Recent photometric studies have revealed that surface spots that produce flux variations are present on virtually all L and T dwarfs. Their likely magnetic or dusty nature has been a much-debated problem, the resolution to which has been hindered by paucity of diagnostic multi-wavelength observations. To test for a correlation between magnetic activity and photometric variability, we searched for Ha emission among eight L3-T2 ultra-cool dwarfs with extensive previous photometric monitoring, some of which are known to be variable at 3.6 mu m or 4.5 mu m. We detected Ha only in the non-variable T2 dwarf 2MASS J12545393-0122474. The remaining seven objects do not show Ha emission, even though six of them are known to vary photometrically. Combining our results with those for 86 other L and T dwarfs from the literature show that the detection rate of Ha emission is very high (94%) for spectral types between L0 and L3.5 and much smaller (20%) for spectral types. >= L4, while the detection rate of photometric variability is approximately constant (30%-55%) from L0 to T8 dwarfs. We conclude that chromospheric activity, as evidenced by H alpha emission, and large-amplitude photometric variability are not correlated. Consequently, dust clouds are the dominant driver of the observed variability of ultra-cool dwarfs at spectral types, at least as early as L0.

In preparation for the upcoming all-sky data releases of the Gaia mission we compiled a catalog of known hot subdwarf stars and candidates drawn from the literature and yet unpublished databases. The catalog contains 5613 unique sources and provides multi-band photometry from the ultraviolet to the far infrared, ground based proper motions, classifications based on spectroscopy and colors, published atmospheric parameters, radial velocities and light curve variability information. Using several different techniques we removed outliers and misclassified objects. By matching this catalog with astrometric and photometric data from the Gaia mission, we will develop selection criteria to construct a homogeneous, magnitude-limited all-sky catalog of hot subdwarf stars based on Gaia data.

Turbulent mixing of chemical elements by convection has fundamental effects on the evolution of stars. The standard algorithm at present, mixing-length theory (MLT), is intrinsically local, and must be supplemented by extensions with adjustable parameters. As a step toward reducing this arbitrariness, we compare asteroseismically inferred internal structures of two Kepler slowly pulsating B stars (SPBs; M similar to 3.25M circle dot.) to predictions of 321D turbulence theory, based upon well-resolved, truly turbulent three-dimensional simulations that include boundary physics absent from MLT. We find promising agreement between the steepness and shapes of the theoretically predicted composition profile outside the convective region in 3D simulations and in asteroseismically constrained composition profiles in the best 1D models of the two SPBs. The structure and motion of the boundary layer, and the generation of waves, are discussed.

We report the first detailed chemical abundance analysis of the exoplanet-hosting M-dwarf stars Kepler-138 and Kepler-186 from the analysis of high-resolution (R similar to 22,500) H-band spectra from the SDSS-IV-APOGEE survey. Chemical abundances of 13 elements-C, O, Na, Mg, Al, Si, K, Ca, Ti, V, Cr, Mn, and Fe-are extracted from the APOGEE spectra of these early M-dwarfs via spectrum syntheses computed with an improved line list that takes into account H2O and FeH lines. This paper demonstrates that APOGEE spectra can be analyzed to determine detailed chemical compositions of M-dwarfs. Both exoplanet-hosting M-dwarfs display modest sub-solar metallicities: [Fe/H](Kepler-138) = -0.09 +/- 0.09 dex and [Fe/H](Kepler-186) = -0.08 +/- 0.10 dex. The measured metallicities resulting from this high-resolution analysis are found to be higher by similar to 0.1-0.2 dex than previous estimates from lower-resolution spectra. The C/O ratios obtained for the two planet-hosting stars are near-solar, with values of 0.55 +/- 0.10 for Kepler-138 and 0.52 +/- 0.12 for Kepler-186. Kepler-186 exhibits a marginally enhanced [Si/Fe] ratio.

We compile observations of early-type binaries identified via spectroscopy, eclipses, long-baseline interferometry, adaptive optics, common proper motion, etc. Each observational technique is sensitive to companions across a narrow parameter space of orbital periods P and mass ratios q. =. M-comp/M-1. After combining the samples from the various surveys and correcting for their respective selection effects, we find that the properties of companions to O-type and B-type main-sequence (MS) stars differ among three regimes. First, at short orbital periods P less than or similar to 20. days (separations a less than or similar to 0.4 au), the binaries have small eccentricities e... 0.4, favor modest mass ratios < q > less than or similar to 0.5, and exhibit a small excess of twins q. >. 0.95. Second, the companion frequency peaks at intermediate periods log P (days). approximate to. 3.5 (a approximate to 10 au), where the binaries have mass ratios weighted toward small values q. approximate to 0.2-0.3 and follow a Maxwellian " thermal" eccentricity distribution. Finally, companions with long orbital periods log P (days). approximate to 5.5-7.5 (a approximate to 200-5000 au) are outer tertiary components in hierarchical triples and have a mass ratio distribution across q. approximate to 0.1-1.0 that is nearly consistent with random pairings drawn from the initial mass function. We discuss these companion distributions and properties in the context of binary-star formation and evolution. We also reanalyze the binary statistics of solar-type MS primaries, taking into account that 30% +/-. 10% of single-lined spectroscopic binaries likely contain white dwarf companions instead of low-mass stellar secondaries. The mean frequency of stellar companions with q. >. 0.1 and log P (days). <. 8.0 per primary increases from 0.50. +/- 0.04 for solar-type MS primaries to 2.1. +/- 0.3 for O-type MS primaries. We fit joint probability density functions f (M-1, q, P, e) not equal f (M-1) f (q) f (P) f (e) to the corrected distributions, which can be incorporated into binary population synthesis studies.

