Thesis (Ph. D.)--University of Chicago, Department of Astronomy & Astrophysics, June 2002. / Includes bibliographical references. Also available on the Internet.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1964. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 73-74).

Thesis (Ph. D.)--Ohio State University, 2005. / Title from first page of PDF file. Includes bibliographical references (p. 137-142).

This dissertation presents a detailed study of many aspects of the Horizontal Branch (HB) phase of stellar evolution. A classical technique of stellar structure analysis is summarized, and applied to Zero-Age Horizontal Branch (ZAHB) models. The chief conclusions from this work are firstly, that the total mass of the envelope sensitively affects the luminosity of the hydrogen-burning shell and the equilibrium of the helium-rich core. Secondly, the rapid progression of models across the Hertzsprung-Russell diagram with decreasing mass is the result of important changes in the hydrostatic structure of the stars. Thirdly, the luminosity-metallicity relationship of the Zero Age models results from the change in the core equilibrium luminosity with the CNO abundance of the shell region, together with the decrease in stellar mass at fixed effective temperature. The change in the mass-temperature relation with CNO is found to be the most important determining factor in the Horizontal Branch stellar distribution, and therefore is the most appropriate ‘first parameter’ for HB morphology. The evolution of the stars is then considered, and the analysis of the interior structures provides a reclassification of HB track morphology into three categories, depending on whether the model contains an outer convection zone or a radiative outer envelope, and on the luminosity of the hydrogen-burning shell. Lastly, the question of the formation of red-giant stars is considered; the general conclusions of this part of the study support the arguments presented by Yahil and van den Horn (1985). Next, the evolution of the convective core of HB stars is reviewed, together with a detailed account of the numerical techniques developed for modelling semi-convection. The problems associated with the late phase of HB evolution are also discussed. A brief review of the physical inputs and numerical methods used in the interior is presented, focussing on the calculation and implementation of the Equation of State. The calculations performed for this study are then presented in detail. The effects of oxygen enhancement on zero-age sequences are illustrated for a range in metallicity, and theoretical relations between luminosity and metallicity for the ZAHBs are demonstrated. The evolutionary tracks computed are illustrated and summarized in extensive tabulations in the Appendices. The final chapter reproduces previously published studies of globular clusters. The first of these investigates the globular cluster NGC104 (47 Tucanae). By fitting the theoretical models to recent CCD photometry of the cluster, it was found that its initial helium content must have been close to 24% by mass. In addition, the best fits show that models for [Fe/H] = -0.65 provide an excellent match to the horizontal branch, if (m - M )v ≈ 13.44, and thereby yield consistency over the entire color-magnitude diagram of the cluster. The second study presents an investigation of the horizontal branch of M15. Detailed matches of our theoretical sequences to the cluster observations indicate that high envelope helium abundances are incompatible with the observed morphology. It is found that there is a clear preference for values of 0.21 ≾ Y ≾ 0.25, independent of the value of [O/Fe]. The precision of the method is reduced by uncertainties in the observations and in the available synthetic temperature-bolometric-correction relations. The oxygen enhanced zero-age HB models are found to have a period-colour relationship which is almost identical to that of their scaled-solar counterparts, but they reduce significantly the predicted double-mode variable masses. Importantly, it is found that, for reasonable assumptions about the reddening to M15, there is no discrepancy between the predicted and observed periods for the RR Lyrae variables. However, the period shift between M3 and M15 can be explained by canonical models only if the helium abundance in both clusters is low (Yhb ~ 0.21), and the bulk of the RR Lyrae star population in M15 is at late stages of evolution. These conclusions are reconsidered in the light of the new calculations presented here. / Graduate

Observations of AB Doradus, a nearby, rapidly-rotating K0 dwarf are analysed, and the surface magnetic field is shown to be approximated by a potential field. Evolving the surface magnetic field according to diffusion and the observed differential rotation still yields good correlation between the calculated and observed radial field after 30 days, contradictory to the results of Barnes et al. (1998), leading to the conclusion that there is an additional cause for the evolution of the magnetic field. The chromospheric magnetic field is modelled as a potential field with a source surface. Using the stability criteria g.B = 0 and B.V(g.B) < 0, places where prominences can be stable are investigated. For agreement with the results of Donati et al. (2000)- that prominences form preferentially near the equatorial plane and at and beyond corotation- it is necessary to add a quasidipolar field of maximum strength ~20G.

