Yeung Yuk Ming. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2002. / Includes bibliographical references (leaves 82-85). / Abstracts in English and Chinese. / Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Motivations --- p.1 / Chapter 1.2 --- Historical background --- p.2 / Chapter 1.3 --- Outline of this thesis --- p.3 / Chapter 2 --- Mode classifications --- p.5 / Chapter 2.1 --- Fluid modes --- p.6 / Chapter 2.1.1 --- f mode (fundamental mode) --- p.6 / Chapter 2.1.2 --- p mode (pressure mode) --- p.7 / Chapter 2.1.3 --- g mode (gravity mode) --- p.7 / Chapter 2.2 --- w mode (spacetime mode) --- p.8 / Chapter 3 --- Oscillations of neutron stars --- p.10 / Chapter 3.1 --- The equilibrium configurations of neutron star models --- p.10 / Chapter 3.1.1 --- Newtonian stars --- p.10 / Chapter 3.1.2 --- Relativistic stars --- p.11 / Chapter 3.2 --- Perturbation of the star's equilibrium --- p.14 / Chapter 3.2.1 --- Axial perturbation equation --- p.15 / Chapter 3.2.2 --- Boundary conditions --- p.16 / Chapter 3.2.3 --- Numerical techniques --- p.16 / Chapter 3.2.4 --- The Quasinormal modes of stars --- p.18 / Chapter 4 --- Excitation and detection of QNMs --- p.20 / Chapter 4.1 --- Studies of excitation of stellar QNMs --- p.20 / Chapter 4.2 --- Detection of QNM ringing --- p.21 / Chapter 4.3 --- Parameter estimation --- p.23 / Chapter 5 --- Oscillations of realistic neutron stars --- p.28 / Chapter 5.1 --- Motivations of study --- p.28 / Chapter 5.2 --- Realistic equations of state --- p.29 / Chapter 5.3 --- Axial QNM --- p.36 / Chapter 6 --- Logarithmic perturbation method --- p.38 / Chapter 6.1 --- Introduction --- p.38 / Chapter 6.2 --- Logarithmic perturbation theory --- p.39 / Chapter 6.3 --- Evaluation of perturbation formulae --- p.42 / Chapter 6.3.1 --- The first-order perturbation --- p.42 / Chapter 6.3.2 --- The second-order perturbation --- p.47 / Chapter 6.4 --- Comparison of LPT calculations with exact results --- p.47 / Chapter 6.5 --- Discussion --- p.51 / Chapter 7 --- Scaled coordinate logarithmic perturbation method --- p.53 / Chapter 7.1 --- Rescaling of the axial perturbation equations --- p.54 / Chapter 7.1.1 --- The original axial perturbation equations --- p.55 / Chapter 7.1.2 --- The rescaled axial perturbation equations --- p.56 / Chapter 7.2 --- The formalism of SCLPT --- p.57 / Chapter 7.2.1 --- Logarithmic perturbation theory --- p.59 / Chapter 7.2.2 --- First-order perturbation --- p.63 / Chapter 7.2.3 --- Second-order perturbation --- p.65 / Chapter 7.3 --- Comparison of the perturbation calculation with the exact result --- p.67 / Chapter 7.4 --- Discussion --- p.70 / Chapter 8 --- Conclusion --- p.73 / Chapter 8.1 --- Summary --- p.73 / Chapter 8.2 --- Outlook --- p.74 / Chapter A --- Units conventions --- p.76 / Chapter B --- Numerical technique in Leaver's series --- p.77 / Chapter C --- Method of numerical derivatives --- p.80 / Bibliography --- p.82

Galactic nuclei are important for studies of galaxy evolution, stellar dynamics and general relativity. Many have Supermassive Black Holes (SMBHs) (with one million to one billion times the mass of the sun) that affect the large scale properties of their hosts. They are also the densest known stellar systems, and produce unique electromagnetic and gravitational wave sources via close encounters between stars and compact objects. For example, stars that wander too close to an SMBH are tidally disrupted, producing a bright flare known as a TDE. This thesis investigates the gas and stellar environments in galactic nuclei. In Chapters 2 and 3, we develop an analytic model for the gas environment around quiescent SMBHs. In the absence of large scale inflows, winds from the local stellar population will supply most of the gas. The gas density on parsec scales depends strongly on the star formation history, and can plausibly vary by four orders of magnitude. In Chapter 3, we use this model to constrain the presence of jets in a large sample of TDE candidates. In Chapter 4 we construct observationally motivated models for the distributions of stars and stellar remnants in our Galactic Center. We then calculate rates of various collisional stellar interactions, including the tidal capture of stars by stellar mass black holes. This process produces ~100 black hole LMXBs in the central parsec of the Galaxy (comparable to the number inferred from recent X-ray studies).