We present a detailed search and analysis of chemically stratified hybrid (traces of helium and hydrogen) white dwarfs in the Sloan Digital Sky Survey (SDSS). Only one stratified white dwarf, PG 1305-017, was known prior to this analysis. The main objective is to confirm the existence of several new stratified objects. We first describe our new generation of stratified model atmospheres, where a thin hydrogen layer floats in diffusive equilibrium on top of a more massive helium layer. We then present the results of our search for hot (T-eff > 30,000 K) white dwarfs with a hybrid spectral type among the similar to 38,000 white dwarf spectra listed in the SDSS. A total of 51 spectra were retained in our final sample, which we analyze using spectroscopic fits to both chemically homogeneous and stratified model atmospheres. We identify 14 new stratified white dwarfs in the SDSS sample. From these results, we draw several conclusions regarding the physical processes that might explain the presence of helium in the atmospheres of all the stars in our sample.

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Tsui Lung Kwan = 致密星的準簡正模 : 以微擾方程為基礎之研究 / 徐隆焜. / Thesis submitted in: August 2003. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2004. / Includes bibliographical references (leaves 98-100). / Text in English; abstracts in English and Chinese. / Tsui Lung Kwan = Zhi mi xing de zhun jian zheng mo : yi wei rao fang cheng wei ji chu zhi yan jiu / Xu Longkun. / Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Oscillating Stars and Gravitational Waves --- p.1 / Chapter 1.2 --- Outline of the Thesis --- p.3 / Chapter 2 --- Stellar Structure and Mode Classifications --- p.5 / Chapter 2.1 --- Relativistic Stars --- p.5 / Chapter 2.2 --- Equation of State --- p.6 / Chapter 2.3 --- Mode Classifications --- p.9 / Chapter 3 --- Axial w-mode Oscillations --- p.16 / Chapter 3.1 --- Equations of Stellar Oscillations --- p.16 / Chapter 3.2 --- Evaluation of QNMs --- p.18 / Chapter 3.3 --- General features of QNMs --- p.20 / Chapter 4 --- Universal behavior of axial QNMs --- p.22 / Chapter 4.1 --- BBF universal curve --- p.22 / Chapter 4.2 --- Scaled Coordinates --- p.26 / Chapter 4.3 --- Universality of Vrw(r*) inside stars --- p.29 / Chapter 4.3.1 --- Searching for the universality inside stars --- p.29 / Chapter 4.3.2 --- Simple analysis --- p.35 / Chapter 4.3.3 --- The importance of R* --- p.38 / Chapter 5 --- Scaled Coordinates Logarithmic Perturbation Theory --- p.40 / Chapter 5.1 --- The scaled axial oscillation equations --- p.41 / Chapter 5.2 --- The Formalism of SCLPT --- p.41 / Chapter 5.2.1 --- Perturbation parameter --- p.42 / Chapter 5.2.2 --- Perturbation-dependent boundary condition --- p.43 / Chapter 5.2.3 --- The first order perturbation --- p.44 / Chapter 5.2.4 --- The second order perturbation --- p.48 / Chapter 6 --- Cubic-Quintic Model of Neutron Stars --- p.53 / Chapter 6.1 --- Cubic-Quintic Model (CQM) --- p.53 / Chapter 6.2 --- The fluid functions --- p.54 / Chapter 6.3 --- The metric coefficients --- p.56 / Chapter 6.4 --- The tortoise radius R* --- p.58 / Chapter 6.5 --- The QNM frequencies --- p.60 / Chapter 7 --- The Chain of Approximations --- p.63 / Chapter 8 --- Inversion of QNMs --- p.72 / Chapter 8.1 --- The modified SCLPT --- p.72 / Chapter 8.2 --- The Inverting Scheme --- p.76 / Chapter 8.3 --- Application of the Inverting Scheme --- p.79 / Chapter 8.4 --- Improved Inverting Scheme --- p.80 / Chapter 9 --- Conclusion --- p.87 / Chapter 9.1 --- Summary of Our Work --- p.87 / Chapter 9.2 --- Outlook --- p.88 / Chapter A --- Linear and Quadratic Models --- p.90 / Chapter A.1 --- Linear Model (LM): Mass oc r --- p.90 / Chapter A.2 --- Quadratic Model (QM): Mass oc r2 --- p.93 / Chapter B --- The relationship between R* and the mass distribution --- p.96 / Bibliography --- p.98

