A DIATOM MODEL OF DUST IN THE TRAPEZIUM NEBULA ∗

(Letter to the Editor) Q. MAJEED, N.C. WICKRAMASINGHE, F. HOYLE and S. AL-MUFTI Department of Applied Mathematics and Astronomy, University College, Cardiff, U.K.

Abstract. Measurements are reported of the 5–35 µm infrared spectrum of a mixed diatom culture dispersed in a CsI disc. These data are used to compute the flux from a diatom model of dust in the Trapezium nebula, where dust grain temperatures are assumed to be distributed in the range 230–130 K. Good agreement with the observational data is found for the model.

We have argued for several years that the infrared spectrum of dust observed in the Trapezium nebula could serve as a touchstone for interstellar grain models (Wickramasinghe, 1974; Hoyle and Wickramasinghe, 1977; Hoyle et al., 1982). On this basis several classes of grain model have been sifted out: mineral grains which were at first thought to be the cause of the 8–12 µm emission feature were later shown to be difficult to maintain, and so were grains comprised of relatively simple organic polymers such as polyoxymethylene. In sharp contrast we found that polysaccharides could match the Trapezium data over the 8–30 µm waveband (Hoyle and Wickramasinghe, 1977). The best fit obtained thus far over the limited 8–12 µm spectral region was for a mixed culture of diatoms, a class of microorganisms that incorporate polymers based on Si-O units within their structure. In our earlier investigations relating to diatoms the situation remained uncertain, however, for wavelengths λ > 14 µm due to the lack of laboratory data (Hoyle et al., 1982). In the present Letter we report an extension of this earlier work to a wavelength λ ≈ 35 µm. The mixed culture of diatoms used here is the same as that for which our earlier results were obtained in the 8–13 µm waveband, so the results over this limited wavelength interval remain essentially unchanged. To obtain good quality infrared spectra further in the infrared, up to λ = 35 µm, it was necessary to use CsI instead of KBr as the disc material for the reason that the latter material has strong absorptions near 28 µm. A quantity of the dry, purified diatom culture weighing 0.6 mg was pressed into a disc in the usual way and an infrared spectrum over the 5–40 µm wavelength region was read out using a 780 series Perkin-Elmer spectrophotometer. From the known CsI disc diameter of 1.3 cm and the mass of diatoms used, we calculate the mass absorption coefficient κ(λ) which is plotted in Figure 1. The function κ(λ) given in Figure 1 can now be used to calculate relative fluxes from the diatom model. Instead of using a single temperature for the dust we use a distribution of temperatures such as would occur in an optically thin, spherically∗ 1988, Astrophys. Space Sci. 140, 205–207.

Astrophysics and Space Science is the original source of publication of this article. It is recommended that this article is cited as: Astrophysics and Space Science 268: 225–227, 1999. © 2000 Kluwer Academic Publishers. Printed in the Netherlands.

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Figure 1. The mass absorption coefficient of the mixed diatom culture.

Figure 2. Flux curves for diatom model computed according to Equation (1) and normalised to agree with the observational data at the ‘10 µm band’ peak.

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symmetric cloud of grains of uniform density illuminated by a central source. For such a distribution the flux under suitable conditions can be shown to be given by Z Tmax Fλ ∝ κ(λ)T −7 Bλ (T ) dT , (1) Tmin

where Bλ (T ) is the Planck function and Tmax , Tmin are the grain temperatures at the inner and outer surfaces of the shell. For Tmax we consider temperatures just below that at which proliferation of diatoms might be thought possible, for instance within cometary objects that contain impurity molecules to depress the freezing point of water. Under terrestrial conditions in the Antarctic ices, certain types of ice-diatoms are known to replicate at temperatures below 273 K. For the upper temperature we fix Tmax = 230 K and for the lower temperature we vary Tmin to obtain the best fits to the flux data from the Trapezium nebula. Our results are shown in Figure 2. Both the cases Tmin = 130 K and Tmin = 140 K could be seen to give very good correspondence with the observations of Forrest et al. (1976). These results lend further strong support to the biological model of interstellar dust.

References Forrest, W.J., Gillett, F.C. and Stein, W.A.: 1976, Astrophys. J. 208, L133. Hoyle, F. and Wickramasinghe, N.C.: 1977, Nature 266, 241. Hoyle, F., Wickramasinghe, N.C. and Al-Mufti, S.: 1982, Astrophys. Space Sci. 86, 63. Wickramasinghe, N.C.: 1974, Nature 353, 462.