IRC+10420 is a massive (> 20M☉), very luminous (> 〖10〗^6L☉) star that is in the rare phase of evolution from the red supergiant to the luminous blue variable or Wolf-Rayet phase. Previous observations reveal that the circumstellar envelope is rich in molecular gas, and can be detected out to a radius of about 8” = 6.0 X 〖10〗^17 cm. Observations in CO also reveal that the global massloss rate of IRC+10420 has changed dramatically over the last 6000 years, comprising two major episodes of mass loss lasting for about 1000 and 4000 years respectively separated by period of very low mass-loss rate lasting for about 1000 years. Surprising, previous observation in SiO(J = 2 - 1) revealed a ring-like enhancement at a radius of about 1” (7.5X 〖10〗^16 cm) from the star, contrary to the expectation that SiO molecules should be frozen onto dust grains very close to the star (within ～ 〖10〗^16cm). This ring-like enhancement has been attributed to a large-scale shock produced by interactions between faster and slower moving portions of the expanding envelope. In this thesis, we mapped the circumstellar envelope in SiO(J = 1 - 0) to better constrain the physical conditions (gas density, temperature and SiO abundance) in the SiO-emitting gas. We find a similar ring-like enhancement in SiO(J = 1 - 0) but located further out at a radius of about 2” (1.5 X 〖10〗^17 cm), and confirm that the SiO emission extends as far out as the CO envelope. The computed SiO(J = 2-1)/SiO(J = 1-0) line ratio significantly exceeds unity at radius out to about the location of the ring-like enhancement (2”), and drops to a value of about unity beyond this radius. From a one-dimensional non-local thermodynamic equilibrium model, we explore the physical conditions that can reproduce the observed brightness temperatures in both SiO(J = 10) and SiO(J = 2-1) as well as their line ratio as a function of radius. The SiO-emitting gas is required to have a density that is much higher (from a factor of a few to about two orders of magnitude) than has been inferred for the CO-emitting gas at the same radii. The required surface filling factor of the SiO-emitting gas depends on their unknown gas-phase SiO abundance; for an abundance of ～〖10〗^(-5), as inferred just above the photospheres of lowmass evolved stars, the surface filling factor of these condensations range from ～0.001 to ～0.1. Thus, the SiO emission from the envelope of IRC+10420 most likely originates from dense condensations that are immersed in more diffuse gas that produces the bulk of the observed CO emission. We reason that the SiO-emitting condensations correspond to the dust clumps detected in reflected light with the Hubble Space Telescope. These dust clumps are distributed from near the star out to a radius of 2”, spanning the same extent as the peaks of SiO- (and CO-) emitting envelope. We show that these dust clumps are expanding in every direction away from the stars at a velocity that is significantly higher than the CO-emitting gas, and anticipate that shocks thus generated heats up the dust clumps to release SiO into the gas phase. / published_or_final_version / Physics / Master / Master of Philosophy

The history of an actively star-forming region, containing the young, open cluster NGC 2264, is examined in this thesis using UBVRIHα photographic plate material. After digitization and removal of the variable background from these plates, automatic detection and photometry algorithms yield magnitudes for a set of candidate low-mass cluster members. Using the photographic plates and a supplementary video camera Hα survey, these candidates are selected on the basis of their exhibiting significant Hα emission, large amplitude variability, or anomalously blue colors. To study the age spread in NGC 2264, a theoretical H-R diagram is constructed. Infrared photometry of 33 candidate members permits an evaluation of the bolometric luminosities of these stars; the unreddened (V - R) and (V - I) colors yield effective temperatures. The theoretical mass tracks and isochrones of Cohen and Kuhi (1979) are adopted and extrapolated to lower luminosities and temperatures. The effects of circumstellar dust and gas shells on the location of the NGC 2264 stars in the H-R diagram are examined. No systematic biases in the derived bolometric luminosities and temperatures are found. The optical and infrared excesses of these stars are found to be uncorrelated, requiring separate mechanisms for their generation. A gaseous envelope is most likely the source of the optical excess; thermal emission from hot dust most probably leads to the observed infrared excesses. An age spread for the low-mass NGC 2264 stars of ≳10⁷ years is indicated. Star formation in NGC 2264 is found to have proceeded sequentially as a function of mass and time. Low-mass star formation began ∼2-3 x 10⁷ years ago, peaked ∼4-5 x 10⁶ years ago, and declined thereafter. Star formation at successively higher masses has started, peaked, and then declined at successively younger ages. No significant differences are seen between the field and NGC 2264 luminosity functions to the faintest levels observed (Mᵥ ≈ +11). There is no evidence for a low-mass turnover in NGC 2264.