Lau, Hoi Kwan. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2010. / Includes bibliographical references (leaves 208-212). / Abstracts in English and Chinese. / Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Gravitational wave astronomy --- p.1 / Chapter 1.2 --- Stellar pulsation and gravitational radiation --- p.3 / Chapter 1.3 --- Outline --- p.5 / Chapter 2 --- Hydrostatic stellar structure --- p.8 / Chapter 2.1 --- Structural equation --- p.9 / Chapter 3 --- Finite temperature equations of state of nuclear matter --- p.13 / Chapter 3.1 --- Finite temperature ordinary nuclear matter --- p.13 / Chapter 3.2 --- Strange Quark Matter --- p.15 / Chapter 3.3 --- Equilibrium and Dynamic EOS --- p.16 / Chapter 4 --- Stellar pulsation and gravitational radiation --- p.19 / Chapter 4.1 --- Linearized theory of general relativity --- p.19 / Chapter 4.2 --- Stellar oscillation --- p.25 / Chapter 4.3 --- Quasi-normal Mode --- p.28 / Chapter 4.3.1 --- f mode --- p.29 / Chapter 4.3.2 --- p mode --- p.29 / Chapter 4.3.3 --- g mode --- p.30 / Chapter 4.3.4 --- w mode --- p.31 / Chapter 5 --- Gravitational wave spectrum of hot compact stars --- p.32 / Chapter 5.1 --- Numerical results --- p.32 / Chapter 5.1.1 --- Temperature effect on QNM --- p.32 / Chapter 5.1.2 --- Temperature effect and QS model --- p.38 / Chapter 5.1.3 --- QNM shift due to phase transition --- p.41 / Chapter 5.2 --- Summary and prospective --- p.48 / Chapter 6 --- Universality of fundamental mode and spacetime mode --- p.50 / Chapter 6.1 --- Review --- p.50 / Chapter 6.2 --- Generic proposal of universalities --- p.53 / Chapter 6.2.1 --- Moment of Inertia --- p.54 / Chapter 6.2.2 --- Gravitational wave spectrum --- p.57 / Chapter 6.3 --- Universality on moment of inertia --- p.63 / Chapter 6.4 --- Origin of universality --- p.70 / Chapter 6.4.1 --- Tolman VII model --- p.71 / Chapter 6.4.2 --- Polytropic Model --- p.76 / Chapter 6.5 --- Application of universality --- p.82 / Chapter 6.6 --- Summary --- p.89 / Chapter 7 --- Quark star properties and gravity mode oscillation --- p.92 / Chapter 7.1 --- Introduction --- p.92 / Chapter 7.2 --- g mode frequencies of quark stars --- p.94 / Chapter 7.2.1 --- Temperature profile and p mode frequency --- p.96 / Chapter 7.2.2 --- Strange quark mass and Yp mode frequency --- p.104 / Chapter 7.3 --- Summary --- p.108 / Chapter 8 --- Gravitational radiation excitation by infalling shell --- p.111 / Chapter 8.1 --- Introduction --- p.111 / Chapter 8.2 --- Formalism --- p.116 / Chapter 8.2.1 --- Connection between star and vacuum --- p.117 / Chapter 8.2.2 --- Matter source --- p.121 / Chapter 8.2.3 --- Geodesic --- p.124 / Chapter 8.2.4 --- Source of infalling dust shell --- p.126 / Chapter 8.2.5 --- Green's function --- p.127 / Chapter 8.3 --- Gravitational Wave excitation by collapsing shell --- p.130 / Chapter 8.4 --- Features of radiation --- p.138 / Chapter 8.4.1 --- Power spectrum --- p.138 / Chapter 8.4.2 --- Wave function --- p.144 / Chapter 8.4.3 --- Energy of excitation --- p.147 / Chapter 8.5 --- Non-adiabatic oscillation --- p.153 / Chapter 8.5.1 --- Mathematical Background --- p.154 / Chapter 8.5.2 --- Numerical results --- p.158 / Chapter 8.6 --- General relativistic simulation --- p.163 / Chapter 8.6.1 --- Technical briefing --- p.163 / Chapter 8.6.2 --- Numerical results --- p.166 / Chapter 8.7 --- Summary --- p.174 / Chapter 9 --- Conclusion and remarks --- p.178 / Chapter A --- Unit conversions --- p.183 / Chapter B --- Series expansion of quark star EOS --- p.185 / Chapter C --- Accuracy of simplified mode extraction scheme --- p.188 / Chapter D --- Computation of moment of inertia --- p.193 / Chapter E --- Comment of exactness of inference scheme --- p.195 / Chapter E.1 --- Precision of the mass inferred --- p.195 / Chapter E.2 --- Accuracy of universality combinations --- p.199 / Chapter F --- Calculation of sound speed --- p.202 / Chapter G --- Mode extraction of non-adiabatic oscillation --- p.204 / Bibliography --- p.208