We present Atacama Large Millimeter Array (ALMA) observations of (CO)-C-12 2-1 emission from circumstellar material around the massive star Eta Carinae (eta Car). These observations reveal new structural details about the cool equatorial torus located similar to 4000 au from the star. The CO torus is not a complete azimuthal loop, but rather, is missing its near side, which appears to have been cleared away. The missing material matches the direction of apastron in the eccentric binary system, making it likely that eta Car's companion played an important role in disrupting portions of the torus soon after ejection. Molecular gas seen in ALMA data aligns well with the cool dust around eta Car previously observed in mid-infrared (IR) maps, whereas hot dust resides at the inner surface of the molecular torus. The CO also coincides with the spatial and velocity structure of near-IR H-2 emission. Together, these suggest that the CO torus seen by ALMA is actually the pinched waist of the Homunculus polar lobes, which glows brightly because it is close to the star and warmer than the poles. The near side of the torus appears to be a blowout, associated with fragmented equatorial ejecta. We discuss implications for the origin of various features north-west of the star. CO emission from the main torus implies a total gas mass in the range of 0.2-1 M-circle dot (possibly up to 5 M-circle dot or more, although with questionable assumptions). Deeper observations are needed to constrain CO emission from the cool polar lobes.

The Hubble Space Telescope archive contains a large collection of images of eta Carinae, and this paper analyses those most suitable for measuring its expanding Homunculus Nebula. Multiple intensity tracings through the Homunculus reveal the fractional increase in the overall size of the nebula; this avoids registration uncertainty, mitigates brightness fluctuations, and is independent of previous methods. Combining a 13 yr baseline ofWide Field Planetary Camera 2 images in the F631N filter, with a 4 yr baseline of Advanced Camera for Surveys/ High Resolution Channel images in the F550M filter, yields an ejection date (assuming linear motion) of 1847.1 (+/- 0.8 yr). This result improves the precision, but is in excellent agreement with the previous study by Morse et al., that used a shorter time baseline and a different analysis method. This more precise date is inconsistent with ejection during a periastron passage of the eccentric binary. Ejection occurred well into the main plateau of the Great Eruption, and not during the brief peaks in 1843 and 1838. The age uncertainty is dominated by a real spread in ages of various knots, and by some irregular brightness fluctuations. Several knots appear to have been ejected decades before or after the mean date, implying a complicated history of mass-loss episodes outside the main bright phase of the eruption. The extended history of mass ejection may have been largely erased by the passage of a shock through clumpy ejecta, as most material was swept into a thin shell with nearly uniform apparent age.

