EVAPORATION AND CONDENSATION KINETICS OF CORUNDUM: THE ORIGIN OF THE 13 μm FEATURE OF OXYGEN-RICH AGB STARS
EVAPORATION AND CONDENSATION KINETICS OF CORUNDUM: THE ORIGIN OF THE 13 μm FEATURE OF OXYGEN-RICH AGB STARS
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DOI：
10.1088/0067-0049/218/1/2
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发表时间：
2015-05-01
影响因子：
8.7
通讯作者：
Ozawa, Kazuhito
中科院分区：
文献类型：
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作者：
Takigawa, Aki;Tachibana, Shogo;Ozawa, Kazuhito
刚玉被预测为最丰富的难熔星际尘埃，并被认为是富氧渐近巨星分支恒星红外光谱中13微米特征的候选者。结晶尘埃具有反映各向异性晶体结构和形成条件或过程的形态。并且可以发出特有的红外线特征。在本研究中，我们在真空中进行了不同晶向刚玉的蒸发和冷凝实验，以确定控制形态各向异性的关键动力学参数：蒸发和冷凝系数(α(E)和α(C))、蒸发和冷凝的动力学阻碍程度。根据实验结果对刚玉凝聚体的可能形状进行了估计，以评价刚玉凝聚体的红外特征。刚玉的蒸发系数在1600-1785℃范围内为0.02-0.2，随温度升高而增大，表现出明显的各向异性。Ae沿晶体c、a和M轴的顺序是α(M)(E)>；α(E)(A)>；α(C)(E)，与温度无关。在1575℃和类似于4的过饱和度比下，沿c、a和m轴的凝结系数分别为0.04-0.06、0.06-0.08和0.1-0.2。从我们的实验中预计，凝聚的环绕恒星的刚玉的形态是扁平的，并稍微扁平地向c轴移动。并与尚未发现偏心形状的极前刚玉的事实相一致。扁平刚玉的质量吸收系数在13微米处出现一个峰值，没有伴随的强吸收峰。这些结果有力地表明，在星周环境中以各向异性凝聚的刚玉是富氧演化恒星周围不明13微米特征的载体。
Corundum is predicted to be the most abundant refractory circumstellar dust and has been suggested as a candidate emitting the 13 mu m feature in infrared spectra of oxygen-rich asymptotic giant branch stars. Crystalline dust has morphologies reflecting the anisotropic crystal structure and formation conditions or processes. and may emit characteristic infrared features. In this study, evaporation and condensation experiments of corundum along different crystallographic orientations were performed in vacuum to determine key kinetic parameters controlling morphological anisotropy: evaporation and condensation coefficients (alpha(e) and alpha(c)), the degrees of kinetic hindrance on evaporation and condensation. Plausible shapes of corundum condensates were estimated from the experimental results to evaluate the infrared features of corundum condensates. The evaporation coefficients of corundum are 0.02-0.2 at 1600-1785 degrees C, which increase with temperature and show notable anisotropy. The order of ae along the crystallographic c, a, and m. axes is alpha(m)(e) >> alpha(e) (a) > alpha(c)(e) irrespective of temperature. The obtained condensation coefficients along the c, a, and m. axes at 1575 degrees C and a supersaturation ratio of similar to 4 are 0.04-0.06, 0.06-0.08, and 0.1-0.2, respectively. The morphology of condensed circumstellar corundum expected from our experiments is oblate and slightly flattened to the c. axis. and is consistent with the fact that no presolar corundum with eccentric shapes has been found. The mass absorption coefficient of oblate corundum slightly flattened to the c. axis shows a peak at 13 mu m without any accompanying strong peaks. These results strongly indicate that corundum condensed anisotropically in circumstellar environments is a carrier of the unidentified 13 mu m feature around oxygen-rich evolved stars.