White dwarfs are the final remnants of low- and intermediate-mass stars. About 95% of main- sequence stars will end their evolutionary pathways as white dwarfs and, hence, the study of the white dwarf population provides details about the late stages of the life of the vast majority of stars.Since white dwarfs are long-lived objects, they also constitute useful objects to study the structure and evolution of our Galaxy. For instance, the initial-final mass relationship, which connects the final mass of a white dwarf with the initial mass of its main-sequence progenitor, is of paramount importance for different aspects in modern astrophysics. This function is used for determining the ages of globular clusters and their distances, for studying the chemical evolution of galaxies, and also to understand the properties of the Galactic population of white dwarfs. Despite its relevance, this relationship is still relatively poorly constrained.The main aim of this thesis is the study of the initial-final mass relationship. For such purpose we have used two different approaches. From an observational perspective, the statistical significance of the current initial final mass relationship can be improved by performing spectroscopic observations of white dwarfs for which some important parameters are available. Since this approach involves the use of theoretical stellar evolutionary tracks the resulting initial-final mass relationship is, in fact, semi-empirical. In this thesis we present a promising method which consists in using common proper motion pairs comprised of a white dwarf and a FGK star. It is sound to assume that the members of the system were born simultaneously and with the same chemical composition. Moreover, these stars are well separated and it can be considered that they have evolved as isolated stars, since mass exchange between them is unlikely. Thus, a careful analysis of the observational data of both members of each pair allows us to derive the initial and final masses of the white dwarf components, something which is totally impossible when white dwarfs are isolated. Considering the new data that we have obtained with this work and the observational data currently used to define the initial-final mass relationship we have carried out a revision of this relationship, giving some clues on its dependence on different parameters, especially on metallicity.The second approach to improve the initial-final mass relationship involves an indirect measurement, which has been carried out by studying its influence on one of the powerful tools related to the white dwarf population, the white dwarf luminosity function. We have computed a series of luminosity functions using different theoretical initial-final mass relationships, and also, considering the semi-empirical relation derived in this thesis. We have compared these computations with the available observational data in order to evaluate the validity of each of these relations.In order to increase the statistical significance of the white dwarf luminosity function and to improve the initial-final mass relationship it is necessary to extend the amount of accurate and reliable observational data. For this reason part of the thesis is devoted to the Alhambra Survey, which is a good example of the new deep surveys currently under development. These observational projects will detect thousands of new white dwarfs, some of them belonging to common proper motion pairs, which could be eventually used to extend our analysis. Thus, we have performed an exhaustive study to optimize the identification procedure of the white dwarf candidates which will be eventually detected by the Alhambra survey.

