Observational Constraints on The Evolution of Dust in ...

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176 Spectral Energy Distributions of the Young Stars with Disks in Serpens Figure 7.4 – Fractional disk luminosity (L disk /L star) derived for the accreting stars (based on Hα data, solid black line) and non-accreting stars (dot-dashed black line) in Serpens, compared to a sample of weak-lined T Tauri stars (WTTS, dotted gray line, Cieza et al. 2007) and a sample of debris disks (Debris, dashed gray line, Chen et al. 2005). 7.3.1 Comparison with Herbig Ae/Be Stars Meeus et al. (2001) found that the disks around higher mass Herbig Ae/Be stars can be divided into two groups, according to the disk geometry: group I comprises sources with considerable IR excess, associated with a flared geometry; group II consists of little IR excess, associated with a geometrically thin midplane, shadowed by the puffed-up disk inner rim. Meeus et al. (2001) showed that the distributions of fractional disk luminosities for the two groups are different, with mean of 0.52 for group I and 0.17 for group II. Figure 7.5 compares the two groups of Herbig Ae/Be stars with the young stars in Serpens, separated in disk geometry according to the ratio between the fluxes at 30 and 13 µm (F 30 /F 13 , Oliveira et al. 2010). It is seen that the geometry separation between flared and flat disks at F 30 /F 13 = 1.5 for T Tauri stars is not reflected with an accompanying separation in L disk /L star , which is the case for group I and II of the Herbig Ae/Be stars (dotted red and dashed blue lines, respectively). Although both the flared and flat disks span the same range, the peaks of the distributions are different, yielding distinctive median fractional disk luminosities: 〈L disk /L star 〉 is 0.21 for the flared disks and 0.17 for flat disks. It can be noted from Figure 7.5 that the great majority of disks around Herbig Ae/Be stars are concentrated in a narrow range of fractional disk luminosities, right at the border between disks dominated by accretion luminosity and passively irradiated disks, showing a bimodal distribution for group I and II. The T Tauri stars, on the other hand, span a much wider range of L disk /L star . In the center of the figure, the peaks of the distribution for flared and flat disks around T Tauri stars are not so different from those of group I and II of Herbig Ae/Be stars, albeit with greater overlap. The most striking difference between T Tauri and Herbig Ae/Be stars are both tails of the distribution. The lack of relatively very faint and very bright disks
Evolution of Dust in Protoplanetary Disks 177 Figure 7.5 – L disk /L star derived for the flared (solid black line) and flat (dot-dashed black line) disks in Serpens (top), compared to the sample of Herbig Ae/Be of Meeus et al. (2001) (bottom). Objects belonging to group I (flared, dotted gray line) and group II (self-shadowed, dashed gray line) are shown separately. around Herbig Ae/Be stars could be a bias effect due to the considerably lower number of such disks observed, when compared to their lower mass counterparts. Another possibility is that indeed disks around higher mass stars evolve faster. That would mean that the relatively very bright phase of disk evolution happens when the disks are still embedded in a gaseous envelope and consequently not visible, while the lack of the faint end of the distribution would imply on a very fast evolution from flat disks to no disks at all, being only visible again in the debris stage. 7.4 Connection Between Stars and Disks While the late-type (K and M) population of Serpens spans a wide variety in disk shapes, the early-type (A, F and G) stars catch the attention. Two of the 9 earlytype stars (#52 and 114) are surrounded by so-called cold disks, i.e. disks depleted of warm dust close to the star but otherwise massive (Oliveira et al. 2010; Merín et al. 2010). The majority, however, show very little IR excess (#70, 80, 98, 120, 131, 139, and 145) consistent with a transition from class II to class III. Assuming the stars in
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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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52 YSOs in the Lupus Molecular Clou
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4 A Spitzer Survey of Protoplanetar
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A. Background Sources 111 Table 4.3
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A. Background Sources 113 Figure A.
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118 Discovery of a Dusty Planetary
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