Carbonates in space - The challenge of low temperature data

TL;DR

This study measures the low-temperature infrared optical constants of calcite and dolomite to better understand their spectral features in space, revealing temperature-dependent changes crucial for identifying carbonate minerals in astronomical environments.

Contribution

The paper provides new low-temperature optical constants for calcite and dolomite, enabling more accurate modeling of their infrared spectra in space conditions.

Findings

Calcite's 44 μm band sharpens and intensifies below 200K.

Calcite at low temperatures can explain the 92 μm band in planetary nebulae.

Dolomite is unlikely to be the carrier of the 60-65 μm band.

Abstract

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