Protoplanetary disc evolution and dispersal

In this thesis I have studied how discs around young stars evolve and disperse. In particular, I build models which combine viscous evolution with photoevaporation, as previous work suggests that photoevaporation can reproduce the observed disc evolution and dispersal time-scales. The main question this thesis attempts to address is: Can photoevaporation provide a dominant dispersal mechanism for the observed population of young stars? Photoevaporation arises from the heating that high energy (UV and X-ray) photons provide to the surface layers of a disc. Before I started this work, only photoevaporation from a pure EUV radiation field was described within a hydrodynamic framework. Therefore, I start by building a hydrodynamic solution to the pure X-ray photoevaporation problem, and then extend this solution to the entire high energy spectrum. This hydrodynamic model leads me to conclude that it is the X-ray radiation field that sets the mass-loss rates. These mass-loss rates scale linearly with X-ray luminosity, are independent of the underlying disc structure and explicitly independent of stellar mass. I build a radiation-hydrodynamic algorithm, based on previous work, to describe the process of X-ray heating in discs. I then use this algorithm to span the full range of observed parameter space, to fully solve the X-ray photoevaporation problem. I further extend the algorithm to roughly approximate the heating an FUV radiation field would have on the photoevaporative flow, as well as separately testing the effect an EUV radiation field will have. These numerical tests are in agreement with the hydrodynamic model derived. Specifically, it is the X-rays that are driving the photoevaporative flow from the inner disc. Armed with an accurate description of the photoevaporative mass-loss rates from young stars, I consider the evolution of a population of disc-bearing, young ($0.7$\msun) stars, in order to asses photoevaporation's role as a dispersal mechanism. This study shows that the observed spread in X-ray luminosity of young stars is sufficient to drive the dispersal of the entire population of discs, reproducing both the required time-scales and the required spread in observables (disc lifetime, accretion rate). I also show that a large fraction of the observed population of `transition' discs are consistent with being created through photoevaporation. Having shown photoevaporation can provide a dominant dispersal mechanism for a population of discs, I attempt to describe some direct observable consequences of photoevaporation, both through gas tracers and dust emission. During this work, the first direct evidence of a photoevaporative flow emerging from a young star was obtained from TW Hya, in the NeII 12.8$\mu$m line. Therefore, I discuss this result within the framework of the X-ray photoevaporation model. Furthermore, I suggest that emission from the photoevaporative flow is the origin of the unexplained, blue-shifted, OI 6300\AA~ line observed around all young stars that possess discs. I then described the properties of the dust particles that may be entrained within the photoevaporative flow. The total dust mass in the flow is found to be small compared to the disc, although such a region becomes observable once the disc presents as edge-on, obscuring the central star and hot inner disc. I discuss the emission from these regions and compare them to the sample of currently imaged edge-on discs. The presented photoevaporation model reproduces all of the current observations, and I discuss some predictions it makes with regard to future observations. Finally, photoevporation may have some intriguing consequences on planet formation and dust evolution that warrant further investigation

Abscissa

Senior Project submitted to The Division of Languages and Literature of Bard Colleg

Colonial Anxiety and Identity: Ethnic Networks as Cultural Supports in Colonial South Asia and Sumatra

This study was inspired by research into the personal correspondence of colonial servants in Sumatra and South Asia, and the realisation that their articulation of negative emotions such as anxiety or fear are ill-fitted to the current wider understanding of colonial anxiety. This article argues that the progress of colonial empires was widely shaped by negative emotions such as these, yet there were also methods used by colonial servants to deal with such negative experiences. The core example of this has been the case studies of Robert Cowan and Alexander Hall; these men’s letter archives display their usage of correspondence networks as part of their coping strategy. It is argued that these specifically ethnic, and at times gendered, correspondence networks formed a cultural bulwark which was used to cope with aspects of colonial anxiety. The method of this study therefore was epistolary examination to gather evidence and construct arguments. The archives of Cowan and Hall were compared and examined side by side to identify common patters and content. These were then considered in tandem with the current wider understanding of colonial anxiety. Based on the evidence gathered, it has been concluded that ethnic networks such as those examined could mitigate aspects of colonial anxiety. At the same time, these also demonstrate the great potential for future interdisciplinary studies involving personal histories tied to both Sumatra and South Asia

Variance Approximation Approaches For The Local Pivotal Method

The problem of estimating the variance of the Horvitz-Thompson estimator of the population total when selecting a sample with unequal inclusion probabilities using the local pivotal method is discussed and explored. Samples are selected using unequal inclusion probabilities so that the estimates using the Horvitz-Thompson estimator will have smaller variance than for simple random samples. The local pivotal method is one sampling method which can select samples with unequal inclusion probability without replacement. The local pivotal method also balances on other available auxiliary information so that the variability in estimates can be reduced further. A promising variance estimator, bootstrap subsampling, which combines bootstrapping with rescaling to produce estimates of the variance is described and developed. This new variance estimator is compared to other estimators such as naive bootstrapping, the jackknife, the local neighborhood variance estimator of Stevens and Olsen, and the nearest neighbor estimator proposed by Grafstrom. For five example populations, we compare the performance of the variance estimators. The local neighborhood variance estimator performs best where it is appropriate. The nearest neighbor estimator performs second best and is more widely applicable. The bootstrap subsample variance estimator tends to underestimate the variance

Incentives to learn

The authors report results from a randomized evaluation of a merit scholarship program for adolescent girls in Kenya. Girls who scored well on academic exams received a cash grant and had school fees paid. Girls eligible for the scholarship showed significant gains in academic examination scores (average gain 0.15 standard deviations). There was considerable sample attrition and no significant program impact in the smaller of the two program districts, but in the other district girls showed large gains (average gain 0.22-0.27 standard deviations), and these gains persisted one full year following the competition. There is also evidence of positive program externalities on learning-boys (who were ineligible for the awards) also showed sizable average test gains. Both student and teacher school attendance increased in the program schools.Primary Education,Gender and Development,Poverty and Social Impact Analysis,Education Finance,Access&Equity in Basic Education