All Figures

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{2339fg1.ps} \end{figure}$ Figure 1: The positions of our stars in the HR diagram. The solid curves indicate the pre-main sequence evolutionary tracks of stars of different masses; the dashed curve represents the birthline for an accretion rate of 10 $_{\rm --5}$ $\ensuremath {{M}_{\odot }}$ yr^-1 (both are taken from Palla & Stahler 1993).
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{2339fg2.ps} \end{figure}$ Figure 2: The derived stellar mass estimates vs. the pre-main-sequence age estimates derived from a comparison of the position in the HR diagram to PMS evolutionary tracks of Palla & Stahler (1993). Our sample shows a clear lack of disks around relatively old stars of 3-4 $\ensuremath {{M}_{\odot }}$ , and of disks around relatively young stars less massive than about 2.5 $\ensuremath {{M}_{\odot }}$ .
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In the text

$\begin{figure} \par\includegraphics[angle=90,width=8.4cm,clip]{2339fg3.ps} \end{figure}$ Figure 3: Classification of the sources based on global SED properties. We plot the ratio of the near-infrared and infrared luminosity (see Sect. 2.2) vs. the IRAS m₁₂-m₆₀ color (defined as $m_{12}-m_{60}=-2.5 \log \mathcal{F}_{12}/\mathcal{F}_{60}$ , where $\mathcal{F}_{12}$ and $\mathcal{F}_{60}$ are the fluxes at 12 and 60 $\mu$ m as listed in the IRAS point source catalogue). Sources classified as group I according to ME01 are in the lower right part of the diagram, group II sources are in the upper left part. Group Ia sources are indicated with crosses, group Ib sources with triangles, and group II sources with diamonds. The dashed curve indicates our division line between the two groups (Sect. 2.2).
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In the text

$\begin{figure} \par\includegraphics[angle=90,width=16.5cm,clip]{2339fg4a.ps} \end{figure}$ Figure 4: N-band spectra of the sources in our sample. The ISM silicate extinction efficiency, plotted in the upper left panel, was taken from Kemper et al. (2004). The AB Aur spectrum was taken by ISO (van den Ancker et al. 2000). Also plotted are the best fits to the spectra (grey curves, see Sect. 5.2).
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In the text

$\begin{figure} \par\includegraphics[angle=90,width=16.5cm,clip]{2339fg4b.ps} \end{figure}$ Figure 4: continued.
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In the text

$\begin{figure} \par\includegraphics[width=8.4cm,clip]{2339fg5.ps} \end{figure}$ Figure 5: The N-band spectrum of HD 179218 ( upper panel), and the measured mass absorption coefficients of ortho enstatite taken from Chihara et al. (2002) ( lower panel). The wavelengths of the most prominent emission bands are indicated by the dotted lines. In this object, enstatite grains are an important constituent of the grain population that causes the 10 $\mu$ m feature.
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In the text

$\begin{figure} \par\includegraphics[width=8.4cm,clip]{2339fg6.ps} \end{figure}$ Figure 6: The flux ratio of the normalized spectra at 11.3 and 9.8 micron (a measure for the amount of processing that the material has undergone) versus the peak/continuum ratio of the silicate feature (a measure for the typical grain size). Group I sources are represented by triangles, group II sources by diamonds. In the upper right corner of the figure we have indicated the typical uncertainties in the displayed quantities.
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In the text

$\begin{figure} \par\includegraphics[width=8.5cm,clip]{2339fg7.ps} \end{figure}$ Figure 7: The mass absorption coefficients of the various templates used in the fitting procedure (upper 5 panels). We use grains with volume equivalent radii of 0.1 $\mu$ m (solid lines) and 1.5 $\mu$ m (dotted lines). In the lower panel we show the template used for the PAH emission, which is normalized such that the maximum flux in the 8 to 13 micron region equals unity. For a detailed discussion see Sect. 5.1.
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In the text

$\begin{figure} \par\includegraphics[width=8.3cm,clip]{2339fg8.eps} \end{figure}$ Figure 8: The templates used in the fitting procedure (solid curves) together with the resulting best fits using all other templates (dotted curves). The spectra are all normalized such that the maximum value of the template equals unity. In the left column we show the templates for the small grains (0.1 $\mu$ m), in the right column those of the large grains (1.5 $\mu$ m).
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{2339fg9_new.ps} \end{figure}$ Figure 9: The relation between grain size and crystallinity found in our spectral fits. Vertically, we plot the mass fraction of crystalline grains (forsterite and enstatite). Horizontally, the mass fraction of large (1.5 $\mu$ m) grains is plotted. Since group Ib sources do not display a silicate feature, they are not shown here. For a discussion see Sects. 5.3 and 6.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{2339fg10.ps} \end{figure}$ Figure 10: The fraction of the dust mass contained in silica grains vs. the mass fraction contained in forsterite grains. Following Bouwman et al. (2001) we also plot the theoretical annealing behavior of two different amorphous magnesium silicates, smectite dehydroxylate (SMD; Mg₆Si₈O₂₂; upper dashed line) and serpentine dehydroxylate (SD; Mg₃Si₂O₇; lower dashed line). The solid line represents the expected annealing behavior of a mixture of these two silicates, consisting of 4% SMD and 96% SD, which was found by Bouwman et al. (2001) to give the best fit to their data. Our data are in better agreement with a pure SMD initial composition.
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In the text

