Observational Constraints on The Evolution of Dust in ...

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88 Spitzer Survey of Protoplanetary Disk Dust in Serpens addition, the objects with PAH emission have standard F 30 /F 13 flux ratios, falling in the ‘flat’ and ‘flared’ regions of Figure 4.3. Only object 120 shows a silicate feature (at 20 µm, as discussed in Section 4.3.3.3), in addition to PAHs. This alludes to an anti-correlation between PAHs and silicates that could be simply explained in terms of contrast, i.e., it is hard to detect PAH features when a strong 10 µm silicate feature is present (Geers et al. 2006). In this sense, PAHs should be more easily detected in disks without silicates, since the falling spectrum provides an increase in the feature to continuum ratio improving contrast. 4.3.3.3 Silicate Emission Silicate emission has been observed around numerous circumstellar disks (e.g., van Boekel et al. 2003; Przygodda et al. 2003; Kessler-Silacci et al. 2006; Furlan et al. 2006; Bouwman et al. 2008; Olofsson et al. 2009). Some of this material has different characteristics in disks compared to primitive interstellar material. The strength and the shape of silicate features (at 10 and 20 µm) contain important information on the dust sizes in the surface layer of the disk. The 89 disk sources in this sample were inspected for their silicate characteristics. Their spectra can be seen in Figures 4.6–4.8 (completed with Figures 4.4 and 4.5), arranged in order of decreasing strength of the 10 µm feature and decreasing α 2−24µm spectral slope. No extinction correction has been applied to these spectra (see Section 4.3.3.5). Five of these sources (5% of the disk population) have spectra that are featureless, i.e., they have neither the 10 nor the 20 µm silicate features in emission, although they present IR excess (objects 38, 42, 59, 83, and 87). It is found that when the 10 µm feature appears, the 20 µm band is also present (by visual inspection of the continuumsubtracted spectra, see Section 4.3.3.4). The contrary is not true, as objects 13, 20, 24, 32, 62, 64, 69, 70, 109, 116, 127, 139, 143 and 145 have an observed 20 µm feature but no apparent 10 µm feature (also reported by Kessler-Silacci et al. (2006) for T Cha, SR21 and HD 135344B). Also, object 120 shows the 20 µm band and no 10 µm feature, as this region is dominated by PAH features in this source (see Section 4.3.3.2). When present, the 10 µm feature (S 10µm peak µm (S 20µm peak ) one as is common in disk sources. ) is usually stronger than the 20 Crystalline silicate features (as discussed by, e.g., Bouwman et al. 2001, 2008; Acke & van den Ancker 2004; van Boekel et al. 2004; Apai et al. 2005; Olofsson et al. 2009; Sargent et al. 2009) are seen in several sources. A detailed analysis in terms of statistics and crystalline fraction of these features is non-trivial and beyond the scope of this paper, but will be subject of a future publication. 4.3.3.4 The Silicate Strength-Shape Relation As first shown for Herbig Ae stars (van Boekel et al. 2003), there is a dependence between grain size and silicate shape and strength for both 10 and 20 µm features: as the sizes of the grains grow, the silicate features appear more and more flattened
Evolution of Dust in Protoplanetary Disks 89 Figure 4.6 – IRS spectra of the disk sources in Serpens. Object number is indicated in each panel. In gray are the original spectra, while in black a binned version (using a Savitzky–Golay (Savitzky & Golay 1964) filter of order 0 and degree 3) is overplotted. and shift to longer wavelengths. A statistical analysis of these two features can be done using the following quantities: S 10µm peak is a measure of the strength of the feature centered at 10 µm and S 11.3/S 9.8 is a proxy for the shape of the feature (the smaller this value, the more peaked the feature is); S 20µm peak measures the strength of the 20 µm feature, while S 23.8/S 19 is a proxy for this feature shape. Following Kessler-Silacci et al. (2006), S λ is defined as: S λ = 1 + 1 2δ In this paper, we use δ = 0.1 µm. ∫ λ+δ λ−δ F λ − C λ C λ dλ. (4.1) Kessler-Silacci et al. (2006) found a global trend (best fit to observations) for both features (Figure 9 in their paper and Figure 4.9 in this one), indicating that the feature shape and strength are physically related, as expected if the silicate features
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Observational <str
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Promotiecommissie Promotor: Co-Prom
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Cover Artwork: Roderik Overzier
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viii Chapter 3. YSOs in the Lupus M
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x Publications 217 Acknowledgments
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2 Introduction planets may eventual
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4 Introduction Figure 1.1 - Illustr
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6 Introduction 1.3.1.1 Disk Observa
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20 Introduction Gorti, U., Dullemon
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Evolution of Dust in Protoplanetary
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