Compositional dependence of infrared absorption spectra of crystalline silicate

Free access article

Issue		A&A Volume 399, Number 3, March I 2003


Page(s)		1101 - 1107
Section		Formation, structure and evolution of stars
DOI		http://dx.doi.org/10.1051/0004-6361:20021831

A&A 399, 1101-1107 (2003)
DOI: 10.1051/0004-6361:20021831

II. Natural and synthetic olivines

C. Koike¹, H. Chihara^{1, 2}, A. Tsuchiyama², H. Suto³, H. Sogawa¹ and H. Okuda⁴

¹ Kyoto Pharmaceutical University, Yamashina, Kyoto 607-8412, Japan
² Department of Earth and Space Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
³ Subaru Telescope, National Astronomical Observatory of Japan, Hilo, Hawaii, USA
⁴ Gunma Astronomical Observatory, Agatsuma, Gunma 377-0702, Japan

(Received 30 July 2002 / Accepted 10 December 2002)

Abstract
The mass absorption spectra of crystalline olivine particles with different ${\rm Mg/(Mg + Fe)}$ ratios, between forsterite ( ${\rm Mg_{2}SiO_{4}}$ , Fo) and fayalite ( ${\rm Fe_{2}SiO_{4}}$ , Fa), were measured for synthetic and natural samples in the mid- and far-infrared regions. The main strong peaks show a systematic shift to longer wavelengths as the ${\rm Mg/(Mg + Fe)}$ ratio decreases. In the 10-17 $\mu$ m region, these trends are very clear, and the peak positions are a good indicator of the composition of dust grains. In the 20-100 $\mu$ m region, the trends are also rather clear, though they are slightly complicated; the intensity and width of the various bands become weak and broad, respectively, or the bands disappear as the concentration of Fo decreases. However, the trends are clear only in a limited composition range near Fo and Fa; the peak positions of 30, 50, and 69 $\mu$ m bands of forsterite shift linearly as the Fo concentration decreases to about ${\rm Fo_{70}}$ . Those of the double band in the 50 $\mu$ m region for Fe-rich olivine shift linearly as the Fa concentration increases from ${\rm Fo_{20}}$ to Fa. When the spectral signature of Mg-rich or Fe-rich olivine can be observed, the peak positions in the 20-100 $\mu$ m region are a good indicator of the composition of dust grains.

Key words: methods: laboratory -- interplanetary medium -- star: circumstellar matter -- ISM: dust, extinction -- ISM: lines and bands -- infrared: ISM

Offprint request: C. Koike, koike@mb.kyoto-phu.ac.jp

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