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The discovery of superluminous supernovae in the past decade challenged our understanding of explosive stellar death. Subsequent extensive observations of superluminous supernova light curves and spectra has provided some insight for the nature of these events. We present observations of one of the most luminous self-interacting supernovae ever observed, the hydrogen-rich SN 2008am discovered by the Robotic Optical Transient Search Experiment Supernova Verification Project with the ROTSE-IIIb telescope located in the McDonald Observatory. We provide theoretical modeling of superluminous supernova light curves and fit the models to a number of observed events and similar transients in order to understand the mechanism that is responsible for the vast amounts of energy emitted by these explosions. The models we investigate include deposition of energy due to the radioactive decays of massive amounts of nickel-56, interaction of supernova ejecta with a dense circumstellar medium and magnetar spin-down. To probe the nature of superluminous supernovae progenitor stars we study the evolution of massive stars, including important effects such as rotation and magnetic fields, and perform multi-dimensional hydrodynamics simulations of the resulting explosions. The effects of rotational mixing are also studied in solar-type secondary stars in cataclysmic variable binary star systems in order to provide an explanation for some carbon-depleted examples of this class. We find that most superluminous supernovae can be explained by violent interaction of the SN ejecta with >1 Msun dense circumstellar shells ejected by the progenitor stars in the decades preceding the SN explosion.

Gamma-ray bursts (GRBs) are the mysterious, short and intense flashes of gamma-rays in the space, and are believed to originate from the rare, explosively devastating, stellar events that happens at cosmological distances. Enormous progress has been made from four decades of GRB research endeavor but the ultimate understanding of their origins has yet to arrive. Recently revealed features in their early afterglows broadened the opportunity space for exploration. We have carried out extensive studies on various physical processes in GRBs. We showed that the distribution of electrons' energy spectral index in GRBs and other relativistic sources is inconsistent with the prediction from the first-order Fermi theory of the shock particle acceleration. We investigated the photon scattering processes within the relativistic outflow that produces the GRB and calculated the resultant emission flux from it. We showed the scattering of the GRB prompt photons by the circum-burst dust, although an attractive possibility, can not explain the puzzling plateau component in the GRB afterglow light curve. We made meaningful constraint on the GRB prompt emission radius, R [greater-than or equal to] 10¹⁴, by studying the synchrotron self absorption for a small sample of bursts with good data. We showed that a late jet, which is thought to be producing the late X-ray flares in GRB afterglows, will produce detectable emissions from its interactions with other components in the explosive event of GRB, and identification of these emissions could verify the existence of the late jet and further prove the massive star origin of long-duration GRBs.

Photometry of the metal-poor globular cluster M30 is presented in B, V, R and I bandpasses. A color-magnitude diagram created from this photometry indicates that accurate magnitude measurements were obtained for stars from the red giant branch down to approximately 2.5 magnitudes fainter than the main sequence turn-off. Time-series photometry is presented for six RR Lyrae type variable stars, three of which are newly discovered. Four variable stars of other types, three of them newly discovered, are also discussed. A metallicity of [Fe/H]=-2.02 was adopted for this study. Using the RR Lyrae stars' mean colors at minimum light, a reddening of E(B-V)=0.053+-0.010 was found for this cluster as well as an extinction value of Av=0.165+-0.031. A distance modulus of mu=14.504+-0.127 and the corresponding distance of 7.958+-0.147 kpc was also computed using the RR Lyrae stars' mean magnitudes. Isochrone fitting of the color-magnitude diagram yielded a cluster age of 15.8 +-1.8 Gyr.

