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138 From Protoplanetary Disks to Planetary Systems 6.4.2 Disk Geometry The amount of IR-excess in a disk is directly related to its geometry (Kenyon & Hartmann 1987; Meeus et al. 2001; Dullemond et al. 2001). Specifically using the IRS spectra, disk geometry can be inferred from the flux ratio between 30 and 13 µm (F 30 /F 13 , Brown et al. 2007; Oliveira et al. 2010; Merín et al. 2010). A flared geometry (1.5 F 30 /F 13 5), with considerable IR excess and small dust, allows the uppermost dust layers to intercept stellar light at both the inner and outer disk. For flat disks (F 30 /F 13 1.5) with little IR excess, only the inner disk can easily intercept the stellar radiation as the outer disk is shadowed. Moreover, cold or transitional disks are interesting objects that present inner dust gaps or holes, producing a region with little or no near-IR excess (5 F 30 /F 13 15). It is interesting to explore the effect of disk geometry on both the mean mass-average grain sizes and crystallinity fractions of the disks studied. Figure 6.4 – Top: Flaring index F 30 /F 13 , used as a proxy for disk geometry, versus warm (left) and cold (right) mass-averaged mean grain sizes. Bottom: F 30 /F 13 versus warm (left) and cold (right) crystallinity fractions. The YSOs in Serpens (black dots), Taurus (gray squares), Upper Sco (gray stars), and η Cha (gray triangles) are compared. (A color version of this figure is available in the online journal) Figure 6.4 shows F 30 /F 13 as a proxy for disk geometry compared with the mean mass-averaged grain sizes and crystallinity fractions for both components and all regions studied here. No preferential grain size (correlation coefficient τ = -0.14, P = 0.02, and τ = 0.07, P = 0.33 for warm and cold components, respectively) nor
Evolution of Dust in Protoplanetary Disks 139 Figure 6.5 – Crystalline fraction of the warm and cold components in Serpens (black dots) and Taurus (gray squares). (A color version of this figure is available in the online journal) crystallinity fraction (τ = 0.09, P = 0.10 for the warm, and τ = -0.19, P = 0.01 for the cold component) is apparent for any given disk geometry. Similar scatter plots result for the mean mass-average grains sizes for only amorphous (τ = -0.12, P = 0.08 for the warm, and τ = 0.13, P = 0.11 for the cold component), or only crystalline grains (τ = 0.08, P = 0.17 for the warm, and τ = -0.13, P = 0.10 for the cold component). Furthermore, no clear separation is seen between the different regions studied. The statistically relevant samples in Serpens and Taurus define a locus where the majority of the objects is located in each plot, which is followed by the lower number statistics for older regions. Figure 6.4 therefore shows not only that grain size and crystallinity fraction are not a function of disk geometry, but also that younger and older regions show similar distributions of those two parameters. 6.4.3 Crystallinity Fraction The crystallinity fractions derived from the warm and cold components (C Warm and C Cold , respectively) for Serpens and Taurus are show in Figure 6.5. No strong trend of warm and cold crystallinity fractions increasing together is seen (τ = 0.10, with P = 0.10 for the entire sample). This fact implies that, if an unique process is responsible for the crystallization of dust at all radii, this process is not occurring at the same rate in the innermost regions as further out in the disk. This is opposite to the conclusion of Watson et al. (2009), who derive a correlation between inner and outer disk crystallinity from the simultaneous presence of the 11.3 and 33 µm features. The opacities of the crystalline species are more complex than those two features alone, making the analysis here more complete than that of Watson et al. (2009). Our finding that the fraction of crystalline material in disk surfaces varies with radius can constrain some of the mechanisms for formation and distribution of crystals. A wider spread in crystallinity fraction is observed for the cold component than
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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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8 Introduction 1.3.1.3 Processes Af
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10 Introduction of them (e.g., β P
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12 Introduction responsible for dis
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14 Introduction Figure 1.4 - Eviden
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16 Introduction stars (due to spect
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18 Introduction the potential to ch
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20 Introduction Gorti, U., Dullemon
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2 Optical Characterization of a New
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Evolution of Dust in Protoplanetary
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4 A Spitzer Survey of Protoplanetar
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209 interagem gravitacionalmente, a
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218 Publications 8. c2d Spitzer-IRS
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220 Thesis Acknowledgments Liesbeth
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