The results of a multi-faceted search for spectral line emission from galaxies at high redshift are presented. Deep two-dimensional spectra of four blank sky fields were taken at the Multiple Mirror Telescope (MMT) at wavelengths corresponding to redshifted Lyα emission in the range 2.7 < z < 4.7, and the resulting non-detections are used to place upper limits on the space density of randomly distributed Lyα emitters. Although the predictions of a conservative model of galaxy formation incorporating cold dark matter are not excluded in a statistical sense, the search would have detected star-forming dwarf galaxies comparable to the Magellanic Clouds at z = 3, given minimal extinction by dust. The Goddard Fabry-Perot Imager, a piezo-electrically controlled tunable narrow-band filter system with a stand-alone CCD system, was developed and tested, and used at the Kitt Peak National Observatory (KPNO) 4 m telescope to image the damped Lyα absorbing clouds towards three high-redshift QSOs in the light of redshifted Lyα. A companion galaxy to one of the damped systems was discovered and confirmed with followup spectroscopy, and its properties are presented and discussed; the companion exhibits strong Lyα and weak C IV λ1549 and He II λ1640 emission lines, and is apparently producing stars at a rate SFR ∼ 5 - 10M(⊙) yr⁻¹. The implications of the companion's proximity to the damped cloud are analyzed in view of the previous non-detections, and a lower limit to the spatial correlation function of the damped Lyα systems with galaxies is given. Spectroscopic limits on Lyα emission from seven damped systems, including some known to have low chemical and dust abundances, imply low levels of star formation, SFR ≲ 1 M(⊙) yr⁻¹, but extinction by dust in some cases may cause an underestimation of these rates. With near-infrared spectrographs at the MMT and the KPNO 4 m, eight damped Lyα systems were searched for spectral lines characteristic of star formation regions but redshifted from the optical into the near-infrared. A possible detection of (O II) λ3727 and Hβ from one system implies a star formation rate on the order of 100 M(⊙) yr⁻¹, though the remainder of the observations produced non-detections compatible with the Lyα emission limits. The implications for galaxy formation and evolution at high redshift are discussed.

No description available.

No description available.

We investigate stellar models for main-sequence and horizontal-branch stars constructed using the Carson opacities and make comparisons with models based on the Cox-Stewart opacities. A Henyey code based on the prescription of Kippenhahn et al (1967) is used for most of the calculations of stellar structure and evolution. In the equation of state we treat ionisation equilibrium and non-relativistic degeneracy for separate temperature-density regimes. The opacity is obtained by 4-dimensional linear interpolation in the Carson opacity tables. Nuclear energy generation rates are taken from Fowler et al (1975) and neutrino losses from the approximation due to Beaudet et al (1967). Electron-screening factors are from Reeves (1965). The standard local mixing-length theory of Bohm-Vitense (1958) is used to treat non-adiabatic convection, although some models are calculated with modifications due to Deupree et al (1979, 1980). We neglect semi convection. The Carson opacities have only a small effect on the position of ZAHB models, but this may be metallicity dependent. The drop in the hydrogen- shell luminosity due to the helium-core expansion during HB evolution is greater than that obtained with the Cox-Stewart opacities. Allowing for the inclusion of semi convection and convective overshooting, we find that adoption of the Carson opacities leads to a reduction of approximately 25% in the HB lifetimes. For a given range of values for the masses and envelope helium abundances of stars on a synthetic HB, the width in effective temperature is increased, and in luminosity the width is decreased. The dependence of the core luminosity on the falling core helium abundance is increased by approximately 16%. Studies of main-sequence stars lead to agreement with Stothers' (1974a, 1974b, 1976) results for homogeneous models constructed with the Carson opacities. The evolution of main-sequence stars of intermediate mass is unaffected by the change in the opacity. Two evolutionary sequences (for 1 Mo stars) suggest that the main-sequence lifetimes of low mass stars may be reduced by as much as 30%. Combined with a shift in the ZAMS position this will move isochrones for low mass stars towards lower effective temperatures and densities. If studies of red-giant evolution indicate little change in the luminosity level of the horizontal branch, globular cluster ages determined from the position of the main-sequence turnoff point may be substantially reduced (possibly by as much as 50%). This could save a conflict between observed values for globular cluster ages and a value for the Hubble constant of 90. Studies of the apsidal motion constant, k2, for evolved MS stars shows that the discrepancy between observed values of k1 for eclipsing binary systems and theoretical values obtained from homogeneous stellar models may be resolved by considering the evolution of the binary components. CO Lac is an exception to this result, but analysis of the observations suggests that a redetermination of the orbital semi-amplitudes may resolve the conflict.