$\begin{figure} \par\includegraphics[width=8.5cm,clip]{2339fg11.ps} \end{figure}$ Figure 11: The mass fraction of crystalline material contained in enstatite vs. the total mass fraction of crystalline material. A value of 0 on the vertical axis indicates that all the crystalline silicates present in the disk are in the form of forsterite, while a value of 1 means all crystalline silicates are in the form of enstatite.
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In the text

$\begin{figure} \par\includegraphics[width=8.4cm,clip]{2339fg12.ps} \end{figure}$ Figure 12: The mass fraction of large grains in the crystalline grain population vs. the mass fraction of large grains in the amorphous component. When the average size of the amorphous grains is large, the majority of the crystalline material resides in large grains as well.
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In the text

$\begin{figure} \par\includegraphics[width=8.6cm,clip]{2339fg13.ps} \end{figure}$ Figure 13: The mass fraction of dust in crystalline grains vs. the stellar mass. Higher mass stars show an on average higher fraction of crystalline grains than do lower mass stars.
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In the text

$\begin{figure} \par\includegraphics[width=8.6cm,clip]{2339fg14.ps} \end{figure}$ Figure 14: The mass fraction of dust residing in large grains vs. the stellar mass. All stars with a mass $M \protect \ga 2.5$ $\ensuremath {{M}_{\odot }}$ have a mass fraction of large grains above 85%.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{2339fg1.ps} \end{figure}$	Figure 1: The positions of our stars in the HR diagram. The solid curves indicate the pre-main sequence evolutionary tracks of stars of different masses; the dashed curve represents the birthline for an accretion rate of 10 $_{\rm --5}$ $\ensuremath {{M}_{\odot }}$ yr^-1 (both are taken from Palla & Stahler 1993).
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$\begin{figure} \par\includegraphics[angle=90,width=16.5cm,clip]{2339fg4a.ps} \end{figure}$	Figure 4: N-band spectra of the sources in our sample. The ISM silicate extinction efficiency, plotted in the upper left panel, was taken from Kemper et al. (2004). The AB Aur spectrum was taken by ISO (van den Ancker et al. 2000). Also plotted are the best fits to the spectra (grey curves, see Sect. 5.2).
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$\begin{figure} \par\includegraphics[angle=90,width=16.5cm,clip]{2339fg4b.ps} \end{figure}$	Figure 4: continued.
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$\begin{figure} \par\includegraphics[width=8.4cm,clip]{2339fg5.ps} \end{figure}$	Figure 5: The N-band spectrum of HD 179218 ( upper panel), and the measured mass absorption coefficients of ortho enstatite taken from Chihara et al. (2002) ( lower panel). The wavelengths of the most prominent emission bands are indicated by the dotted lines. In this object, enstatite grains are an important constituent of the grain population that causes the 10 $\mu$ m feature.
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$\begin{figure} \par\includegraphics[width=8.3cm,clip]{2339fg8.eps} \end{figure}$	Figure 8: The templates used in the fitting procedure (solid curves) together with the resulting best fits using all other templates (dotted curves). The spectra are all normalized such that the maximum value of the template equals unity. In the left column we show the templates for the small grains (0.1 $\mu$ m), in the right column those of the large grains (1.5 $\mu$ m).
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{2339fg9_new.ps} \end{figure}$	Figure 9: The relation between grain size and crystallinity found in our spectral fits. Vertically, we plot the mass fraction of crystalline grains (forsterite and enstatite). Horizontally, the mass fraction of large (1.5 $\mu$ m) grains is plotted. Since group Ib sources do not display a silicate feature, they are not shown here. For a discussion see Sects. 5.3 and 6.
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$\begin{figure} \par\includegraphics[width=8.5cm,clip]{2339fg11.ps} \end{figure}$	Figure 11: The mass fraction of crystalline material contained in enstatite vs. the total mass fraction of crystalline material. A value of 0 on the vertical axis indicates that all the crystalline silicates present in the disk are in the form of forsterite, while a value of 1 means all crystalline silicates are in the form of enstatite.
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$\begin{figure} \par\includegraphics[width=8.4cm,clip]{2339fg12.ps} \end{figure}$	Figure 12: The mass fraction of large grains in the crystalline grain population vs. the mass fraction of large grains in the amorphous component. When the average size of the amorphous grains is large, the majority of the crystalline material resides in large grains as well.
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$\begin{figure} \par\includegraphics[width=8.6cm,clip]{2339fg13.ps} \end{figure}$	Figure 13: The mass fraction of dust in crystalline grains vs. the stellar mass. Higher mass stars show an on average higher fraction of crystalline grains than do lower mass stars.
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$\begin{figure} \par\includegraphics[width=8.6cm,clip]{2339fg14.ps} \end{figure}$	Figure 14: The mass fraction of dust residing in large grains vs. the stellar mass. All stars with a mass $M \protect \ga 2.5$ $\ensuremath {{M}_{\odot }}$ have a mass fraction of large grains above 85%.
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