s-過程原子核合成能解釋隕石中鈣同位素的異常嗎?

Abstract

自 1970 年代以來,隕石中的鈣同位素就開始被研究。相較於地球 的同位素,科學家發現這些鈣同位素普遍存在一些異常,其中就包括 了鈣 -48。透過 Ia 型超新星模型,這些鈣 -48 同位素異常能夠獲得解 釋。另一方面,太陽前顆粒的研究也顯示了鈣 -42,43,44 和鈦同位素異 常現象。Amari 等人(2000 年)和 Nittler 等人(2008)都不約而同都 透過漸近巨星分支模型來解釋這些同位素異常。 後來,這些原本只在太陽前顆粒中出現的鈣同位素異常,在隕石中 的鈣鋁包裹體(CAIs)中也被發現了。這些發現的鈣同位素異常分別 為 0.29 ± 0.16ε(43Ca/44Ca), 1.11 ± 5.51ε(46Ca/44Ca)和 4.48 ± 0.29ε (48Ca/44Ca)。因此,我們試圖模仿一個漸近巨星分支模型來解釋這些 存在於 CAIs 中的鈣同位素異常現象。 在這個模型中,我們透過不同的參數,尤其是中子密度來模擬各種 結果。大致上這些模擬結果都得出一致的結果:鈣 -42,43 可以透過慢 中子捕獲作用來大量產生。雖然這模型製造出少量的鈣 -46,不過在同 位素研究中常用的正規化處理後,我們會得到明顯多餘的鈣 -46。即便 如此,只要我們限制中子撞擊的反應時間,這個鈣 -46 的異常就可以 獲得有效控制。此外,如果我們考慮與星際物質的混合作用,那麼就 可以推論處那些輕質量、以碳 -13 為中子來源的漸近巨星分支恆星, 能夠合理解釋這些鈣同位素異常現象。

From 1970s, the calcium isotopes of meteorites have been studied. Compare with the terrestrial abundance, there exists some anomalies, including 48Ca, within these isotopes. With a neutron-rich Type Ia supernovae model, the 48Ca and others neutron rich isotopic anomalies had be solved. On the other hand, the pre-solar grains studies also indicated both calcium(42Ca, 43Ca, 44Ca) and titanium anomalies. Amari et al.2000 and Nittler et al.2008 succeed to explain these anomalies through an asymptotic giant branch(AGB) star model to some degree. The 43Ca anomalies, which has been found in presolar grains and solved by an AGB star model, was found in the calcium-aluminium inclusions(CAIs) recently. The excess of 43Ca/44Ca, 46Ca/44Ca & 48Ca/44Ca in the CAIs were found to be 0.29±0.16, 1.11±5.51, 4.48±0.29 respectively. For that reason, an AGB model is suggested to explain the CAIs measurement results. In such a model, different parameter, especially the main factor, neutron density was tuned in the simulation. All of the simulation generated a consistent result - less neutron-rich isotopes(42Ca, 43Ca) can be created through the s-process. Even through this model create less 46Ca anomalies, a significant excess of 46Ca can be seen after the strategy of normalize to solar abundance which is often used to analyse isotopic abundance. Nevertheless, a short time neutron exposure will leave 46Ca no effect. In another case, if the mixing model is considered, the 13C neutron source AGB stars will be a good candidate for this 46Ca anomalies.