This dissertation explores the consequences of atmospheric dynamics for observations of substellar mass objects (SMOs). Discussed first is the growth of cloud particles of various compositions in brown dwarfs of different surface gravities and effective temperatures. The structure of these objects is calculated with a one-dimensional radiative transfer model. To determine particle sizes, the timescales for microphysical growth processes, including nucleation, coagulation, and coalescence, are compared to the timescale for gravitational sedimentation. The model also allows for sustained uplifting of condensable vapor in convective regions. The results show that particle sizes vary greatly over the range of objects studied. In most cases, clouds on brown dwarfs do not dominate the opacity. Rather, they smooth the emergent spectrum and partially redistribute the radiative energy. The focus then shifts to extrasolar giant planets (EGPs). Results are presented from a three-dimensional model of atmospheric dynamics on the transiting Jupiter-like planet HD 209458b. As a close-in orbiter (known as a “roaster”), HD 209458b is super-heated on its dayside. Due to tidal locking of the interior, the dayside hemisphere faces the star in perpetuity, which leads to very different dynamics than is seen on Jupiter. The flow is characterized by an eastward supersonic jet (u ∼ 4 km s⁻¹) extending from the equator to the mid-latitudes. Temperature contrasts are ∼ 500 K at the photosphere. At 220 mbar, winds blow the hottest regions downstream from the substellar point by ∼ 60°, with direct implications for the infrared light curve. These simulations are extended to the study of carbon chemistry in HD 209458b’s atmosphere by coupling the CO/CH₄ reaction kinetics to the dynamics. Disequilibrium results from slow reaction rates at low temperatures and pressures. Effective vertical quenching near the ∼ 3 bar level leads to uniformly high concentrations of CO at the photosphere, even in cool regions where CH₄ is strongly favored thermodynamically. Observations are underway to detect these signatures of meteorology on HD 209458b and similar planets.

To calculate realistic models of objects with Ni in their atmospheres, accurate atomic data for the relevant ionization stages need to be included in model atmosphere calculations. In the context of white dwarf stars, we investigate the effect of changing the Ni IV-VI bound-bound and bound-free atomic data on model atmosphere calculations. Models including photoionization cross-section (PICS) calculated with AUTOSTRUCTURE show significant flux attenuation of up to similar to 80 per cent shortward of 180 angstrom in the extreme ultraviolet (EUV) region compared to a model using hydrogenic PICS. Comparatively, models including a larger set of Ni transitions left the EUV, UV, and optical continua unaffected. We use models calculated with permutations of these atomic data to test for potential changes to measured metal abundances of the hot DA white dwarf G191-B2B. Models including AUTOSTRUCTURE PICS were found to change the abundances of N and O by as much as similar to 22 per cent compared to models using hydrogenic PICS, but heavier species were relatively unaffected. Models including AUTOSTRUCTURE PICS caused the abundances of N/O IV and V to diverge. This is because the increased opacity in the AUTOSTRUCTURE PICS model causes these charge states to form higher in the atmosphere, more so for N/O V. Models using an extended line list caused significant changes to the Ni IV-V abundances. While both PICS and an extended line list cause changes in both synthetic spectra and measured abundances, the biggest changes are caused by using AUTOSTRUCTURE PICS for Ni.

The aim of this thesis is to revisit the properties of white dwarf stars in the Solar neighbourhood (distance > 100 pc), in particular their magnetic fields, the occurrence of binarity and their space density. This thesis presents observations and analysis of a sample of white dwarfs from the southern hemisphere. Over 80 objects were observed spectroscopically, and 65 of these were also observed with a spectropolarimeter. Many of the white dwarfs observed belong to the Solar neighbourhood, and can be used to study the star formation and evolution in this region. Our spectropolarimetric measurements helped constrain the fraction of magnetic white dwarfs in the Solar neighbourhood. Combining data from different surveys, I found a higher fraction of these objects in the relatively old local population than in other younger selections such as the Palomar-Green survey which suggests magnetic field evolution in white dwarfs, or different sets of progenitors. The progenitors of magnetic white dwarfs have been assumed to be Ap and Bp stars, however I find that the properties and number of Ap and Bp stars would only explain white dwarfs with magnetic fields larger than 100 MG. The number of known white dwarfs is believed to be complete to about 13 pc, however the sample is certainly incomplete to 20 pc from the Sun. To identify new white dwarfs in the Solar neighbourhood, some possibly magnetic or in binaries, numerous candidate white dwarfs from the Revised NLTT catalogue have been observed, which resulted in the discovery of 13 new white dwarfs, with 4 of these having a distance that places them within 20 pc of the Sun. The candidates were selected using a V − J reduced-proper-motion diagram and optical-infrared diagram. A total of 417 white dwarf candidates were selected, 200 of these have already been spectroscopically confirmed as white dwarfs. Spectroscopic confirmation is required for the remaining 217 candidates, many of these are likely to belong to the Solar neighbourhood. Four close binaries consisting of a white dwarf and a cool companion were also observed, for which atmospheric and orbital parameters were obtained. The photometry for two of these binary systems, BPM 71214 and EC 13471-1258 shows that the secondary stars are filling their Roche lobes, and combined with their orbital parameters, these systems are very good candidates for hibernating novae. The time of their previous interaction or the extent of this interaction are unknown. The two other binary systems, BPM 6502 and EUVE J0720-31.7 are post-common envelope binaries. BPM 6502 is not expected to interact within a Hubble time, however EUVE J0720-31 is expected to become a cataclysmic variable within a Hubble time. The atmospheric parameters of the white dwarfs were determined using model atmosphere codes which were modified for the present study to include convective energy transfer, self-broadening and Lyman satellite features.

