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Measuring stellar parallax, position and proper motion is the task of astrometry. With the development of new and much more accurate equipment, different noise sources are likely to affect the very precise measurements made with future instruments. Some of these sources are: stellar surface structures, circumstellar discs, multiplicity and weak microlensing. Also exoplanets may act as a source of perturbation. In this thesis I present an investigation of stellar surface structures as a practical limitation to ultra-high-precision astrometry. The expected effects in different regions of the HR-diagram are quantified. I also investigate the astrometric effect of exoplanets, since their astrometric detection will be possible with future projects such as Gaia and SIM PlanetQuest. Stellar surface structures like spots, plages and granulation produce small surface areas of different temperatures, i.e. of different brightness, which will influence integrated properties such as the total flux (zeroth moment of the brightness distribution), radial velocity and photocenter position (first moments of the brightness distribution). Also the third central moment of the brightness distribution, interferometrically observable as closure phase, will vary due to irregularities in the brightness distribution. All these properties have been modelled, using both numerical simulations and analytical methods, and statistical relations between the variations of the different properties have been derived. Bright and/or dark surface areas, randomly spread over the stellar surface, will lead to a binomial distribution of the number of visible spots and the dispersion of such a model will be proportional topN, where N is the number of spots or surface structures. The dispersion will also be proportional to the size of each spot, A. The dispersions of the integrated properties are therefore expected to be/ ApN. It is noted that the commonly used spot filling factor, f / AN, is notthe most relevant characteristic of spottiness for these effects. Both the simulations and the analytic model lead to a set of statistical relations for the dispersions or variations of the integrated properties. With ,e.g. knowledge of the photometric variation, m, it is possible to statistically estimate the dispersions for the other integrated properties. Especially interesting is the variation of the observed photocenter, i.e. the astrometric jitter. A literature review was therefore made of the observed photometric and radial-velocity variations for various types of stars. This allowed to map the expected levels of astrometric jitter across the HR diagram. From the models it is clear that for most stellar types the astrometric jitter due to stellar surface structures is expected to be of order 10 μAU or greater. This is more than the astrometric displacement typically caused by an Earth-sized exoplanet in the habitable zone of a long-lived main-sequence star, which is about 1–4 μAU. Only for stars with extremely low photometric variability (< 0.5 mmag) and low magnetic activity, comparable to that of the Sun, will the astrometric jitter be of order 1 μAU, sufficient to allow astrometric detection of an Earth-sized planet in the habitable zone. While stellar surface structure may thus seriously impair the astrometric detection of small exoplanets, it has in general negligible impact on the detection of large (Jupiter-size) planets.

Thesis (Ph.D.)--George Mason University, 2009. / Vita: p. 234. Thesis director: John F. Wallin. Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Computational Sciences and Informatics. Title from PDF t.p. (viewed Oct. 11, 2009). Includes bibliographical references (p. 224-233). Also issued in print.

Thesis. 1978. Ph.D.--Massachusetts Institute of Technology. Dept. of Physics. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Includes bibliographical references. / Ph.D.

Observations were made of 309 binary systems between RA 17h - 07h and Dec -70 degrees to -60 degrees. The images were calibrated against pairs measured for CCD astometry. Measurements were made of separations, position angles and differential magnitudes. Uncertainties in position angle and separation were 7.8o/p and 0.16 arcseconds respectively. The position angles, separations and differential V magnitudes were compared with WDS figures. It was determined that only 29% _+ 5% of the observed pairs had undergone significant movement since the last previous measurement, which agrees with the results of a survey of the WDS. A statistical study of 1q77 pairs found that the spread of calculated separations agreed with those found by other authors, while the distribution of periods, while falling within the range(s) calculated by other authors, were significantly longer. A short-arc fitting program was tested using data derived from the published orbit of Centauri before being applied to seven observed pairs. Orbits could be calculated for six of the seven pairs. For four of these pairs the observed movement may be due to proper motion rather than orbital motion / Master of Science (Hons)

Thesis (Ph. D.)--University of Texas at Austin, 2002. / Vita. Includes bibliographical references. Available also from UMI Company.

Not available / text

A catalogue of faint blue stars at the North Galactic Pole, compiled from the literature, is presented. Spectral classifications for catalogue stars within 3° of the pole have been obtained from U.K.S.T. objective prism and St. Andrews grism plates. Photometric data on the uvbyβ system is presented for 572 U-F8 stars at the South Galactic Pole, with radial velocities being given for 161 of these stars. From this South Galactic Pole data the interstellar reddening towards the Pole is shown to be negligible, in agreement with the findings of other authors. A number of photometrically odd stars are isolated, including several intermediate Population II, Population II and Am stars. From available data at both Poles the relative proportions of various population groups as a function of height are discussed. There is an apparent excess of PI A over iPII stars out to Ikpc., relative to the numbers expected on the basis of the 'thick disk' of iPII stars reported by Gilmore and Reid (1983). The w-velocity distributions of Pop.I A and F stars within 200pc. of both Poles are shown to be well fitted by gaussians and these gaussians are shown to be the same for both Poles. The Pop.I A stars are shown to have a mean w-velocity of 0,6 kms⁻¹ (rms 11.1 kms.⁻¹) and the corresponding F stars to have a mean w-velocity of -2.9 kms⁻¹ (rms 10.9 kms⁻¹), implying negligible net streaming through the galactic plane.

