Understanding the formation and evolution of planetary systems is one of the most fundamental challenges in astronomy. To directly image and study young exoplanets and the circumstellar disks they... Show moreUnderstanding the formation and evolution of planetary systems is one of the most fundamental challenges in astronomy. To directly image and study young exoplanets and the circumstellar disks they form from, dedicated high-contrast imaging instruments are built. Several of these instruments have polarimetric modes that are particularly powerful to reach the large contrasts required to directly image these objects as well as to characterize them. This thesis aims to improve the polarimetric sensitivity, accuracy, and capabilities of high-contrast imaging polarimeters for the detection and characterization of exoplanets and circumstellar disks. In addition, this thesis presents the first direct detections of linear polarization from self-luminous planetary mass companions. The focus of this thesis is mostly on ground-based high-contrast imaging, in particular with the instrument SPHERE-IRDIS at the Very Large Telescope. This thesis covers many aspects of high-contrast imaging polarimetry, ranging from theoretical work, calibrations, and the development of new observing techniques to actual scientific polarimetric measurements and astrophysical interpretation. Show less
Mason, C.; Putten, P.W.H. van der; Duijn, M. van 2020
Computer simulations have been used to model psychological and sociological phenomena in order to provide insight into how they affect human behavior and population-wide systems. In this study,... Show moreComputer simulations have been used to model psychological and sociological phenomena in order to provide insight into how they affect human behavior and population-wide systems. In this study, three agent-based simulations (ABSs) were developed to model opinion dynamics in an online social media context. The main focus was to test the effects of ‘social identity’ and ‘certainty’ on social influence. When humans interact, they influence each other’s opinions and behavior. It was hypothesized that the influence of other agents based on ingroup/outgroup perceptions can lead to extremism and polarization under conditions of uncertainty. The first two simulations isolated social identity and certainty respectively to see how social influence would shape the attitude formation of the agents, and the opinion distribution by extension. Problems with previous models were remedied to some extent, but not fully resolved. The third combined the two to see if the limitations of both designs would be ameliorated with added complexity. The combination proved to be moderating, and while stable opinion clusters form, extremism and polarization do not develop in the system without added forces. Show less
This study goes beyond the analysis of the causes of the emergence of populist actors in the party system, focusing instead on those cases in which populism and its counterpart anti-populism,... Show moreThis study goes beyond the analysis of the causes of the emergence of populist actors in the party system, focusing instead on those cases in which populism and its counterpart anti-populism, translate into an ideological and discursive divide that contributes to structuring a certain party system. When populism/anti-populism emerges as a political cleavage, the factors behind parties’ political choices in general, and electoral coalition preferences in particular, can be affected. For this new cleavage to start to polarize, a change in the political opportunity structure is needed. In fact, when the political opportunity structure opens as a consequence of events external to the party system new actors may enter the system, producing a change in the dynamic of competition. Moreover, considering also the organizational density of the parties in the system, defined as the power of penetration of a given party, both in terms of intensity and reach, this dissertation can shed light on the likely duration not only of the parties but also of the cleavage. Show less
We live in a magnetic universe with magnetic fields spanning an enormous range of spatial and temporal scales. In particular, magnetic fields at the scale of a galaxy are known as galactic magnetic... Show moreWe live in a magnetic universe with magnetic fields spanning an enormous range of spatial and temporal scales. In particular, magnetic fields at the scale of a galaxy are known as galactic magnetic fields and are the focus of this PhD thesis. These galactic magnetic fields are very important since they affect the dynamics of the interstellar gas as well as the gas distribution. The presence of these magnetic fields induces a certain type of radiation to occur at radio frequencies known as synchrotron radiation. The observed polarization properties of this synchrotron radiation then serves to record the imprint of these magnetic fields. The goal of this thesis has been to infer the structure of the magnetic field across various spatial scales in our own Galaxy as well as the strength and structure of the magnetic field in other galaxies using radiopolarimetric observations. Show less
DNA-hosted silver clusters (Ag:DNAs) have attracted a lot of attention due to their small size (~20 atoms), wide range of applications in chemistry and biology, and sequence-dependent optical... Show moreDNA-hosted silver clusters (Ag:DNAs) have attracted a lot of attention due to their small size (~20 atoms), wide range of applications in chemistry and biology, and sequence-dependent optical tunability. Most of the previous studies are focused on the ensemble of emitters in solution. However, little is known about the optical properties of individual emitters, which is a crucial step towards understanding of their real nature, otherwise lost in ensemble averaging. We show that the excitation and emission spectra of individual emitters are broad even at 1.7 K (FWHM ~25 nm). Also, polarization measurements indicate that the excitation is not strongly dependent on the polarization of excitation light, whereas the emission is highly linearly polarized. Furthermore, from time-resolved measurements, we can conclude that the emission of single emitters can be fitted with single exponential decay curve, whereas the emitters organized with nanometer precision on the DNA scaffolds show double–exponential decay. This indicates the interaction between densely packed Ag:DNAs. Finally, we show that the DNA tubes can be used as a nano-contact glue between the colloidal particles functionalized with short DNA strands. Show less
My PhD research focuses on the maser emission which occurs in different regions of the circumstellar envelopes (CSEs) of evolved stars and can be studied at high angular resolution using radio... Show moreMy PhD research focuses on the maser emission which occurs in different regions of the circumstellar envelopes (CSEs) of evolved stars and can be studied at high angular resolution using radio interferometers. These masers are useful probes of the dynamics and kinematics of the outflow from AGB stars. Moreover, the masers can be important tracers of the magnetic field strength and morphology at various distances from the central stars. It is expected that the magnetic field plays an important role in transforming spherically symmetric AGB stars into a-spherical PNe. Theoretical modeling indicated that magnetically collimated jets may be responsible for the formation of the a-spherical PNe. We performed polarimetric observations of the masers (OH, H2O and SiO) in different classes of evolved stars using several radio interferometers as well as single dish radio and sub-mm telescopes, to study the role of the magnetic field throughout the AGB evolution. Show less