Thesis presented in fulfilment of the requirements for the degree of Doctor of Philosophy at the University of the Witwatersrand, Johannesburg, 2017. / This thesis investigates in detail the structure of models of turbulent convec tion with phenomenological closures for the eddy-viscosity. It explores the merits of replacing the canonical Mixing Length Theory of stellar convection with more realistic models of fluid turbulence that take into account the full spectrum of eddy sizes. The author provides a detailed exposition of the fun damental assumptions and the modus operandi of various approaches to the treatment of convective energy-transfer in stars. He focuses in particular on spectral descriptions of the convective process. The structure of several clo sure models developed by various authors are investigated, and he identifies and elucidates those aspects of these closures that lead to an improved descrip tion of convective turbulence in the stellar interior. The author also develops an implementation within the public-domain code, called Modules for Experi ments in Stellar Astrophysics, of two of these models and reports and discusses the results of his numerical experiments. / XL2018

暗物質乃構成宇宙的主要部分。而暗物質，即自身為不可見之物，無論自我湮滅與否，皆可能於各方面影響天體的特性。 / 在本論文中，我們首先研究一個混入非自我湮滅費米子暗物質的核心，並藉二流體方法，探討它如何影響中子星的靜態平衡結構。我們發現若達到足夠大的暗物質質量比重時，便可產生一種新型的致密星一顆由暗物質支配、半徑有數公里的中子星。然後，我們研究該核心導致中子星的線性變化，並應用此星體模型解釋已觀測的特細中子星，也考察此暗物質對中子星的冷卻過程的影響，和不同粒子質量的暗物質的效果。最後，我們討論該核心的徑向震動，從中發現新一種震動模式。 / 接下來，我們集中討論混入暗物質的核心如何干涉主序星的星體演化。一般而言，此核心皆縮短主序星的壽命，並改變演化間唯量，譬如氫融合及氦融合的最低所需質量。但就一個擁有可觀質量的暗物質核心而言，我們發現幾種唯象的改變包括主序星壽命的延長，高質量恆星的橫向分支的消失，以及星體演化的新路徑。由此我們得出，把暗物質放入考慮之列，能令星體演化的討論變得更有系統，也是不可或缺的。再者，我們論證前述的星體，即由暗物質支配的中子星，亦可能從類近的演化中產生。 / 最後，我們轉向研究，在牛頓水動力學中的球體對稱情況下，一個點質量的暗物質核心如何改變白矮星徑向運動的特性。於可行的模擬運算下，此核心的額外引力只局部改變星體的特性，如初始靜態平衡下的密度分佈的外型，而星體的徑向震動模式及震動頻率則不受影響。 / Dark matter constitutes a large proportion of matter in the universe and it is believed that dark matter, self-annihilating or not, can bring observational change to astrophysical objects in different aspects, even though it cannot be observed directly. / In chapter 2, we first study the effect of an admixed dark matter core, consisted of non-self-annihilating fermionic dark matter particles, to the equi-librium structure of a neutron star using the two-fluid formalism. We find that a new subclass of compact star, dark matter dominated neutron star, is formed for a sufficiently large mass fraction of dark matter, which has only a few-kilometer-sized visible radius. Also, we study the linear response of a neutron star towards a dark matter core; we apply the model to explain several observed neutron stars with extraordinary small radii and we study how the dark matter core affects the cooling properties of neutron stars. We examine the effects caused by the choice of dark matter particle mass. Then, we study the radial oscillation modes of a dark matter admixed neutron star, in which a new class of modes is present due to the admixture of dark matter. At last, we extend our study from neutron stars to strange stars, in which we discover that the response of a strange star to the presence of an admixed dark matter core is qualitatively different from a neutron star. / In chapter 3 we focus on the effect of the dark matter core to the stellar evolution of main-sequence stars. We find that in general the existence of a dark matter core reduces the stellar lifetime and quantitative change in the stellar evolution, such as the decrement of the threshold mass for hydro¬gen and helium fusion. But for sufficiently massive dark matter core, several qualitative changes are observed: they include an extension in main-sequence lifetime, absence of horizontal branch for high-mass stars and some new paths in the stellar evolution. From the study we find that the consideration of dark matter is indispensable in a systematic discussion of stellar evolution. Also we argue that dark matter dominated neutron star can be formed following similar stellar evolution processes. / In chapter 4 we turn to the impact of a point-mass dark matter core on the radial oscillation of a white dwarf by doing Newtonian hydrodynamical simulations in spherically symmetric scenarios. We find that, within feasible simulations, the effects of an extra gravity from the dark matter core is highly local. It creates a change in the initial static equilibrium density profile, but its radial oscillation patterns and oscillation frequencies remain unchanged. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Leung, Shing Chi = 非自我湮滅暗物質對致密星及主序星的影響 / 梁成志. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2012. / Includes bibliographical references (leaves 119-131). / Abstracts also in Chinese. / Leung, Shing Chi = Fei zi wo yan mie an wu zhi dui zhi mi xing ji zhu xu xing de ying xiang / Liang Chengzhi. / Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Dark Matter: The mysterious piece in the Universe --- p.1 / Chapter 1.1.1 --- The puzzle of missing mass --- p.1 / Chapter 1.1.2 --- Dark Matter in Particle Physics --- p.2 / Chapter 1.1.3 --- Alternatives to Dark Matter --- p.4 / Chapter 1.2 --- Probing the Nature of Dark Matter --- p.5 / Chapter 1.3 --- Neutron Star Astrophysics --- p.8 / Chapter 1.3.1 --- Equilibrium Structure and Radial Oscillation Modes --- p.8 / Chapter 1.3.2 --- Non-radial Oscillation Modes --- p.9 / Chapter 1.3.3 --- Use of NS as a DM probe --- p.10 / Chapter 1.3.4 --- Rotating Neutron Stars --- p.11 / Chapter 1.4 --- Hydrodynamical Simulation --- p.12 / Chapter 1.4.1 --- Algorithm --- p.12 / Chapter 1.4.2 --- Applications in Astrophysics --- p.14 / Chapter 1.5 --- Structure of the thesis --- p.15 / Chapter 2 --- Astrophysical Properties of DANS --- p.17 / Chapter 2.1 --- Formalism --- p.17 / Chapter 2.1.1 --- Static Equilibrium Structure of DANS --- p.17 / Chapter 2.1.2 --- Choice of EOS --- p.19 / Chapter 2.1.3 --- Moment of inertia --- p.20 / Chapter 2.1.4 --- Equations for radial oscillations --- p.21 / Chapter 2.1.5 --- Equations for NS cooling --- p.23 / Chapter 2.1.6 --- EOS for strange quark matter --- p.32 / Chapter 2.2 --- Static Equilibrium Properties of DANS --- p.35 / Chapter 2.2.1 --- General Properties of DANS --- p.35 / Chapter 2.2.2 --- Linear Response of DANS --- p.39 / Chapter 2.2.3 --- Effects of DM particle mass --- p.41 / Chapter 2.3 --- Radial Oscillations of DANS --- p.44 / Chapter 2.3.1 --- One-fluid limit --- p.44 / Chapter 2.3.2 --- Oscillation modes of DANS --- p.45 / Chapter 2.3.3 --- Effects of DM particle mass --- p.51 / Chapter 2.4 --- Neutron Star Cooling --- p.52 / Chapter 2.4.1 --- Introduction --- p.52 / Chapter 2.4.2 --- Numerical Result in Neutrino Emission --- p.53 / Chapter 2.4.3 --- Time-dependent calculation --- p.54 / Chapter 2.4.4 --- Discussion --- p.58 / Chapter 2.5 --- Strange Star --- p.59 / Chapter 2.5.1 --- Introduction --- p.59 / Chapter 2.5.2 --- Strange Star Properties --- p.60 / Chapter 2.5.3 --- Discussion --- p.63 / Chapter 2.6 --- Chapter Summary --- p.65 / Chapter 3 --- Effects of Non-self-annihilating Dark Matter on Stellar Evolution --- p.66 / Chapter 3.1 --- Formulation --- p.66 / Chapter 3.1.1 --- Effect of Dark Matter --- p.66 / Chapter 3.1.2 --- Convergence test --- p.67 / Chapter 3.2 --- Results --- p.72 / Chapter 3.2.1 --- High mass stars --- p.74 / Chapter 3.2.2 --- Intermediate mass stars --- p.76 / Chapter 3.2.3 --- Low mass stars --- p.79 / Chapter 3.2.4 --- Brown Dwarfs --- p.82 / Chapter 3.3 --- Discussion --- p.85 / Chapter 3.3.1 --- Absence of Horizontal Branch --- p.85 / Chapter 3.3.2 --- Extended Main-sequence Lifetime --- p.87 / Chapter 3.3.3 --- Lower Mass Thresholds for Hydrogen Fusion and Helium Fusion --- p.88 / Chapter 3.3.4 --- New Stellar Evolution Path --- p.90 / Chapter 3.4 --- Chapter Summary --- p.91 / Chapter 4 --- Dynamical Simulation of Hybrid White Dwarf --- p.93 / Chapter 4.1 --- Formalism --- p.93 / Chapter 4.2 --- Convergence Test --- p.94 / Chapter 4.3 --- Numerical Results --- p.96 / Chapter 4.4 --- Chapter summary --- p.98 / Chapter 5 --- Conclusion --- p.101 / Chapter A --- DANS as an Alternative to Strange Stars --- p.105 / Chapter B --- Verification of separated TOV equations --- p.107 / Chapter C --- Code for numerical evolution --- p.109 / Chapter D --- Notation Convention and Units --- p.111 / Chapter E --- Discussion on the smearing of DM core --- p.113 / Chapter F --- Unit conversion --- p.118 / Bibliography --- p.119