Element abundance trends with overall metallicity contain vital clues to the formation and evolution of the Galaxy. Abundances may be used to elucidate nucleosynthesis mechanisms and to ascertain rates of Galactic enrichment. To obtain accurate abundances, several crucial inputs such as high-quality spectroscopic observations, rigorous calculations of line transfer, and precise atomic data (e.g. transition probabilities) are necessary. The current work endeavors to improve abundance values for key elements with a four-fold approach: accumulation of hundreds of high-resolution stellar spectra in order to commence a systematic and thorough Manganese abundance derivation in cluster and halo field stars; re-determination of the neutral chromium oscillator strengths and application of this data to stellar abundance analyses; modification of a radiative line transfer code in order to yield accurate abundances from evolved stars; and semi-empirical derivation of transition probabilities to allow for the utilization of spectral features in the red visible and infrared wavelength ranges for abundance determinations. The first comprehensive investigation of manganese in globular clusters is done in this work. A subsolar Mn abundance trend for both halo globular cluster and field stars is found. The analysis shows that for the metallicity range -0.7>(Fe/H)>-2.7 stars of 19 globular clusters have a a mean relative abundance of <(Mn/Fe)>= -0.37±0.01 (σ=0.10), a value in agreement with that of the field stars: <(Mn/Fe)>= -0.36± 0.01 (σ=0.08). Remarkably, the <(Mn/Fe)> ratio remains constant in both stellar populations over a 2 orders of magnitude span in metallicity. Next, the present study employed branching fraction measurements from Fourier transform spectra in conjunction with published radiative lifetimes to determine transition probabilities for 263 lines of neutral chromium. These laboratory values are used to derive a new photospheric abundance for the Sun: log [element of](Cr I)⊙= 5.64±0.01 (σ=0:07). In addition, oscillator strengths for singly-ionized chromium recently reported by the FERRUM Project are employed to determine: log [element of](Cr II)⊙ = 5.77±0.03 (σ= 0.13). No indications of departures from LTE are found in the neutral chromium abundances. The current work then takes advantage of the fact that transition metals exhibit relatively pure LS coupling and employs standard formulae to yield semi-empirical oscillator strengths. These data were then compared to experimental gf values in order to assess accuracy. Finally, this study undertakes a new abundance investigation of the RGB and RHB stars of the M15 globular cluster. A detailed examination of the both the metallicity and n capture elements is performed. This work appears to confirm that star-to-star abundance variations do occur among the M15 giants (which was initially observed by Sneden et al. 1997, 2000). / text

We present an equatorial (± 30◦ Decl.) sample of all known single (within 4′′) mid M-dwarfs (M2.5V-M8.0V) extending out to 10 pc. For this sample of 58 stars, we provide photometry, low dispersion optical (6000−9000 ̊A) spectra from which spectral types are determined, Hα equivalent widths, and gravity sensitive NaI indices. For 45 of these 58 stars, strict limits are placed on the presence of companions, based on precise infrared radial velocities. Our spectroscopic results indicate that on average, we rule out the existence of companions with masses of 1.5 MJUP or greater in 10 day orbital periods around slowly rotating (vsini < 6.5 km s−1) M-dwarfs. Similarly, strict limits are placed on the presence of companions to 53 out of the 58 stars with astrometry. Our astrometric results show that, on average, we rule out the presence of companions with masses greater than 9 MJUP with an orbital period of 8 years. These results establish these stars as the nearest set of single mid M-dwarfs. Two additional stars, GJ 867B and LHS 1610, were initially included in this program, but later discovered to be spectroscopic binaries (SB). The binary GJ 867BD is a wide (24.5') companion to the M2 dwarf GJ 867AC. With this discovery, the GJ 867 system (d =8.82 ± 0.08 pc) becomes one of only four quadruple systems with in 10 pc of the Sun and the only among these with all M-dwarf (or cooler components). To measure how the rotational velocities vary with spectral type, we assembled a list of all known single (within 3′′) mid M-dwarfs that have trigonometric parallaxes within 25 pc and reside between −30◦ and +65◦ Decl from the RECONS sample. From this list of 402 stars, only 169 stars have previously reported vsini values. We obtained spectroscopic measurements for an additional 75 stars. Of those, 17 have vsini values above our detection threshold of 3 km s−1. Our data are consistent with the trend of more low mass M-dwarfs having high projected rotational velocity values than high mass M-dwarfs.