A photometric study of the extreme helium stars has detected three new variables (BD+1°4381, BD-1°3438, LSIV-1°2) and confirmed the suspected variability of two others (LSII+33°5, BD-9°4395). The timescale of the variations in BD+1°4381 (~"21 days), BD-1°3438 (5-8 days), LSIV-1°2 (~11 days) and LSII+33°5 (3-4 days), and the presence of colour changes concomitant with the luminosity variations, indicates that they are radial pulsators. In contrast, BD-9°4395 is believed to be pulsating non-radially. Photometric investigations of the extremely hydrogen-deficient binaries Upsilon Sagittarii, CPD-58°2721 and KS Persei are presented. KS Persei is suspected of having a ~5-day periodicity in addition to the ~30 day previously reported. A frequency analysis of the complex light curve of CPD-58°2721 shows it may be reconstructed from two sine-waves with periods of 9.3 and 14.1 days. For Upsilon Sagittarii, the results indicate that the variability previously ascribed to eclipses can instead be understood in terms of radial pulsation of the visible component, with a period of ~20 days. Additional rapid, ~0.02 mag luminosity variations with a period of 239 seconds are reported. Their origin is uncertain, and they are the shortest form of variability reported for this type of object. Theoretical modelling of the light curve indicates an early B-type main-sequence secondary with a mass of ~11M[sub] ⊙ , much higher than previously thought. A possible future merging of the binary components would result in a Type I supernova. There is no evidence of eclipses in any of the stars. A fine abundance analysis of CPD-58°2721 yielded T[sub]eff=14,000K, log g=1.25, n[sub]H:n[sub]He=0.005, n[sub]N:n[sub]c=40 and suggests an overabundance of heavy metals when compared to related objects. Its spectrum shows marked changes in the strengths of low excitation metallic lines which are evidence of temperature changes during pulsation. Spectroscopy of the surrounding nebulosity shows it to have an emission spectrum typical of an HII region. Radial-velocity measurements indicate it is probably part of the surrounding Eta Carina nebula, whilst the star is more distant.

In this thesis we investigate the importance of various elements of the input physics and other parameters which affect the structure and evolution of models of low mass stars. Of the elements of input physics, the nuclear generation rates, the electron screening effect on thermo-nuclear reactions, and the conductive opacities are adopted from the formulation or data tables by other contributors. The low temperature opacities are taken from the data calculated by Carson and Sharp, as well as those by Alexander. In our study, we establish a sophisticated model for the determination of the equation of state. It is formulated by a new method based on the theory of the grand canonical ensemble. The interatomic interactions, which are responsible for the nonideal effects and pressure ionization, are treated carefully in the equation of state. In addition, and consistent with the equation of state, we also evaluate the radiative opacity according to the average atom model for heavy elements and the hydrogen-like model for hydrogen and helium. Negative hydrogen absorption and free electron scattering are also included. A computing code is constructed to calculate the radiative opacity data which are required in the study of low mass stars. To investigate the importance of each element of the input physics, we exclude it or replace it with an alternative for the model calculations. The parameters of the stellar code, such as the element abundances, the surface condition and the mixing length ratio are as well investigated by alternative values or formulation. In the computation, all elements of input physics, except the energy generation rates, are incorporated into the main code by means of data tables. A bicubic interpolation is used for their input. Our numerical calculations cover the zero age main sequence of the stars ranging from a solar mass down to the hydrogen burning minimum mass. The calculated results of the zero age models for the Standard Population I and Population II indicate good agreement with the observed data for the objects with effective temperatures above 3000 K, or with masses greater than 0,15 M[sub]sun. Below this limit, in agreement with other theoretical work, there is still a small discrepancy between the theoretical models and the observed data. The investigations of the effects of the input physics and model parameters show that some of them affect considerably the minimum mass limit for hydrogen burning on the lower main sequence. The low temperature opacities, the nonideal effects in the EOS and the conductive opacities lead to a limiting mass ranging from 0.08 M[sub]sun to 0.15 M[sub]sun although they do not affect the models with masses above 0.15 M[sub]sun obviously. The investigations also show that the massive models of the lower main sequence (i.e. those with mass near the sun) are dependent very much on the element abundances and the mixing length ratio while the lower mass models are not. The lower main sequence models are found to be insensitive to the surface condition (photospheric model or atmospheric model) used. We also perform the calculation of the evolutionary models of the lower main sequence from the zero age up to an age of 1010 years. A perfect theoretical model of the solar evolution is obtained when the atmospheric model is used as surface condition, the element abundances are chosen to be 0.70/0.28/0.02 and the mixing length ratio to be 1.5. The investigations show that the element abundances are the most important parameters in the determination of the solar models. The mixing length ratio is the second most important one being much more important than the physical model used as surface condition. The evolutionary models of the masses below 0.8 M[sub]sun are found to have only small changes in their properties within 1010 years. None of them can deplete their central hydrogen in that time. We find one problem in that the models with masses around 0.1 M[sub]sun have oscillating solutions for both the zero age models and evolutionary models. According to our investigation, the nonideal effects in the EOS can be responsible. These oscillating solutions imply the existence of more than one stable configuration for stellar masses.