太空中的碳酸盐-低温数据的挑战
Carbonates in space - The challenge of low temperature data
摘要：
碳酸盐作为星尘发射带的可能载体已经被反复讨论过。然而，迄今为止提出的频带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，它们在低温下的介电函数的变化是相关的。本文从方解石和白云石在300、200、100和10 K下的反射光谱导出了它们的红外光学常数，并计算了不同颗粒形状的小颗粒光谱。结果表明：（1）方解石和白云石的吸收效率因子与颗粒形状密切相关。这是由于CaCO 3和CaMg[CO 3]2的光学常数的高峰值。ii）方解石和白云石的远红外特性也非常显著地取决于温度。低于200 K，FIR共振的带强度发生明显的锐化和增加。iii）鉴于方解石在200- 100 K的44 mum带的内在强度和锐化，PNE中没有这个带-从红外空间观测数据推断-需要尘埃温度低于45 K。iv）在如此低的温度下的方解石颗粒可以解释“92”μ m带，而我们的数据排除了白云石作为60-65 μ m带的载体。这里给出的光学常数可在电子数据库http://www.astro.uni-jena.de/Laboratory/OCDB中公开获得。
英文摘要：
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 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 IR optical constants of calcite and dolomite from reflectance spectra - measured at 300, 200, 100 and 10K - and calculated small particle spectra for different grain shapes, with the following results: i) The absorption efficiency factors both of 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. ii) The far infrared properties of calcite and dolomite depend also very significantly on the temperature. Below 200K, a pronounced sharpening and increase in the band strengths of the FIR resonances occurs. iii) In view of the intrinsic strength and sharpening of the 44 mum band of calcite at 200-100K, the absence of this band -- inferred from Infrared Space Observatory data -- in PNe requires dust temperatures below 45K. iv) Calcite grains at such low temperatures can account for the '92' mum band, while our data rule out dolomite as the carrier of the 60-65 mum band. The optical constants here presented are publicly available in the electronic database http://www.astro.uni-jena.de/Laboratory/OCDB
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