Observational Constraints on The Evolution of Dust in ...

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86 Spitzer Survey of Protoplanetary Disk Dust in Serpens Figure 4.4 – IRS spectra of the cold disks in Serpens. Object number is indicated in each panel. In gray are the original spectra, while in black a binned version is overplotted. The dotted line represents the stellar photosphere, for comparison. wavelengths, characteristic of cold disks. Eight objects (9, 21, 69, 82, 90, 113, 114, and 122) in this sample are classified as cold disks based on 5 F 30 /F 13 15, corresponding to 8.5% of the disk population. These spectra are shown in Figure 4.4. For clarity, the original spectra are shown in gray, while in black a binned version is overplotted. The binning was done using a Savitzky-Golay (Savitzky & Golay 1964) filter of order 0 and degree 3, for illustrative purposes only. This is the most unbiased survey of cold disks to date, based entirely on spectroscopy in the critical mid-IR range, and should thus give a representative fraction of such disks. These objects are also part of a larger cold disk sample, studied by Merín et al. (2010), where the authors have selected cold disks from photometric criteria in all five c2d clouds. 4.3.3.2 PAH Emission PAHs are large molecules whose emission is excited by UV radiation out to large distances from the star. Thus, these features are good diagnostics of the amount of stellar UV intercepted by the disk surface. Almost half of the sample of Herbig Ae/Be stars (young stars of intermediate masses) studied by Meeus et al. (2001) and Acke & van den Ancker (2004) show PAH emission. Geers et al. (2006), on the other hand, show that PAH emission is not as common among low-mass YSOs as it is for more massive stars. The authors argue that the lack of features is not only due to their weaker UV radiation fields, but also to a lower PAH abundance compared with the general ISM. In the current sample, five sources have clear PAH emission, presented in Figure 4.5. Although a great variation in shapes and strengths is seen, all sources show the features at 6.2, 7.7, 8.6, 11.2 and 12.8 µm. Objects 52, 97, and 131 also show a feature that could be the PAH band at 16.4 µm, although the feature does not match perfectly in position. The central stars in these five sources were studied with optical spectroscopy in Oliveira et al. (2009). Spectral types and luminosities were derived, yielding ages and
Evolution of Dust in Protoplanetary Disks 87 Figure 4.5 – IRS spectra of the disk sources which show clear PAH features. The dashed lines indicate the positions of the most prominent bands (at 6.2, 7.7, 8.6, 11.2, 12.8 and 16.4 µm). The object number and its spectral type (from Oliveira et al. 2009) are indicated at the top right of each panel. Note that the ratios of different line strengths differ between sources. masses for each object. Objects 52, 98, 120, and 131 have masses between 1.9 and 2.7 M ⊙ (spectral type G3 to A2), while object 97 has lower mass, of around 0.45 M ⊙ (M2). Objects 97 and 98 are spatially very close to each other and have very similar PAH features. IRAC Band 4 (8 µm) images show that there is extended nebulosity surrounding both positions so that the PAHs are likely associated with this nebula rather than the disks. Discarding these 2 sources, this sample indicates that PAHs are present in 3.2% of the disk population studied, which is below the 11%–14% of PAH detection rate found by Geers (2007). This discrepancy could be due to the two samples being differently selected, with the current sample having a larger fraction of late-type stars. The percentage of PAHs present in disks in Serpens is, however, very comparable with that in Taurus (4% of the disk population, Section 4.4.3). Interestingly, none of the cold disks show PAHs, different from expected if grain settling enhances PAH features (Dullemond et al. 2007) and observed for other cold disk samples studied which do show PAHs (Brown et al. 2007; Merín et al. 2010). 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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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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209 interagem gravitacionalmente, a
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