早期太陽系裡鈣同位素驚奇的異常

Abstract

本實驗是藉由增加Ca電流強度至1.2×10nA來改善Ca同位素的精確度之測量。把42Ca/44Ca用來修正質量的分化後，40Ca/44C、43Ca/44C、46Ca/44C以及48Ca/44C的兩個標準差分別爲1.5、0.3、7.5 及 0.7ε (ε ≡ 0.01%)。這些改善後的精確度不但可以確定在普通的CAIs (Ca-Al-Inclusions) 裡48Ca的異常值高達5.1ε，同時也可尋找隕石裡Ca同位素的變化。在此之前，48Ca的異常只有在CAIs裡可以看見，但令人驚訝的是：實驗結果顯示，與地球上的樣品相比，非球粒隕石的48Ca是呈現負的，達－2.3ε。因爲非球粒隕石是被假設爲從數百公里之巨大的融化形態小行星凝固而成，我們並不期待可測出任何的異常。在另一方面，46Ca的含量仍然是與地球上樣品一致，而很多48Ca的異常與50Ti、54Cr有相關性。這個相關性指出這些核種是從少見的neutron rich nuclear statistic equilibrium (n-rich NSE) 產生，而不是藉由中子的捕獲。然而，實際狀況仍然不清楚。至於這些由少見的Ia型超新星產生的成份，如何在太陽系中非球粒隕石凝固之後仍不被混合，目前較被支持的推論傾向於：在非球粒隕石凝固後，類地行星形成之前，在太陽系鄰近地區有一顆超新星爆炸，太陽系接受其噴發物質，所導致而成。

We have achieved higher precision for the measurement of Ca isotopic ratio by increasing the beam intensity to around 1.2nA. 42Ca/44Ca was normalized in order to correct the remaining ratios for fractionation. The two standard deviations in reproducibility for 40Ca/44Ca, 43Ca/44Ca, 46Ca/44Ca and 48Ca/44Ca were 1.5, 0.3, 7.5 and 0.7ε (ε ≡ 0.01%) respectively. The improved precision enabled us to not only confirm 48Ca anomalies up to 5.1ε in ordinary CAIs (Ca-Al-Inclusions) in meteorites but also search for Ca isotopic variation in bulk meteorites. Prior to this, 48Ca anomalies were only endemic to CAIs in carbonaceous chondrites. Surprisingly, our experiment revealed 48Ca deficit up to －2.3ε in achondrites relative to terrestrial samples. Because achondrites were presumably solidified from large scale melting on planetary bodies hundreds of km in size, they were not expected to present any detectable heterogeneities. On the other hand, the widespread 48Ca anomalies that correlate with 50Ti and 54Cr without large effects on the tiny 46Ca indicate that these nuclides originated from the rare neutron rich nuclear statistic equilibrium (n-rich NSE), not neutron capture. However, it remains unclear. As to how to keep this nucleosynthetic component made only in rare subset of type Ia supernovae from completely mixed in the solar system after the achondrites being solidified, it lends some support to the speculation that the solar system received some injected material from a nearby supernova that happened to explode after the solidification of achondrites but before the formation of terrestrial planets.