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$\begin{figure} \includegraphics[width=8.8cm]{0137fig1.eps} \end{figure}$ Figure 1: Critical grain sizes of iron spheres where the collision rate $\Gamma _{\rm i}$ of the free electrons in the material is equal to the collision rate at the surface shown for $\zeta =1$ , 10, and 100. Below the curves the collision rate is dominated by surface scattering.
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In the text

$\begin{figure} \mbox{\includegraphics[width=8.7cm]{0137fig2.eps} \hspace*{2mm} \includegraphics[width=8.7cm]{0137fig3.eps} } \end{figure}$ Figure 2: Real and imaginary part of the dielectric function for individual pure small iron spheres at energies above 1 eV. For comparison also the real and imaginary parts of $\epsilon ^{b}$ are shown as thin solid lines. The part of $\epsilon _2^{b}$ shown as thin dotted line are extrapolated values used for the Kramers-Kronig relation to derive $\epsilon _1^{b}$ . The real part of the dielectric function is always negative except for the three branches shown. The short and the long dashed thin lines in the right hand figure refer to the dielectric function $\delta\epsilon_2^f=\mbox{Im}(\epsilon-\epsilon^{b})$ of an iron sphere with $a=0.001~\mu{\rm m}$ and $a=1.0~\mu{\rm m}$ , respectively.
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In the text

$\begin{figure} \mbox{\includegraphics[width=8.7cm]{0137fig4.eps} \hspace*{2mm} \includegraphics[width=8.7cm]{0137fig5.eps} } \end{figure}$ Figure 3: Real and imaginary part, given as conductivity $\sigma =\epsilon _2\omega /4\pi$ , of the dielectric function $\epsilon$ above 1eV at a temperature of 298 K. The chosen values of the imaginary part and the result for the real part after Kramers-Kronig analysis are shown as solid lines. The values differ somewhat from the published values. The derived values of the real part of the dielectric function lie between those from Johnson & Cristy (1974) and those from Moravec et al. (1976). The real part $\epsilon _1^b={\rm Re}(\epsilon -\delta \epsilon ^f)$ (shown as dashed-dotted line) is the direct result derived with the Kramers-Kronig relation from the imaginary part $\epsilon ^b_2=\epsilon _2-\delta \epsilon _2^f$ .
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In the text

$\begin{figure} \includegraphics[width=8.8cm]{0137fig6.eps} \end{figure}$ Figure 4: Derived optical properties of spherical iron grains. Shown are the absorption coefficients $Q_{\rm abs}(\lambda ,a)$ , scattering coefficients $Q_{\rm sca}(\lambda ,a)$ , and the scattering parameter $g_{\rm sca}(\lambda,a)=\left<\cos(\theta)\right>$ at 25 K for various grain radii from $10~\AA{}$ to $10~\mu{\rm m}$ with a difference in radius of $\Delta\log{a[\mu{\rm m}]}=0.5$ . Thick lines are labeled with the corresponding grain radii in microns.
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In the text

$\begin{figure} \includegraphics[width=8.8cm]{0137fig7.eps} \end{figure}$ Figure 5: Effect of the temperature on the absorption behaviour of spherical iron grains. For various temperatures, the absorption coefficients of iron grains with radii 1.0 and $0.01~\mu{\rm m}$ for $\mu =1$ (thick lines) are shown. For comparison the absorption coefficients due to magnetisation effects for magnetic single and multi-domain iron grains well below the Curie temperature ( $744^{\circ}~{\rm K}$ ) are given (thin dashed and solid line).
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In the text

$\begin{figure} \mbox{\includegraphics[width=8.7cm]{0137fig8.eps} \hspace*{2mm} \includegraphics[width=8.7cm]{0137fig9.eps} } \end{figure}$ Figure 6: Angle-averaged absorption cross section $\left <C_{\rm abs}\right >$ of prolate (a>b) and oblate (a<b) iron grains with volumes equivalent to the volume of a sphere with radius $r_{\rm v}=0.01~\mu{\rm m}$ . For comparison also the absorption of a sphere is given. Solid and dashed lines correspond to absorption where the size and temperature dependence of the dielectric function has been taken into account and has been ignored, respectively. The dotted line shows the electric dipole absorption of a sphere for the case of temperature and size independent dielectric function.
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In the text

$\begin{figure} \includegraphics[width=8.8cm]{0137figa.eps} \end{figure}$ Figure 7: Equilibrium temperatures of spherical iron grains in the ISM for three different radiation strengths ( $\chi =0.1$ , $\chi =1$ and $\chi =10$ ). Dashed and dotted lines correspond to calculations in which we included and ignored the temperature and size dependence of the dielectric function.
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In the text