Located at 2 pc, the L7.5+ T0.5 dwarfs system WISE J104915.57-531906.1 (Luhman 16 AB) is the third closest system known to Earth, making it a key benchmark for detailed investigation of brown dwarf atmospheric properties, thermal evolution, multiplicity, and planet-hosting frequency. In the first study of this series - based on a multicycle Hubble Space Telescope (HST) program - we provide an overview of the project and present improved estimates of positions, proper motions, annual parallax, mass ratio, and the current best assessment of the orbital parameters of the A-B pair. Our HST observations encompass the apparent periastron of the binary at 220.5 +/- 0.2 mas at epoch 2016.402. Although our data seem to be inconsistent with recent ground-based astrometric measurements, we also exclude the presence of third bodies down to Neptune masses and periods longer than a year.

The romance is essentially an account of the lives and adventures of two knight-errants, Ysaie le Triste, and his son, Marcq l'Essilliet. In many instances, the author has been content to reproduce conventional Arthurian adventure motifs. His knights set forth on perilous quests, participate in numerous tournaments, and are occasionally permitted to enter the realm of the Fairies which lies on the fringe of their own Arthurian world. By the end of the romance, both knights have won the love of fair princesses. The author has also included in his story the biography of a hideous dwarf-character, named Troncq, who accompanies each of the knights on his quests and acts as his valet. He has permitted this strange character to play the principal role in the numerous comic episodes with which he has interspersed his narrative. The dwarf's role is, in fact, as lengthy as that of either of his masters. Nevertheless, it is only towards the end of the story that the author reveals his true identity. He is a supernatural being who has been obliged by a curse to appear on earth in a grotesque form, so that he might seek there a mortal hero capable of accomplishing various difficult tasks on his behalf. When the knight, Marcq, eventually proves himself equal to these tasks, Troncq experiences metamorphosis and regains his former beauty. The story then concludes with a description of his triumphant return to Fairyland. That the author should have permitted a hideous dwarf-valet to play such an important role in his tale of chivalry represents a drastic departure from Arthurian tradition. A survey of the dwarfs who appear in other romances reveals that they are almost invariably supernumerary characters. It is possible, however, that the author of this romance has given to the role of his dwarf a significance which is not immediately apparent to the modern reader. A study of Troncq's role reveals that his creator was a master of the difficult art of fusing together themes borrowed from very diverse sources. Troncq resembles other Arthurian dwarfs in so far as he is ugly, and plays the humble role of valet. But his true literary counterparts are not to be found in romance, but rather in certain Celtic folklore tales. Like Ysaie le Triste, these tales relate the experiences of supernatural creatures who have been obliged to appear on earth in the form of grotesque dwarfs. Unfortunately, it has not been possible to determine whether the author of the romance borrowed the theme of the ugly dwarf from the Celtic storytellers or whether they were indebted to him for it. For his description of Troncq's experiences on earth, the author has drawn upon his own observation of everyday reality; he has modelled his character after the dwarf-entertainers who are known to have frequented various medieval courts. Interestingly enough, a study of these dwarf-fools reveals that theirs was, in the eyes of their contemporaries, the most degrading function that a man could possibly exercise in society. The significance of Troncq's role is therefore quite obvious. He is a caricature of the human condition. He is a creature of supernatural origin, just as Everyman is a child of God; and the curse which has obliged him to live in exile on earth is symbolic of the mark of Original Sin which Everyman has borne since the Fall. As for his assumption of the humiliating role of entertainer, it represents the state of degradation that all men must endure while they earn their redemption. Troncq's biography and the romance of Ysaie le Triste end upon a note of optimism. The dwarf's triumphant return to the land of the Fairies is obviously intended to symbolize the happy lot awaiting all good Christians on the Day of Judgement. / Arts, Faculty of / French, Hispanic, and Italian Studies, Department of / Graduate

No description available.

