Observational Constraints on The Evolution of Dust in ...

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10 Introduction of them (e.g., β Pictoris and Fomalhaut, Lagrange et al. 2010; Kalas et al. 2008). However, most MS stars show no signs of planets or debris within the current observational limitations, meaning that disks around such stars were completely dispersed, leaving no dust behind to tell the story. Since the landmark discovery of a planet orbiting the solar-like star 51 Peg around 15 years ago (Mayor & Queloz 1995), the ∼500 exoplanets detected to date form a powerful database of the possible end-products of disk evolution. Follow-up studies of planet-host stars have shown that stars that harbor planets are consistently more enriched in metals than those that have shown no sign of orbiting planets (Fischer & Valenti 2005; Udry & Santos 2007). If true, this relation must either be a natural consequence or the direct cause of planet formation. Additionally, the distribution of planet masses and orbital positions in relation to the host stars offer further observational constraints for planet formation models. Only one planetary system, namely our own Solar System, has been studied to the level of detail necessary to provide reliable information on dynamics and composition of its constituents. Remarkably, our Solar System contains a collection of ancient remnants that have preserved the material that composed the early solar system in virtually unaltered form for over 4.5 Gyr. Analysis of chondritic meteorites and interplanetary dust particles, as well as observations of comets, have revealed their structure and mineralogy, pointing to important physical and chemical processes that took place during the early evolution of the protoplanetary disk around the young Sun. For instance, it has been found that these bodies are composed of considerable fractions of crystalline material (Wooden et al. 2007; Pontoppidan & Brearley 2010, and references therein). Looking for clues on how and when such changes occur in the early stage of protoplanetary disks, when planets are still forming, is a very active field of research, and the subject of Chapter 6 in this thesis. 1.4 Disk Diversity and Evolution In the following, a more detailed background on the main topics of this thesis is presented: disk and dust evolution. The launch of Spitzer represented a major step forward in the study of circumstellar disks. Although many disks had been successfully observed from the ground and space (e.g., with the Infrared Space Observatory, ISO), the sensitivity and mapping capability of Spitzer allowed the observation of extremely large numbers of systems. Thanks to the statistically relevant disk samples observed by Spitzer, complemented by UV, optical and (sub-)mm data, a previously unknown variety of disk structures and timescales has been revealed. Before Spitzer, the global understanding of disk evolution was very much based on evidence from a small number of sources. Thus models for disk evolution and planet formation were derived from: i) extrapolation of characteristics of the few brightest disks as universal; ii) evidence from our own Solar System, and the “solar nebula” from which it originated; iii) observational evidence for disk dissipation within some
Evolution of Dust in Protoplanetary Disks 11 few Myr. The dominant pre-Spitzer view was that disks evolve steadily with time, with material either accreting onto the central star, being dissipated by the stellar radiation, or coagulating into bigger and bigger particles that sink to the midplane. One of the most important parameters for the formation of giant planets is the gaseous disk lifetime. However, since small dust is well coupled to the gas and since dust is an easier observable, disk lifetimes are often derived just from the infrared excess. The clock of circumstellar disks is the age of its central star. Using the mean age of a cluster and measuring the near-IR excess of its members, Haisch et al. (2001) showed that the fraction of stars surrounded by disks decreases steadily with mean age, concluding that time is an essential parameter in disk evolution. Since then, with much more comprehensive disk observations, several authors have revisited this relationship (e.g. Hernández et al. 2008; Magnelli et al. 2009) without much change in the results: by 6 – 8 Myr only less than 10% of stars in a given cluster still have disks. Although this irrefutably presents time as an important parameter, it is not the only one. Within a cluster of a given age (whether a young, 1 Myr cluster, or an older, 7 Myr one), a great diversity in infrared excess (and therefore amount of dust still present in the disk) is seen. It is still not clear why some disks are completely dissipated by 1 or 2 Myr, while others may live up to 10 Myr. Many other parameters have been suggested to play an important role in the evolution of disks. For instance, Carpenter et al. (2006) and Kennedy & Kenyon (2009) show evidence of different disk lifetimes depending on the mass of the central star (quicker dispersal for disks around higher mass stars). A diversity of stellar temperatures, luminosities and masses among young stars has been known and studied for decades. Indeed, since stars and disks are connected, it is unlikely that differences in the central sources will not be reflected in their surrounding matter. However, to date no strong evidence has been found as to which processes are determinant on setting the timescale in which a disk will dissipate. Observations of large disk samples have shown that disks in the same cluster, with same assumed age, show very distinct shapes, structures and masses (Furlan et al. 2006; Sicilia-Aguilar et al. 2006; Oliveira et al. 2010). Different amounts of excess infrared radiation, and at different bands, yield different structures for disks. A strong excess is interpreted as a geometrically flared disk, while little excess suggests a flat, almost dissipated disk. A few disks show little or no near-infrared excess, but otherwise a normal disk, with a significant excess at longer wavelengths. These are interpreted as inner gaps or holes (Calvet et al. 2002). Indeed, sub-mm observations of such disks have shown that a gap without small dust really exists, reinforcing the use of the amount of infrared excess as an indicator of disk geometry (Brown et al. 2007). A difference in geometries is demonstrated by a large sample of classical and weak-line T Tauri stars, as well as debris disks, by Cieza et al. (2007). The distribution of excess slope (α excess ) changes as a function of the wavelength where the excess starts (λ turn−off ). If grains were to simply grow and settle, α excess should become increasingly smaller for longer λ turn−off . However, Figure 1.3 suggests that a range of possible processes is responsible for inner disk clearing, which could then be
Page 1: Observational <str
Page 4 and 5: Promotiecommissie Promotor: Co-Prom
Page 6 and 7: Cover Artwork: Roderik Overzier
Page 8 and 9: viii Chapter 3. YSOs in the Lupus M
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Page 12 and 13: 2 Introduction planets may eventual
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60 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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128 From Protoplanetary Disks to Pl
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Nederlandse Samenvatting Inleiding
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193 Figuur 1 - Het ontstaan van een
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199 parameter is. Na 6 tot 8 miljoe
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Resumo em Português Restrições O
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207 Figura 2 - O Telescópio Espaci
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209 interagem gravitacionalmente, a
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213 uma correta separação entre a
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216 Curriculum Vitæ omy Center (ES
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218 Publications 8. c2d Spitzer-IRS
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220 Thesis Acknowledgments Liesbeth
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