The aim of the research reported in this thesis was to construct leaf and canopy photosynthesis models for understorey cocksfoot pasture grown in a 10-11-year old Pinus radiata silvopastoral system. From these models, dry matter (DM) production was predicted based on a numerical description of the biological mechanisms involved in canopy photosynthesis. To do this, a wide range of environmental and management conditions were created through changes in light intensity and regime, temperature, soil moisture, nitrogen (N) and regrowth duration. A unique component of a silvopastoral system is the fluctuating light regime experienced by the understorey plants. The daily photosynthetic photon flux density (PPFD) integral was 55-62% of the open, with periods of full sunlight (1700-1900 µmol m⁻² s⁻¹ PPFD at midday) and severe shade (129-130 µmol m⁻² s⁻¹ PPFD) that changed within 45-120 minutes depending on the solar angle elevation. A similar pattern obtained from artificial slatted structures, also provided a bimodal light regime but with lower light intensity. The resulting DM growth rates ranged from 2 to 154 kg DM/ha/d. These differences were related to differences in canopy leaf area index (LAI) from 0.5 to 8.2 units caused by a reduced tiller population, canopy etiolation and canopy leaf angle. Net photosynthesis rate from seven light intensities (0, 100, 250, 500, 750, 1000 and 2000 µmol m⁻² s⁻¹ PPFD) were measured in the field using an open infrared gas analysis system. These were used to construct light curves for the youngest fully expanded intact leaves. The prediction of DM production was based on an integrated leaf photosynthesis model that uses a non-rectangular hyperbola function to estimate the saturated leaf photosynthetic rate (Pmax), the photosynthetic efficiency (α) and the degree of curvature (θ) in the photosynthetic response of individual leaves. The highest Pmax was 27.4 µmol CO2 m⁻² s⁻¹ in non-limited conditions. This decreased to a minimum of -0.5 µmol CO₂m⁻² s⁻¹ under severe water stress (Ψlp= -16 bar). Values of α ranged from 0.036 µmol CO₂/µmol PPFD in non-limiting conditions to 0.020 µmol CO₂/µmol PPFD at 1.5% N. The degree of curvature of the leaf response curve θ was unaffected by the range of environmental factors and regrowth duration and had a mean value of 0.96 ±0.02. The response of these parameters to different temperature, N, moisture, regrowth and shade conditions were predicted using a multiplicative model for predicting Pmax, a 'law of the minimum factor' model for α and a constant for θ. These parameters were then incorporated into a canopy photosynthesis model with coefficients for respiration, partitioning and the main canopy characteristics that affect light interception (LAI and leaf angle). Based on this model, cocksfoot DM production was predicted for silvopastoral systems in non-limiting situations and where a single, two, three, four or all five factors were limiting for: air temperatures from 2 to 37 °C, water status from Ψlp -0.1 to -16.0 bar (corresponding to a soil volumetric water content to 500 mm depth of 8.5 to 34%), foliage N content from 1.5 to 5.9%, regrowth duration from 20 to 60 days, and time course of shade (severe shade: 5% of open PPFD or moderate shade: 50% of open PPFD) from 1 to 180 minutes and the correspondent induction process (lag in the rise of photosynthesis rate to the maximum value) from 30, 60 and 180 minutes of severe shade. Using this model, it was shown that the continuous light regime of 50% transmissivity throughout a day had higher canopy photosynthesis than for the same intensity but a fluctuating light regime with periods of 90-120 minutes of full sunlight and severe shade (10.4 vs 8.4 g CO₂ m⁻² d⁻¹). This was due to (i) a faster decrease in Pmax and α for plants experiencing 5% of open PPFD compared with 50% of open PPFD; (ii) the lack of an induction process under continuous shade. These results indicate that artificial shade cloth gives a biased representation of the response of understorey pastures in silvopastoral systems. Validations from observed DM production data (from 9 to 134 kg DM/ha/d) were obtained from different environmental and management conditions and indicated that approximately 86% of the variation in cocksfoot growth rate was accounted for by using the model proposed. Therefore, the canopy photosynthesis model proposed in this study provides a powerful and useful tool for understanding and predicting DM production of cocksfoot understorey pastures in silvopastoral systems.