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174 Spectral Energy Distributions of the Young Stars with Disks in Serpens Figure 7.3 – Fractional disk luminosity (L disk /L star) derived for the objects in Serpens (top left), compared to those in Taurus (bottom left), Upper Scorpius (top right) and η Chamaeleontis (bottom right). The dashed line in the Serpens distribution accounts for completeness (see text for details). The boundary for accreting vs. passive disks is put at L disk /L star ∼ 0.2. Thus “accreting disks” does not refer to the information on accretion rates derived from Hα. Furthermore, Figure 7.3 shows the distribution of L disk /L star in the Upper Scorpius and η Chamaeleontis clusters for the entire samples (consisting of objects with IR-excess and those that have little or no excess, Maud et al. in prep). These older regions are known to have lower disk fractions (Hernández et al. 2008), meaning that most of the member stars have already fully dissipated their disks. Figure 7.3 clearly shows this difference in relation to the young Serpens and Taurus clouds, with distributions that peak (and spread) at considerably lower disk luminosities. The vertical dotted lines roughly separate luminosity ratios that can be explained by different mechanisms: accretion disks (L disk /L star > 0.2, Kenyon & Hartmann 1987) and passive disks. “Debris”-like disks are considerably fainter (L disk /L star < 0.001). It is important to note that the L disk /L star distributions in Taurus, Upper Sco and η Cha are fairly complete. That is, for these well studied regions information on the entire young star population is available whether or not the source has IR excess (down to the brown dwarf limit, which ensures completeness for the results derived here for T Tauri stars). Taurus indeed has a brighter disk population than Upper Sco and η Cha. The Serpens sample presented here, on the other hand, is flux-limited and selected based on IR excess. That means that, by definition of the selection criteria, stars without disks and with disks fainter than 3 mJy at 8 µm are not part of the sample and therefore not shown in Figure 7.3. A conservative calculation of the fractional disk luminosity of the 88 missed sources (considering a flux lower than 3 mJy at 8 µm) was performed and is shown on top of the distribution of Serpens in
Evolution of Dust in Protoplanetary Disks 175 Figure 7.3 for reference. Due to the selection criteria, the disk population missed in Serpens should be fainter than that presented here. Harvey et al. (2007b) identified a population of 235 IR-excess sources in Serpens, called YSO candidates. 147 of the original sample were further studied with the IRS spectrograph onboard Spitzer, comprising the sample presented here. This means that about 88 potential young stars with disks are missing. Considering the ∼20 % contamination fraction of background sources in the direction of Serpens (due to its low galactic latitude), conservatively about 70 of these 88 should be young stars that were missed, and which should populate the left part of Figure 7.3. These missed sources could account for the difference between Serpens and Taurus in the faintest bin of L disk /L star in Figure 7.3, but should not be able to shift the peak of the L disk /L star distribution for Serpens. Serpens has not been as well explored in the literature as Taurus or Upper Sco and η Cha, since it suffers from high extinction and is not amongst the closest starforming regions. Data covering a wide range of wavelengths exists but have not yet been published. Specifically, because this new young population in Serpens has been discovered from IR colors, young stars no longer surrounded by disks (class III) are, by definition, missed in the selection criteria for cloud membership. For this reason, the fraction of young stars that are surrounded by disks, determined for instance by combining IR and X-ray observations, is still unknown. However, the agreement in disk distribution with Taurus (as seen in Figure 7.3 and in Oliveira et al. 2010) and difference with Upper Scorpius and η Chamaeleontis could hint that the disk fraction in Serpens must be of the order of that in Taurus. Figure 7.4 shows an additional comparison of the disks in Serpens with a sample of weak-line T Tauri stars (WTTS, Cieza et al. 2007) and a sample of debris disks (Chen et al. 2005). Oliveira et al. (2009) derived mass accretion rates based on the width at 10% of peak intensity of the Hα line, covered by their optical spectra. As previously noted, that sample is smaller than that presented here, due to the nondetection of the faintest objects in their optical spectroscopy program. It can be seen in Figure 7.4 that L disk /L star of the accreting and non-accreting stars in Serpens (solid and dot-dashed black lines, respectively) overlap with the WTTSs (dotted red line). The Serpens population and the WTTSs differ in the distribution tails. The WTTS sample has a faint tail that reaches the faint debris disks population (dashed blue line), while the Serpens population shows a bright tail population. Specifically comparing the accreting and non-accreting sub-samples in Serpens, the two classes overlap greatly, except at the brighter end of the distribution, dominated by accreting objects. This is more clearly seen by looking at the mean fractional disk luminosity 〈L disk /L star 〉 which is 0.21 and 0.11 for accreting and non-accreting objects, respectively. The median fractional disk luminosity 〈L disk /L star 〉 for the entire population of Serpens is 0.20.
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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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118 Discovery of a Dusty Planetary
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