Carbonates in Space: The Challenge of Low‐Temperature Data

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 (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 CaCO 3 and CaMg[CO 3 ] 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 m 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'' m band, while our data rule out dolomite as the carrier of the 60-65 m band. The optical constants presented here are publicly available in the AIU Jena Database of Optical Constants for Cosmic Dust.

Fitting of dust spectra with genetic algorithms

“…An unambiguous determination of the dust composition can constitute a complex challenge, since the spectral behaviour of dust does not only depend on the correct ratio of abundances, but also strongly on such parameters as the dust temperature, the optical depth and the grain shape itself (see e.g. Posch et al 2007).…”

“…In the case of S Pav and CE And Heras & Hony (2005) served as a source for the starting values of the calculations. For TY Dra, o Cet and Z Cyg the values were taken from Posch (2005). Finally, for SV Peg both sources have been used to determine the starting values.…”

“…4 plus the assumed starting values used for the genetic algorithm. The first entry for each star gives the parameters for the fit done by Posch (2005). Thus there are no values for the population and generation entered.…”

“…Finally the best result for each star is shown. CE And and S Pav do not have fits done by Posch (2005) available. For SV Peg the results of 2 additional fits done with PIKAIA are given.…”

“…As starting parameters for the calculation, we assumed the same dust composition as Posch (2005). After some iterations and efforts to adapt the parameters, it turned out that another dust composition including aluminum oxide produces a better fit.…”