Carbonates in Space: The Challenge of Low-Temperature Data
Carbonates in Space: The Challenge of Low-Temperature Data
复制标题
太空中的碳酸盐：低温数据的挑战
DOI：
--
复制
发表时间：
2007
期刊：
影响因子：
--
通讯作者：
T. Henning
中科院分区：
文献类型：
--
作者：
T. Posch;A. Baier;H. Mutschke;T. Henning
碳酸盐曾多次被讨论为星尘发射带的可能载体。然而，目前提出的波段分配主要是基于各自矿物的室温粉末透射光谱。由于非常冷的方解石颗粒被认为存在于原恒星和行星状星云（如NGC 6302）中，因此从天文学的角度来看，它们在低温下介电功能的变化是相关的。我们从300、200、100和10 K的反射光谱中推导出方解石和白云石的红外光学常数，并计算出不同颗粒形状的小颗粒光谱，结果如下：(1)方解石和白云石的吸光效率因子对颗粒形状的依赖性极大。这是由于CaCO3和CaMg[CO3]2的光学常数的峰值较高。(2)方解石和白云石的远红外性质也与温度有很大关系。在200k以下，远红外共振出现明显的锐化和波段强度的增加。(3)考虑到方解石的~44 μm波段在200-100 K时的固有强度和锐化，从红外空间观测数据推断，PNs中没有该波段需要低于45 K的尘埃温度。(4) 92 μm波段为方解石颗粒，60 ~ 65 μm波段为白云石颗粒。这里展示的光学常数可以在AIU耶拿宇宙尘埃光学常数数据库中公开获得。
Carbonates have repeatedly been discussed as possible carriers of stardust emission bands. However, the band assignments proposed so far were mainly based on room-temperature powder transmission spectra of the respective minerals. Since very cold calcite grains have been claimed to be present in protostars and in planetary nebulae (PNs) such as NGC 6302, the changes of their dielectric functions at low temperatures are relevant from an astronomical point of view. We have derived the infrared optical constants of calcite and dolomite from reflectance spectra, measured at 300, 200, 100, and 10 K, and calculated small-particle spectra for different grain shapes, with the following results. (1) The absorption efficiency factors of both calcite and dolomite are extremely dependent on the particle shapes. This is due to the high peak values of the optical constants of CaCO3 and CaMg[CO3]2 . (2) The far-infrared properties of calcite and dolomite also depend very significantly on the temperature. Below 200 K, a pronounced sharpening and increase in the band strengths of the far-infrared resonances occurs. (3) In view of the intrinsic strength and sharpening of the ~44 μm band of calcite at 200-100 K, the absence of this band, inferred from Infrared Space Observatory data, in PNs requires dust temperatures below 45 K. (4) Calcite grains at such low temperatures can account for the "92" μm band, while our data rule out dolomite as the carrier of the 60-65 μm band. The optical constants presented here are publicly available in the AIU Jena Database of Optical Constants for Cosmic Dust.