Microdiamonds in the Spectrum of Carbon Stars

Details Microdiamonds in the Spectrum of Carbon Stars Microdiamonds in the Spectrum of Carbon Stars

Sep 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.480a&link_type=abstract

Meteoritics, vol. 30, no. 5, page 480

Astronomy and Astrophysics

Astrophysics

Astrophysics, Diamonds, Stellar Atmospheres, Stellar Spectra

Scientific paper

Until 1987, dust particles around stars had only been recognized by their appearance in stellar spectra. The possibility of studying unprocessed stellar condensates from primitive meteorites directly in the laboratory has given important information not only about the Solar System formation, but also provided precise data for testing stellar models. For studies of radiative processes in stellar environment, knowledge on opacities of the relevant atoms, molecules and grains are essential. Because the absorption and scattering cross sections of grains are large compared to the corresponding values for atoms and molecules, grains will dominate the mean opacity of a stellar spectrum whenever they are present. In order for the grains to be taken into account in stellar atmosphere computations, knowledge of the optical properties of the relevant grains are needed. We have therefore extracted presolar diamonds from the Allende meteorite by the method described in ref. [1]. The optical properties of the extracted diamond grains were measured in the infrared (400 cm^-1 - 4000 cm^-1) in KBr and in the visual (12200 cm^-1 - 52600 cm^-1) in a water solution. The spectra showed similar features to what have earlier been reported in ref. [2] and [3]. As opposed to the earlier measurements we have converted our spectra to absorption coefficient per unit of diamonds [4] which is necessary in order to compute stellar spectra. The computed model is based on an improved version [5] of the MARCS (Model Atmospheres in Radiative and Convective Scheme) code [6]. It assumes hydrostatic equilibrium and local thermodynamic equilibrium (LTE), but includes effects of sphericity and uses an opacity sampling (OS) treatment of molecular opacities from approximately 60 million spectral lines [7]. Such models have been proven to reproduce well the observed spectral features of carbon stars [8]. The numerical model atmosphere was calculated with the full opacities of CO, C(sub)2, CN, C(sub)2H(sub)2, C(sub)3, HCN and presolar diamonds included. The presolar diamonds were included under the assumption that they form via PAH growth and therefore only C(sub)2H(sub)2 could work as seeds [9]. According to our observed laboratory spectra the micro-diamond absorption features will appear in the stellar spectra at 2.93 micrometers, 3.42 micrometers, 3.50 micrometers, 6.13 micrometers, 7.23 micrometers and 8.93 micrometers. When the full opacity of the five most dominant carbon molecules, are included in the synthetic stellar spectrum too, some of the diamond-features will disappear, due to crowding with the molecular bands. At a resolution of R=60. The most prominent micro-diamond absorption features that can be seen between the molecular band structures are those at 2.93 micrometers, 3.42 micrometers and 6.13 micrometers. Neither of these three features are observable from the Earth, due to absorption in the Terrestrial atmosphere. But with the ISO (Infrared Space Observatory) satellite (to be launched in September 1995) it will be possible to obtain the first stellar spectra at these wavelengths and our models will serve as a base for analyzing such spectra. If the extracted diamonds have preserved the optical properties they had when they formed in a stellar atmosphere, then these spectra can be analyzed based on our new models. Once the computed spectrum [4] can be compared with observed stellar spectra it will lead to either verification or a possible upper limit estimates of the abundances of micro-diamonds in the stellar atmospheres. An observational identification of the stellar source of the presolar grains would lead to improved understanding of the upper layers of stellar atmospheres, of the mass loss process in stars, and of the detailed chemical evolution of our Galaxy. References: [1] Tang M. and Anders E. (1988) GCA, 52, 1235-1244. [2] Lewis R. S. et al. (1989) Nature, 339, 117-121. [3] Colangeli L. et al. (1994) Astron. Astrophys., 284, 583-592. [4] Andersen A. C. et al. (1995) Astron. Astrophys., submitted. [5] Jorgensen U. G. et al. (1992) Astron. Astrophys., 261, 263-273. [6] Gustafsson B. et al. (1975) Astron. Astrophys., 42, 407-432. [7] Jorgensen U. G. (1994) in Molecules in the Stellar Environment (U. G. Jorgensen, ed.), 29-48, Springer-Verlag. [8] Jorgensen U. G. (1989) Astrophy. J., 344, 901-906. [9] Helling C. et al. (1995) Astron. Astrophys., submitted.

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