The Hobby-Eberly Telescope Chemical Abundances of Stars in the Halo (CASH) project aims to characterize the nature of the early universe through the study of metal-poor stars in the stellar halo of the galaxy. Once completed, this will be the largest set of abundances determined for metal-poor stars from high resolution spectra. In this paper, we present chemical abundances and trends of eleven elements for the first ~80 stars of the ~500 star study. These 80 stars serve as a pilot sample to test the automated stellar parameter and abundance determination pipeline newly developed for the CASH project called CASHCODE. Among the pilot sample, two stars with [Fe/H]<-3.5 were discovered and their abundance analysis is discussed. / text

In order to study jets and outflows from Young Stellar Objects (YSOs), I performed an unbiased search on a continuous 33 square degree sized region in Serpens and Aquila using data taken from the UWISH2 survey, which uses the 1-0 S(I) emission line ofH2 as a tracer. I identified 130 molecular hydrogen outflows from YSOs from which 120 (92 %) objects are new discoveries. Distances were measured by foreground star counts with an accuracy of25 %. Outflows were found in groups of 3 - 5 members with a size of about 5 pc. Groups were separated by about half a degree on the sky. About half of the objects were assigned with potential source candidates. Brighter MHOs had a higher probability to have a source candidate assigned to them. I find an over abundance of outflows with position angles between 1300 and 1500 which is almost perpendicular to the Galactic Plane. The fraction of parsec scale outflows is about 25 % which is more than twice compared to the one found in Orion A by Stanke et al. (2002) and Davis et al. (2009). The outflows are not able to provide a sufficient fraction of energy and momentum to support the turbulence levels in their surrounding molecular clouds. The typical dynamical jet age was of the order of 104 yrs, while groups of emission knots are ejected every 103 yrs. This indicates that low level accretion rate fluctuations and not Fu-Ori type events are responsible for the episodic ej ection of material. The luminosity distribution of the outflows shows a power law behaviour with N ex: LH;·9. The Milky Way star formation rate was estimated to more than 1.6 ± 0.4M0 yr-1 The Spectral Index Classification distribution plot of YSOs indicated that the number of outflows increases in line with a values and has a similar distribution to the one from Davis et al. (2009) from Orion A.