$\begin{figure} \includegraphics[width=8.8cm]{0137figb.eps} \end{figure}$ Figure 8: Approximate equilibrium temperatures of spheroidal iron grains with a volume equal to a volume of a sphere with radius $r_{\rm v}=0.01~\mu{\rm m}$ and $r_{\rm v}=0.05~\mu{\rm m}$ as function of the ratio a/b. The solid and the dashed line correspond to calculations where the dependence of the dielectric function on grain size and temperature has been taken into account and has been ignored, respectively.
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In the text

$\begin{figure} \includegraphics[width=8.8cm]{0137figc.eps} \end{figure}$ Figure 9: Temperature distribution of spherical iron grains with radius 0.001 and $0.004~\mu{\rm m}$ heated by the ISRF (MMP). Solid and dashed lines correspond to calculations in which we included and ignored the dependence of the dielectric function on temperature and grain size.
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In the text

$\begin{figure} \includegraphics[width=8.8cm]{0137figd.eps} \end{figure}$ Figure 10: Emission spectra of individual small iron grains. The sizes are chosen to be 10, 20, and 40 Å. The spectra shown as solid lines correspond to calculations in which the size and temperature dependence of the dielectric function has been taken into account. The spectra shown as dashed lines correspond to calculations in which this dependence has been ignored.
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In the text

$\begin{figure} \includegraphics[width=8.8cm]{0137fige.eps} \end{figure}$ Figure 11: Spectral energy distribution (SEDs) of spherical iron grains with a mass of $1~M_{\odot}$ at a distance of 1kpc heated by the ISRF (MMP). The grains are assumed to have a power law ${\rm d}n(a)\propto a^{-3.5}~{\rm d}a$ with a maximum grain size $a_{\rm max}=0.1~\mu{\rm m}$ and different minimum grain sizes $a_{\rm min}$ of 10, 20 and $40~{\rm \AA{}}$ . SEDs in which calculation the temperature and size dependence of the dielectric function was included are shown as solid lines. Dashed lines correspond to SEDs in which this dependence has been ignored.
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In the text

$\begin{figure} \mbox{\includegraphics[width=8.5cm]{0137figf.eps} \includegraphics[width=8.5cm]{0137figg.eps} } \end{figure}$ Figure B.1: Absorption coefficients $Q_{\rm abs}(a,\lambda )$ of metallic iron spheres derived approximating the dielectric function $\epsilon$ through the free electron part $\epsilon ^{\rm f}$ . In the left hand figure the collision rate of the free electrons is taken to be the same as for bulk iron at 298 K. In the right hand figure it is assumed that the collision rate is dominated by scattering events at the grain surface. The absorption coefficients are shown as contours in logarithmic scale (thin solid lines) where the difference between the lines is taken to be $\Delta \log_{10}(Q_{\rm abs}(a,\lambda))=0.5$ . The labels correspond to $\log_{10}(Q_{\rm abs}(a,\lambda))$ . The hatched region shows where the absorption behaviour deviates strongly from the dipole absorption ( $2\pi a/\lambda > 0.5$ ). The thick solid line visualises where the electric dipole absorption is equal to the magnetic dipole absorption. Where the frequency $\omega$ of the photons is equal to the collision rate $\Gamma$ of the free electrons in the grain is shown as long dashed line. The short dashed and dashed dotted line correspond to absorption coefficients where |y|=1 and |y|=3, respectively.

In the text

$\begin{figure} \includegraphics[width=8.8cm]{0137fig1.eps} \end{figure}$	Figure 1: Critical grain sizes of iron spheres where the collision rate $\Gamma _{\rm i}$ of the free electrons in the material is equal to the collision rate at the surface shown for $\zeta =1$ , 10, and 100. Below the curves the collision rate is dominated by surface scattering.
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$\begin{figure} \includegraphics[width=8.8cm]{0137figa.eps} \end{figure}$	Figure 7: Equilibrium temperatures of spherical iron grains in the ISM for three different radiation strengths ( $\chi =0.1$ , $\chi =1$ and $\chi =10$ ). Dashed and dotted lines correspond to calculations in which we included and ignored the temperature and size dependence of the dielectric function.
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$\begin{figure} \includegraphics[width=8.8cm]{0137figc.eps} \end{figure}$	Figure 9: Temperature distribution of spherical iron grains with radius 0.001 and $0.004~\mu{\rm m}$ heated by the ISRF (MMP). Solid and dashed lines correspond to calculations in which we included and ignored the dependence of the dielectric function on temperature and grain size.
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$\begin{figure} \includegraphics[width=8.8cm]{0137figd.eps} \end{figure}$	Figure 10: Emission spectra of individual small iron grains. The sizes are chosen to be 10, 20, and 40 Å. The spectra shown as solid lines correspond to calculations in which the size and temperature dependence of the dielectric function has been taken into account. The spectra shown as dashed lines correspond to calculations in which this dependence has been ignored.
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