We search for planets and brown dwarves around white dwarves (WDs). Finding extra-solar planets is the first step toward establishing the existence and abundance of life in the Universe. The low mass and luminosity of WDs make them ideal stars to search for low mass companion objects. Theoretical predictions generally agree that a star will consume and destroy close-in, low mass planets as it ascends the red giant and asymptotic giant branch evolutionary tracks, but larger mass objects and those further out will survive. The matter ejected from the star as it evolves into a white dwarf may also be accreted onto daughter planets, or may coalesce into a disk from which planets can form. We employ two techniques to search for planets and brown dwarves (BDs) around WDs. A subset of pulsating white dwarf stars have a pulsational stability that rivals pulsars and atomic clocks. When a planet is in orbit around a such a star the orbital motion of the star around the centre of mass is detectable as a change in arrival times of the otherwise stable pulsations. We search for, and find, a sample of suitable pulsators, monitor them for between three and four years, and place limits on companions by constraining the variation consistent with a 2.4M[subscript J] planet in a 4.6 year orbit. We also observe a large sample of WDs to search for a mid-infrared excess caused by the presence of sub-stellar companions. We present evidence for a potential binary system consisting of a WD and a BD on the basis of an observed excess flux at near and mind-infrared wavelengths. We also place limits on the presence of planetary mass companions around those stars and compare our results to predictions of planetary survival theories. Our findings do not support suggestions of planet formation or accretion of extra mass during stellar death.

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

Stars form inside clouds of molecular gas and dust. In the early stages of stellar evolution the remainders of the initial cloud form a circumstellar disk. For the next few million years the disk will slowly dissipate via accretion, outflows, photoevaporation and planet growth while the star makes its way onto the Main Sequence. This stage of a star's life is referred to as the T Tauri phase and is characterised by high-level spectrophotometric variability. This thesis aims to study and map out the environments of T Tauri stars down to the very low mass regime by the means of time-domain monitoring. Different physical processes in the system manifest themselves as variability on different time- scales as well as produce characteristic spectroscopic and photometric features at various wave- lengths. In order to study young stellar objects in depth, the observing campaigns presented in this work were designed to cover a large range of time-scales - minutes, hours, days and months. Combining all the data, this thesis establishes a baseline of over a decade for some objects. The observations also cover a wide range of wavelengths from the optical to the mid-infrared part of the spectrum. The star RW Aur experienced two long-lasting dimming events in 2010 and 2014. This thesis presents a large collection of spectral and photometric measurements carried out just before and during the 2014 event. Spectral accretion signatures indicate no change in the accretion activity of the system. Photometry indicates that parallel to the dimming in the optical the star becomes brighter in the mid-infrared. The observations in this work combined with literature data suggest that the origin of the 2014 event is most likely obscuration of the star by hot dust from the disk being lifted into the disk wind. Very low mass stars (< 0.4 M⊙) are the most common type of star in the Galaxy. In order to understand the early stages of stellar evolution we must study young very low mass stars. This work investigates the photometric and spectroscopic variability of seven brown dwarfs in star forming regions near σ Ori and ε Ori. All targets exhibit optical photometric variability between from 0.1 to over 1.0 magnitude that persists on a time-scale of at least one decade. Despite the photometric variability no change in the spectral type is measured. In the cases where the stars are accreting, modelling of the spectral changes suggest the accretion flow is more homogeneous and less funnelled compared to Sun-like T Tauri stars. The non-accreting variables are more plausibly explained by obscuration by circumstellar material, possibly a ring made out of multiple clouds of dust grains and pebbles with varying optical depths. The star-disk systems studied in this thesis have some broader implications for star and planet formation theory. The case-study of RW Aur has unambiguously demonstrated that the planet- forming environment is very dynamic and can change dramatically on short time-scales, which in turn would have implications for the diversity of planetary systems found in the Galaxy. The Orion stars have shown that the current theory for the T Tauri stage of stellar evolution is valid down to the very low mass regime. The seven dwarfs are a good example for the evolutionary path of circumstellar disks, showing the transition from gas-high, flared accretion disks (σ Ori) to dust-dominated, depleted, structured debris disks (ε Ori).

Lightning is an important electrical phenomenon, known to exist in several Solar System planets. Amongst others, it carries information on convection and cloud formation, and may be important for pre-biotic chemistry. Exoplanets and brown dwarfs have been shown to host environments appropriate for the initiation of lightning discharges. In this PhD project, I aim to determine if lightning on exoplanets and brown dwarfs can be more energetic than it is known from Solar System planets, what are the most promising signatures to look for, and if these "exo-lightning" signatures can be detected from Earth. This thesis focuses on three major topics. First I discuss a lightning climatology study of Earth, Jupiter, Saturn, and Venus. I apply the obtained lightning statistics to extrasolar planets in order to give a first estimate on lightning occurrence on exoplanets and brown dwarfs. Next, I introduce a short study of potential lightning activity on the exoplanet HAT-P-11b, based on previous radio observations. Related to this, I discuss a first estimate of observability of lightning from close brown dwarfs, with the optical Danish Telescope. The final part of my project focuses on a lightning radio model, which is applied to study the energy and radio power released from lightning discharges in hot giant gas planetary and brown dwarf atmospheres. The released energy determines the observability of signatures, and the effect lightning has on the local atmosphere of the object. This work combines knowledge obtained from planetary and earth sciences and uses that to learn more about extrasolar systems. My main results show that lightning on exoplanets may be more energetic than in the Solar System, supporting the possibility of future observations and detection of lightning activity on an extrasolar body. My work provides the base for future radio, optical, and infrared search for "exo-lightning".