Chemical abundances as measured from stellar spectral lines are often subject to uncertainties due to lack of accurate data for inelastic collisions, which is needed for non-local thermodynamic equilibrium (non-LTE) modelling. For cool stars, understanding of collision processes with electrons and hydrogen atoms is required to achieve high precision measurements. In this thesis, I have investigated the role of these collisions on the non-LTE formation of Li and Mg spectral lines in late-type stars. In the case of Li, electron impact excitation processes were calculated using the R-matrix with pseudo states method and the results found to agree well with recent calculations using the convergent close-coupling technique. These modern data were employed in non-LTE calculations by updating an existing model atom, which already included modern data for hydrogen collision processes. Our results were compared with calculations using older semi-empirical approximation calculations and only small differences were found: about 0.01 dex (~ 2%) or less in the abundance corrections. We therefore conclude that the influence of uncertainties in the electron collision data on non-LTE calculations is negligible. Indeed, together with the collision data for the charge transfer process Li + H ↔ Li+ + H- now available, and barring the existence of an unknown important collisional process, the collisional data in general is not a source of significant uncertainty in non-LTE Li line formation calculations. In the case of Mg, electron impact excitation processes were again calculated with the Rmatrix with pseudo states method, and used together with recent hydrogen collision calculations to build and test a model atom, without free parameters, for non-LTE modelling. Both electron and hydrogen collision processes, including charge transfer and excitation, are found to be important thermalising agents in various cases. The modelled spectra agree well with observed spectra from benchmark stars in the optical and infrared. The modelling predicts non-LTE abundance corrections ∆A(Mg)NLTE–LTE in dwarfs, both solar metallicity and metal-poor, to be very small (of order 0.01 dex), even smaller than found in previous studies. In giants, corrections vary greatly between lines, but can be as large as 0.4 dex. Results of calculations in a large grid of 1D model atmospheres are presented, and the implications for studies of Mg discussed. The propagation of uncertainties in the inelastic collision data to those in stellar abundances is investigated, and found to lead to small uncertainties, once again typically less than 0.01 dex (2%), although for few stellar models in specific lines (e.g., metal-poor suns, in the 7691 Å line) uncertainties can be as large as 0.03 dex (7%).

Stellar model atmospheres are a fundamental tool for our understanding of stars. Because the chemical composition of stars cannot be measured directly, the inferred stellar parameters are model dependent. In recent years great progress has been made in the modeling of stellar atmospheres, allowing the relaxation of simplifying assumptions made in previous models. The use of new 3D model atmospheres to infer the solar chemical composition has resulted in a decrease of the solar metallicity. This result has caused some controversy and is being challenged. The main aim of this thesis is to ascertain if the new models of stellar atmospheres are realistic and can be trusted to derive the chemical composition of stars in general, and the Sun in particular. Other objectives also include the study of line formation in the Sun at high spatial resolution, and possible implications in the modeling. The Sun is the ideal test-bench for detailed analyses of stellar atmospheres. With the solar surface being resolved in great detail and at different viewing angles, a wealth of information can be gathered that allows for very robust tests of atmosphere models. The testing detailed here addresses several fronts. On one hand, the model�s temperature structure is directly tested with the classical tests of continuum centre-to-limb variations and absolute fluxes. On the other hand, the line formation is tested at different viewing angles and high spatial and spectral resolution. Here the main focus is on oxygen lines, as oxygen has an important contribution for the total solar metallicity. However, other lines are also tested. High quality data were specifically obtained for these line formation tests, using the Swedish 1-m Solar Telescope. For the temperature structure tests we find a surprisingly good agreement between the 3D model and the observations, surpassing even semi-empirical models. The solar 1D non-LTE models have a very similar behaviour to 1D LTE models, confirming that LTE is a good approximation in the solar photosphere. The 3D theoretical model performs consistently better than its 1D counterparts. The oxygen line formation tests are carried out in great detail, with a careful wavelength calibration, revised atomic data, and allowing for departures from LTE. Again we find a reassuring agreement between the 3D model predictions and the observations, both for the centre-to-limb variation of the lines and the line formation at high spatial resolution. The observations at different viewing angles also allowed the empirical determination of the role of hydrogen collisions with oxygen, important when deriving the oxygen abundance. The tests undertaken here show that the 3D model atmospheres are indeed very realistic. Their predicted temperature structure and velocity fields compare very favourably with observations of the Sun. Together with previous tests, this indicates they can be relied upon to derive the chemical composition of the Sun and similar late-type stars.