Thesis (Ph. D.)--University of Hong Kong, 2001. / Includes bibliographical references (leaves 113-122).

I report the results of a three-year intensive radial-velocity survey of 22 planet-host stars in search of the low-amplitude (K ~5-10 m s⁻¹) signals from additional planets which may be "hiding" in the residuals of the known planet orbital solution. On average, more than 40 radial-velocity observations were obtained for each target using the High-Resolution Spectrograph at the 9.2m Hobby-Eberly Telescope (HET). These high-precision data can be used to rule out additional planets in some of these systems to a detection limit of M sin i ~10-20 Earth masses at a = 0:05 AU. Jupiter-mass planets can be excluded at the 99% level for orbital separations a < 2 AU. No additional planets are evident, and our data do not confirm the planets HD 20367b, HD 74156d, and 47 UMa c. Test particle simulations of these systems with the SWIFT N-body integrator reveal the regions where additional planets could reside in stable orbits. Further simulations with Saturn-mass bodies in these regions are also performed. We note a lack of short-period giant planets in any of these 22 systems, despite dynamical feasibility. The frequency of inner giant planets may be much lower than what was expected based on early discoveries of such objects in systems containing jovian-mass planets. Terrestrial-mass planets may be present in these systems but as yet undetectable. These results suggest that planet formation and migration processes do not favor systems containing both "hot" and "cold" Jupiters. Hence, as detection methods become sensitive to terrestrial-mass planets, systems with architectures like our own Solar system may yet be commonplace. / text

ICESat consisted of the Geoscience Laser Altimeter System (GLAS) and a commercial spacecraft bus. The stability of the GLAS to bus alignment was unknown and significant for GLAS pointing. Pointing control was performed by the bus, and variations of the GLAS alignment were effectively pointing control errors. There were four star trackers making measurements sensitive to this alignment, two on GLAS and two on the bus. Tracker pointing variations during samples from seven years of flight data were estimated using an alignment filter. The states of an alignment filter represent multiple independent attitudes, enabling the fusion of measurements from an arbitrary number of trackers and gyro units. The ICESat alignment filter states were equivalent to four tracker pointing vectors, expressed in both the body and celestial frames. Together with a star catalog, the four pointing vectors were equivalent to predictions of the tracker measurements. The stars provided nearly ideal reference points, but filter performance was improved by detecting and handling deterministic star errors. The primary result was evidence for relatively large pointing variations of the two GLAS trackers, on the order of fifty arcseconds, with both periodic orbital variations and trends on long time scales. There was also evidence of correlations between the variations of the two GLAS trackers, suggesting that they reflected GLAS to bus alignment variations. / text

The positive correlation between planet detection rate and host star iron abundance lends strong support to the core accretion theory of planet formation. However, iron is not the most significant mass contributor to the cores of giant planets. Since giant planet cores are thought to grow from silicate grains with icy mantles, the likelihood of gas giant formation should depend heavily on the oxygen and silicon abundance of the planet formation environment. Here we compare the silicon and oxygen abundances of a set of 76 planet hosts and a control sample of 80 metal-rich stars without any known giant planets. Our new, independent analysis was conducted using high resolution, high signal-to-noise data obtained at McDonald Observatory. Because we do not wish to simply reproduce the known planet-metallicity correlation, we have devised a statistical method for matching the underlying [Fe/H] distributions of our two sets of stars. We find a 99\% probability that planet detection rate depends on the silicon abundance of the host star, over and above the observed planet-metallicity correlation. We do not detect any such correlation for oxygen. Our results would thus seem to suggest that grain nucleation, rather than subsequent icy mantle growth, is the important limiting factor in forming giant planets via core accretion. Based on our results and interpretation, we predict that planet detection should correlate with host star abundance for refractory elements responsible for grain nucleation and that no such trends should exist for the most abundant volatile elements responsible for icy mantle growth. / text

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This dissertation is primarily focused on the topic of M dwarf metallicities and their relevance to the study of extrasolar planets. I begin by describing a method for accurately determining M dwarf metallicities with spectral synthesis based on abundance analyses of visual binary stars. I then apply this technique and present the first spectroscopic metallicities of three M dwarfs with planetary mass companions. The three M dwarf planet hosts have sub-solar metallicities, a surprising departure from the trend observed in FGK-type planet hosting stars. I discuss the implications of this result for extrasolar planet searches around the most numerous stars in the galaxy. I also present measured metallicities for a select group of M dwarfs with precisely determined masses and luminosities. Comparison of these stars' V band magnitudes with the predictions of the current state-of-the-art theoretical models for low-mass stars indicate that the models are deficient, as previously thought. I discuss how the cool star model atmospheres that were developed for the metallicity analysis technique might be used to improve M dwarf structure and evolution models. In addition to M dwarf metallicities, I describe some complementary work to determine the true mass of an extrasolar planet candidate using a combined analysis of high-precision astrometry and radial velocities. I present a dynamical mass for the companion to HD 33636 that indicates it is a low-mass star instead of an exoplanet. This result demonstrates the value of follow-up astrometric observations to determine the true masses of exoplanet candidates detected with the radial velocity method. Finally, I discuss the broader implications of the results presented in this dissertation and the prospect for similar work in the future. / text

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.

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