In this thesis I present the search for ultracool dwarf companions to main sequence stars, subgiants and white dwarfs. The ultracool dwarfs identified here are benchmark objects, with known ages and distances. The online data archives, the two micron all sky survey (2MASS) and SuperCOSMOS were searched for ultracool companions to white dwarfs, where one M9 1 companion to a DA white dwarf is spectroscopically confirmed as the widest separated system of its kind known to date. The age of the M9 1 is constrained to a minium age of 1.94Gyrs, based on the estimated age of the white dwarf from a spectroscopically derived Teff and log g and an initial-final mass relation. This search was extended using the next generation surveys, the sloan digital sky survey (SDSS) and the UK infrared deep sky survey (UKIDSS), where potential white dwarf + ultracool dwarf binary systems from this search are presented. A handful of these candidate systems were followed-up with second epoch near infrared (NIR) imaging. A new white dwarf with a spectroscopic M4 companion and a possible wide tertiary ultracool component is here confirmed. Also undertaken was a pilot imaging survey in the NIR, to search for ultracool companions to subgiants in the southern hemisphere using the Anglo-Australian telescope. The candidates from that search, as well as the subsequent follow-up of systems through second epoch NIR/optical imaging and methane imaging are presented. No systems are confirmed from the current data but a number of good candidates remain to be followed-up and look encouraging. A search for widely separated ultracool objects selected from 2MASS as companions to Hipparcos main-sequence stars was also undertaken. 16 candidate systems were revealed, five of which had been previously identified and two new L0 2 companions are here confirmed, as companions to the F5V spectroscopic system HD120005 and the M dwarf GD 605. The properties of HD120005C were calculated using the DUSTY and COND models from the Lyon group, and the age of the systems were inferred from the primary members. For GD 605B no age constraint could be placed due to the lack of information available about the primary, but HD120005C has an estimated age of 2-4Gyr. In the final part of this thesis I investigate correlations with NIR broadband colours (J - H, H - K and J - K) with respect to properties, Teff , log g and [Fe/H] for the benchmark ultracool dwarfs, both confirmed from the searches undertaken in this work and those available from the literature. This resulted in an observed correlation with NIR colour and Teff, which is presented here. I find no correlation however with NIR colours and log g or [Fe/H], due in part to a lack of suitable benchmarks. I show that despite the current lack of good benchmark objects, this work has the potential to allow UCD properties to be measured from observable characteristics, and suggest that expanding this study should reveal many more benchmarks where true correlation between properties and observables can be better investigated.

This PhD thesis presents work on the atmospheres of both brown dwarfs and exoplanets from an observers viewpoint. The composition and weather of these worlds are explored starting with M-type brown dwarfs and continuing through the L, T and Y spectral sequence, before entering the planetary regime of hot-Jupiters and super-Earths. The similarities and differences between these objects such as their radii, surface gravities, pressures, temperatures and composition are discussed. This thesis presents new results from an extensive near-infrared monitoring survey of a uniform and unbiased sample of 69 L & T dwarfs spanning the L0 to T8 spectral range. Results show that amongst 14 identified variables, nine of them newly identified, variable brown dwarfs are not concentrated at the L - T transition, nor are they observed in a specific colour, or preferentially in binary systems. The thesis also presents narrow-band photometric measurements of the hot-Jupiter HAT-P-1b and the super-Earth GJ~1214b using the 10.4~m Gran Telescopio Canarias (GTC) and the OSIRIS instrument. Results for HAT-P-1b show a strong presence of potassium in the atmosphere caused by a large scale height, possibly due to higher than anticipated temperatures in the upper atmosphere or the dissociation of molecular hydrogen caused by the UV flux from the host star. Results for GJ 1214b, which constitute the first tunable filter measurements of a super-Earth, find no evidence for the presence of methane showing a featureless transmission spectrum consistent with previous studies.

No description available.

We present a detailed analysis of the white dwarf luminosity functions derived from the local 40 pc sample and the deep proper motion catalog of Munn et al. Many previous studies have ignored the contribution of thick disk white dwarfs to the Galactic disk luminosity function, which results in an erroneous age measurement. We demonstrate that the ratio of thick/thin disk white dwarfs is roughly 20% in the local sample. Simultaneously fitting for both disk components, we derive ages of 6.8-7.0 Gyr for the thin disk and 8.7 +/- 0.1 Gyr for the thick disk from the local 40 pc sample. Similarly, we derive ages of 7.4-8.2 Gyr for the thin disk and 9.5-9.9 Gyr for the thick disk from the deep proper motion catalog, which shows no evidence of a deviation from a constant star formation rate in the past 2.5 Gyr. We constrain the time difference between the onset of star formation in the thin disk and the thick disk to be 1.6(-0.4)(+0.3) Gyr. The faint end of the luminosity function for the halo white dwarfs is less constrained, resulting in an age estimate of 12.5(-3.4)(+1.4) Gyr for the Galactic inner halo. This is the first time that ages for all three major components of the Galaxy have been obtained from a sample of field white dwarfs that is large enough to contain significant numbers of disk and halo objects. The resultant ages agree reasonably well with the age estimates for the oldest open and globular clusters.