M.Sc. / Just as the observations of oscillations of ordinary stars can be used to determine their composition and structure, the oscillations of neutron stars could potentially be used to determine the nature of the dense nuclear matter from which they are made. The superfluidity of the interiors of neutron stars is normally probed by observations of pulsar glitches. It turns out that the superfluidity affects the oscillations in a neutron star core. In particular, it results in a class of oscillation modes specifically associated with the superfluid core. Although these modes have not been detected from observations, it is hoped by some that gravitational wave data may be used to probe the superfluidity of neutron star cores. In this dissertation, a simple equilibrium model is used in order to calculate the superfluid modes in the context of newtonian gravity. The equilibrium model that is used is the same combination of the Serot equation of state and the Harrison-Wheeler equation of state that was used formerly by Lee and by Lindblom & Mendell. Numerical calculations of the superfluid modes are done for 20 different neutron star models ranging in mass between 0.5 and 2 solar masses. The frequencies of the oscillations for the 0.5 and 1.4 solar masses agree fairly well with Lee's results, which strongly validates the computer code written for numerical calculation in this work. In all the models, the eigenfrequencies of the super uid or s-modes are found among those of the f and p-modes. For the equation of state that is used, it is shown that the dimensionless frequencies of the p-modes increase with an increase in mass of the neutron star while those of the s-modes decrease with an increase in neutron star mass. The plan of the dissertation is as follows. Chapter 1 gives a short introduction to stellar oscillations and mentions the oscillations of neutron stars. Chapters 2 and 3 provide the general theoretical background of stellar structure and stellar oscillations respectively. Chapter 4 is a review of the equations of state of neutron star matter derived previously in the literature. Chapter 5 provides the method of calculation as well as the results. Chapter 6 provides a discussion of the results. Chapter 7 briefly gives a review of a mathematical framework for fluids that could be used in order to calculate the oscillations in a general relativistic context and then briefly describes the effects of rotation and magnetic fields. Appendix B liststhe source code for the programs used to do the calculations and also explains some of the extra numerical procedures used for the computation.

L-band spectroscopy is a powerful probe of cool low-gravity atmospheres: the P, Q, and R branch fundamental transitions of methane near 3.3 mu m provide a sensitive probe of carbon chemistry; cloud thickness modifies the spectral slope across the band; and H-3(+) opacity can be used to detect aurorae. Many directly imaged gas-giant companions to nearby young stars exhibit L-band fluxes distinct from the field population of brown dwarfs at the same effective temperature. Here we describe commissioning the L-band spectroscopic mode of Clio2, the 1-5 mu m instrument behind the Magellan adaptive-optics system. We use this system to measure L-band spectra of directly imaged companions. Our spectra are generally consistent with the parameters derived from previous near-infrared spectra for these late M to early L type objects. Therefore, deviations from the field sequence are constrained to occur below 1500 K. This range includes the L-T transition for field objects and suggests that observed discrepancies are due to differences in cloud structure and CO/CH4 chemistry.

We consider the question of the magnetic field configuration of central compact objects (CCOs), specifically if their observed spectra allow uniform surface temperatures and carbon atmospheres. Although it is theoretically plausible that young hot neutron stars will deplete their hydrogen and helium atmospheres through diffusive nuclear burning, we find that there is no strong observational evidence to suggest that any particular CCO has a uniform temperature carbon atmosphere. In fact, they all may have small hot spots, similar to what we have measured on the surface of RX J0822−4300, and what has been observed in the cases of two other CCOs, 1E 1207.4−5209 and PSR J1852+0040. We find it is likely that most CCOs have small magnetic inclination angles. We also study the magnetic field configurations of two particular young neutron stars through general relativistic modeling of the X-ray light curves produced by their thermal surface emission. In particular, we have analyzed over a decade of XMM-Newton observations of the central compact object RX J0822−4300 and also the transient magnetar XTEJ1810−197. We show that the CCO RX J0822−4300 has two heated regions with very dif-ferent sizes and temperatures, and we measure a significant deviation angle from a purelyantipodal geometry. This measurement can inform theoretical models of the strength and geometry of the crustal magnetic fields that conduct heat to toward these hot spots. We measure the size, temperature, angular emission pattern and viewing geometry toward the heated surface regions of the magnetar XTE J1810−197 in the years following its 2003 outburst. We demonstrate that, after the size and the temperature of the heated region shrank from what was measured in the initial outburst, the magnetar eventually entered a steady state with the hot spot luminosity powered by magnetic field decay. We find that the magnitude of the flux from the whole surface of XTE J1810−197, combined with several distance estimates, indicates that the mass of XTE J1810−197 must be significantly larger than the canonical 1.4 solar mass neutron star.