Formation of hot subdwarf B stars with neutron star components

Binary population synthesis predicts the existence of subdwarf B stars (sdBs) with neutron star (NS) or black hole (BH) companions. We systematically investigate the formation of sdB+NS binaries from binary evolution and aim to obtain some clues for a search for such systems. We started from a series of MS+NS systems and determined the parameter spaces for producing sdB+NS binaries from the stable Roche-lobe overflow (RLOF) channel and from the common envelope (CE) ejection channel. Various NS accretion efficiencies and NS masses were examined to investigate the effects they have. We show the characteristics of the produced sdB+NS systems, such as the mass of components, orbital period, the semi-amplitude of the radial velocity (K), and the spin of the NS component. In the stable RLOF channel, the orbital period of sdB+NS binaries produced in this way ranges from several days to more than 1000 days and moves toward the short-period (~ hr) side with increasing initial MS mass. the sdB+NS systems that result from CE ejection have very short orbital periods and then high values of K (up to 800km s^-1). Such systems are born in very young populations (younger than 0.3 Gyr) and are potential gravitational wave sources that might be resolved by the Laser Interferometer Space Antenna (LISA) in the future. Gravitational wave radiation may again bring them into contact on a timescale of only ~Myr. As a consequence, they are rare and hard to discover. The pulsar signal is likely a feature of sdB+NS systems caused by stable RLOF, and some NS components in sdB binaries may be millisecond pulsars.Comment: 12 pages, 6 figures, 4 tables. Accepted for publication in A&

Discrete mechanics Based on Finite Element Methods

Discrete Mechanics based on finite element methods is presented in this paper. We also explore the relationship between this discrete mechanics and Veselov discrete mechanics. High order discretizations are constructed in terms of high order interpolations.Comment: 14 pages, 0 figure

Observation of vacancy-induced suppression of electronic cooling in defected graphene

Previous studies of electron-phonon interaction in impure graphene have found that static disorder can give rise to an enhancement of electronic cooling. We investigate the effect of dynamic disorder and observe over an order of magnitude suppression of electronic cooling compared with clean graphene. The effect is stronger in graphene with more vacancies, confirming its vacancy-induced nature. The dependence of the coupling constant on the phonon temperature implies its link to the dynamics of disorder. Our study highlights the effect of disorder on electron-phonon interaction in graphene. In addition, the suppression of electronic cooling holds great promise for improving the performance of graphene-based bolometer and photo-detector devices.Comment: 13 pages, 4 figure

Doctor of Philosophy

dissertationSpontaneous preterm birth (SPTB) is defined as birth before 37 completed weeks gestation that is not secondary to iatrogenic intervention. SPTB is both a pressing personal and public health issue that is not well understood. Previous studies have shown genetics as an important factor to SPTB. Here I present my PhD work on genetic analyses of SPTB. I dissected the problem with: (1) candidate gene, (2) quantitative genetics, and (3) genome-wide approaches. In Chapter 1, I explore why published candidate gene studies are inconsistent with each other. Allele frequency difference across populations provides one reason. I conducted a meta-analysis on the single nucleotide polymorphism (SNP) rs1800795, located in the promoter region of interleukin-6. With population stratification, I unmasked the signal showing that the CC genotype is protective against PTB in women of European descent. This also highlights how positive genetic signals can become obscured when the population structure is not controlled for. In Chapter 2, I use quantitative genetic approaches to decompose the etiologies of SPTB with massive data from the Utah Population Database. A generation effect, which confounds the heritability estimate, was discovered. I then utilized a sibling analysis and partitioned components contributing to the phenotypic variance of SPTB: iv heritability (13.33%), dominance genetic (11.12%), maternal effect (15.23%), and individual environment (60.33%). These findings shed light on the architecture of SPTB pathogenesis and quantify the maternal and fetal contribution to SPTB. In Chapter 3, I perform an unbiased genome-wide association study for SPTB on 22 autosomal chromosomes. The data, which contain cases and controls collected by the Danish National Birth Cohort, were acquired from the National Center for Biotechnology Information Genotypes and Phenotypes Database. No SNP reached genome-wide significance. Although negative, the result provides proof of principle regarding the relatively low heritability of SPTB. Finally, I address insights learned from these analyses that may help guide future SPTB research and may be applicable to other human genetic studies. With these lessons, progress can be made in understanding the genetics of SPTB

Translational aspects of cardiac cell therapy.

Cell therapy has been intensely studied for over a decade as a potential treatment for ischaemic heart disease. While initial trials using skeletal myoblasts, bone marrow cells and peripheral blood stem cells showed promise in improving cardiac function, benefits were found to be short-lived likely related to limited survival and engraftment of the delivered cells. The discovery of putative cardiac 'progenitor' cells as well as the creation of induced pluripotent stem cells has led to the delivery of cells potentially capable of electromechanical integration into existing tissue. An alternative strategy involving either direct reprogramming of endogenous cardiac fibroblasts or stimulation of resident cardiomyocytes to regenerate new myocytes can potentially overcome the limitations of exogenous cell delivery. Complimentary approaches utilizing combination cell therapy and bioengineering techniques may be necessary to provide the proper milieu for clinically significant regeneration. Clinical trials employing bone marrow cells, mesenchymal stem cells and cardiac progenitor cells have demonstrated safety of catheter based cell delivery, with suggestion of limited improvement in ventricular function and reduction in infarct size. Ongoing trials are investigating potential benefits to outcome such as morbidity and mortality. These and future trials will clarify the optimal cell types and delivery conditions for therapeutic effect