No description available.

The evolutionary status of the hot R CrB stars has long remained a matter of conjecture, primarily because of the relative dearth of relevant observational material. Previously the group had been thought to occupy a position intermediate in status between the EHe and R CrB classes as they have (at least at first glance) observational properties in common with both types of object. The photosphere of DY Cen has been quantitatively confirmed to be hydrogen-deficient and photometric variations suggest the star undergoes short period pulsations. Using a period-temperature relation applicable to hydrogen-deficient stars it would appear that this object has similar physical properties to both the EHe and R CrB stars. Narrow band imaging of V348 Sgr has shown that the associated nebula exhibits a bipolar structure and therefore must be closely related to planetary nebulae rather than H II regions. Spectroscopic observations have proved that the star in its present evolutionary state is incapable of ionising the nebula. Several scenarios for this behaviour are briefly discussed. The large helium enrichment found in the nebula indicates that processed material must have been ejected during the last major episode of mass loss. Evidence is presented that suggests a strong hydrogen abundance gradient exists within the nebula. A novel technique has been developed for determining reddening distances. Its main advantage over other similar methods is that both early and late-type stars may be used to establish the reddening-distance relationship. With more development this technique may prove to be an important tool in distance determinations for objects such as planetary nebulae etc. This technique was used to derive a distance of (4. 7±1.0) kpc for V348 Sgr. Using the core-mass relation for hydrogen-deficient stars implies that both V348 Sgr and MV Sgr are lower mass and luminosity objects than EHe and R CrB stars. The evidence presented in this thesis indicates that the hot R CrB group is not a homogeneous one. DY Cen is much more luminous and massive than the other members. The mass and luminosity of V348 Sgr and MV Sgr are consistent with the scenario that both have recently suffered a thermal pulse (causing re-ignition of a helium burning shell) and are currently looping back to the R CrB domain of the H R diagram.