How forms observed low-temperature crystalline silicate:Approach from laboratory experiments
Yuki KimuraInstitute of Low Temperature Science / Hokkaido University, Japan
Astronomical observations have shown the existence of abundant silicate grains aroundevolved stars in the infrared spectra. It has been realized that most of the silicate grains areamorphous phases based on the infrared features. In contrast, characteristic infrared features
attributed to Mg-rich crystalline silicates have been reported in limited number of evolvedstars with high mass loss rate (>10-5 Msun/yr) [1]. The fraction of the crystalline silicates,
particularly forsterite, is typically 10-15 % in the outflows with high mass loss rate ofasymptotic giant branch (AGB) stars [2]. During the formation of silicate grains, whether asilicate grain becomes crystalline or amorphous depends sensitively on its formation
conditions. To elucidate the crystallization condition of amorphous silicate, several
laboratory experiments have been attempted. It has been known that crystalline forsterite
grains are formed at a slower cooling rate (<~700 K/s: [3]) or by later annealing (~1000 K:[4]) of previously condensed amorphous grains. Crystallization has generally been discussed
on the basis of thermal annealing [5]. However, it seems to be unrealistic to find amechanism of partial annealing occurred only at the stars with high mass loss rate, because
the crystallization of silicate is very sensitive in temperature. In addition, some implicationsof low-temperature crystallization have been reported from the astronomical observation ofboth young and evolved stars [6-8]. Generally, the temperatures of the crystalline silicategrains astronomically observed are at as low as 100-250 K. In contrast, warmed amorphoussilicates in a stationary circumstellar disk were observed [9]. Therefore, it can be consideredthat the crystallization of silicates may be not affected by thermal heating.
In this situation, I would like to propose that the astronomically observed crystalline
forsterite can be formed by direct condensation from the vapor phase of magnesium andsilicon oxide. It was found that the crystallinity, which was deduced from the shape of the 10μm feature, of the circumstellar silicates could be determined by the balance between heatgeneration by Mg oxidation and heat dissipation due to radiation. In this case, later annealingof the silicate fraction or the use of a warm substrate for condensation of crystalline silicate is
unnecessary. In addition, to explain the existence of highly crystalline silicate grains (75% oftotal silicate) observed in some silicate carbon stars, the possibility of low-temperature
crystallization is presented based on electron irradiation on amorphous Mg-silicate grains,because the environment of the envelope is very low in temperature for the crystallization ofsilicates due to annealing [10].
References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]

A. Speck et al., 2000, A&A Suppl. Ser. 146, 437.
F. Molster & C. Kemper, 2005, Space Science Reviews 119, 3.
J. A. Tangeman, et al., 2001, Geophysical Research Letters 28, 2517.
Y. Kimura et al., 2007, Workshop on the Chronology of Meteorites and the Early Solar System, LPI
Contribution, No. 1374, p.90-91.
S. L. Hallenbeck, J. A. Nuth, Ш & P. L. Daukantas, 1998, Icarus 131, 198.
F. J. Molster, et al., 1999, Nature 401, 563.
T. K. Okamoto, et al., 2004, Nature 431, 660.
B. Merin, et al., 2007, ApJ 661, 361.
I. Yamamura, et al. “in Proc, 1st ISO workshop on Analytical Spectroscopy” eds. A. M. Heras, et al. (SP419, ESA, Noordwijk, 1997), p. 313-314.
F. J. Molster et al. 2001, A&A 366, 923.