On the Origin of Crystalline Silicate in Circumstellar Envelopesof Oxygen-rich Asymptotic Giant Branch Stars
On the Origin of Crystalline Silicate in Circumstellar Envelopesof Oxygen-rich Asymptotic Giant Branch Stars
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富氧渐近巨分支星星周包层中结晶硅酸盐的起源
DOI：
10.1086/311992
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发表时间：
1999
期刊：
影响因子：
--
通讯作者：
T. Kozasa
中科院分区：
文献类型：
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作者：
H. Sogawa;T. Kozasa
应用结晶动力学理论，研究了富氧渐近巨星(AGB)周围稳态尘埃驱动风中的尘埃形成过程中结晶硅酸盐的形成。在计算中，不仅考虑了均匀刚玉(Al_2O_3)和硅酸盐颗粒的形成，而且考虑了由刚玉核和硅酸盐地幔组成的非均相颗粒的形成，恒星参数M*=1.0M☉，L*=2×10~4 L☉，T*=2500K。根据尘粒形成后的热历史进行的结晶计算表明，当质量损失率大于3×10−5M☉yr-1时，通过硅酸盐在预凝刚玉颗粒上的堆积而引起的热处理，非均质颗粒地幔中的硅酸盐可以完全结晶。另一方面，均匀的硅酸盐颗粒保持无定形。以橄榄石为代表的结晶硅酸盐的辐射传输计算再现了红外空间天文台-短波长谱仪观测揭示的硅酸盐特征的行为。在富氧AGB星周围观察到的结晶硅酸盐反映了星周包层中普遍存在的尘埃颗粒的形成过程，是AGB星演化后期高质量损失率的诊断。
Formation of crystalline silicate is explored within the framework of dust formation in steady state dust-driven winds around oxygen-rich asymptotic giant branch (AGB) stars by applying a kinetic theory of crystallization. In the calculations, formation of not only homogeneous corundum (Al2O3) and silicate grains but also heterogeneous grains consisting of a corundum core and a silicate mantle is taken into account for the stellar parameters M*=1.0 M☉, L*=2×104 L☉, and T*=2500 K. Crystallization calculations based on the thermal history of dust grains after their formation lead to the result that only silicate in the mantles of heterogeneous grains can be completely crystallized through the thermal processing caused by the accretion of silicate on precondensed corundum grains when the mass-loss rate is greater than 3×10−5 M☉ yr-1. On the other hand, the homogeneous silicate grains remain amorphous. The radiative transfer calculations taking olivine as a representative of crystalline silicate reproduce the behavior of silicate features revealed by the Infrared Space Observatory-Short-Wavelength Spectrometer observations. The crystalline silicate observed around oxygen-rich AGB stars reflects the formation process of dust grains prevailing in the circumstellar envelopes and is a diagnostic of high mass-loss rates at the late stage of AGB star evolution.