变反应合成的,那么这个产生率,以及太阳中其他质量相似的p核丰度,就可以得到最好的解释。

英文摘要:

Niobium-92 is an extinct proton-rich nuclide, which decays to 92Zr with a half-life of 37 Ma. This radionuclide potentially offers a unique opportunity to determine the timescales of early Solar System processes and the site(s) of nucleosynthesis for p-nuclei, once its initial abundance and distribution in the Solar System are well established. Here we present internal Nb-Zr isochrons for three basaltic achondrites with known U-Pb ages: the angrite NWA 4590, the eucrite Agoult, and the ungrouped achondrite Ibitira. Our results show that the relative Nb-Zr isochron ages of the three meteorites are consistent with the time intervals obtained from the Pb-Pb chronometer for pyroxene and plagioclase, indicating that 92Nb was homogeneously distributed among their source regions. The Nb-Zr and Pb-Pb data for NWA 4590 yield the most reliable and precise reference point for anchoring the Nb-Zr chronometer to the absolute timescale: an initial 92Nb/93Nb ratio of $(1.4 \pm 0.5) \times 10^{-5}$ at $4557.93 \pm 0.36$ Ma, which corresponds to a 92Nb/93Nb ratio of $(1.7 \pm 0.6) \times 10^{-5}$ at the time of the Solar System formation. On the basis of this new initial ratio, we demonstrate the capability of the Nb-Zr chronometer to date early Solar System objects including troilite and rutile, such as iron and stony-iron meteorites. Furthermore, we estimate a nucleosynthetic production ratio of 92Nb to the p-nucleus 92Mo between 0.0015 and 0.035. This production ratio, together with the solar abundances of other p-nuclei with similar masses, can be best explained if these light p-nuclei were primarily synthesized by photodisintegration reactions in Type Ia supernovae.

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