Wu Jun = 中子星的極性w-模振盪 / 吳俊. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2005. / Includes bibliographical references (leaves 100-102). / Text in English; abstracts in English and Chinese. / Wu Jun = Zhong zi xing de ji xing w-mo zhen dang / Wu Jun. / Abstract --- p.i / Acknowledgement --- p.iii / Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Background introduction and historical review --- p.1 / Chapter 1.2 --- Outline of the thesis --- p.3 / Chapter 1.3 --- Notations and conventions --- p.4 / Chapter 2 --- Equilibrium and oscillations of Relativistic stars --- p.5 / Chapter 2.1 --- Relativistic stars --- p.5 / Chapter 2.1.1 --- Equilibrium configuration --- p.6 / Chapter 2.1.2 --- Equation of state --- p.7 / Chapter 2.2 --- Oscillations of relativistic stars --- p.9 / Chapter 2.2.1 --- Families of fluid modes --- p.10 / Chapter 2.2.2 --- Families of spacetime modes (w-mode) --- p.11 / Chapter 3 --- Polar oscillations of neutron stars --- p.14 / Chapter 3.1 --- Axial oscillations of neutron stars --- p.14 / Chapter 3.2 --- LD formulation --- p.16 / Chapter 3.2.1 --- Equations inside star --- p.17 / Chapter 3.2.2 --- Boundary conditions at r = 0 and r = R --- p.19 / Chapter 3.2.3 --- Perturbations outside star --- p.21 / Chapter 3.3 --- AAKS formulation --- p.22 / Chapter 3.3.1 --- Equations inside the star --- p.23 / Chapter 3.3.2 --- Behavior at the center and the stellar surface --- p.25 / Chapter 3.3.3 --- Evolution outside star --- p.28 / Chapter 3.3.4 --- Connection formula --- p.29 / Chapter 4 --- QNMs of polar oscillations --- p.31 / Chapter 4.1 --- Solution outside star --- p.31 / Chapter 4.2 --- LD approach --- p.32 / Chapter 4.3 --- Hamiltonian constraint --- p.33 / Chapter 4.4 --- Boundary conditions a.t r = R --- p.37 / Chapter 4.5 --- Direct integration scheme (DIS) --- p.42 / Chapter 4.6 --- Two-way integration scheme (TIS) --- p.43 / Chapter 4.7 --- Connect the interior and exterior solutions --- p.45 / Chapter 4.8 --- Numerical results --- p.46 / Chapter 5 --- Polar oscillations without fluid motions --- p.50 / Chapter 5.1 --- Zero pressure variation approximation (ZPVA) --- p.51 / Chapter 5.1.1 --- Evolution formulas --- p.51 / Chapter 5.1.2 --- Boundary conditions --- p.53 / Chapter 5.1.3 --- Approximate QNMs --- p.55 / Chapter 5.2 --- Zero density variation approximation (ZDVA) --- p.55 / Chapter 5.2.1 --- Single equation formulas --- p.58 / Chapter 5.2.2 --- Numerical results --- p.61 / Chapter 5.2.3 --- Summary --- p.62 / Chapter 5.3 --- Application of ZDVA --- p.65 / Chapter 5.3.1 --- Relation between axial and polar w-modes --- p.65 / Chapter 5.3.2 --- Analysis --- p.66 / Chapter 6 --- Universal behavior of polar QNMs --- p.69 / Chapter 6.1 --- Universal behavior of polar w-modes --- p.70 / Chapter 6.2 --- Ordinary CQM of neutron stars --- p.71 / Chapter 6.2.1 --- TOV parameters of a CQM star --- p.71 / Chapter 6.2.2 --- Stability problem of CQM --- p.73 / Chapter 6.2.3 --- EOS near the surface --- p.75 / Chapter 6.3 --- Scaling behavior of polar oscillations --- p.78 / Chapter 6.3.1 --- Scaling behavior of fluid motions --- p.79 / Chapter 6.3.2 --- Scaled wave equations --- p.80 / Chapter 6.4 --- Scaled Cubic-Quintic Model (SCQM) --- p.82 / Chapter 7 --- Conclusion --- p.85 / Chapter 7.1 --- Summary of Our Work --- p.85 / Chapter 7.2 --- Outlook --- p.86 / Chapter A --- Expansion of Hamiltonian constraint around the center --- p.88 / Chapter B --- Factorization integration scheme (FIS) --- p.92 / Chapter C --- Equivalence of two definitions of the Zerilli function --- p.96