26Al is an important radionuclide in astrophysics. Its decay to 26Mg results in the emission of a 1.8 MeV gamma-ray which is detected and mapped across the galaxy, providing evidence of ongoing nucleosynthesis in the universe. Its origin is still not understood, however observations suggest massive stars as a possible main production site. A post processing network calculation study modelled nucleosynthesis in the C/Ne convective-shell before the core collapse of a massive star and found that the 23Na(α,p)26Mg reaction is important for the synthesis of 26Al in this environment. Due to large uncertainties in previous experimental measurements of this reaction, theoretically calculated Hauser-Feshbach cross sections were used to calculate the 23Na(α,p)26Mg reaction rate for the post processing calculations. This theoretical rate has large uncertainties as the statistical model used to calculate the cross sections is not thought to be applicable for the level density of the compound nucleus 27Al. The 23Na(α,p)26Mg reaction is also found to play an important role in the nucleosynthesis of several nuclei in type Ia supernovae explosions by several sensitivity studies. Again these studies used the reaction rate from Hauser-Feshbach statistical model cross-section calculations. A measurement has been made of the 23Na(α,p)26Mg reaction cross section in inverse kinematics using the TUDA scattering chamber at TRIUMF laboratory in Canada. The cross sections were calculated in the energy range Ec.m. = 1.28 - 3.15 MeV and found to be in reasonable agreement with the Hauser Feshbach model calculations. A new reaction rate has been calculated providing tight constraints on the uncertainty in the production of 26Al in the C/Ne convective shell of massive stars due to the 23Na(α,p)26Mg reaction.

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Stellar rotation plays an important role in maintaining the magnetic fields inside the stellar interior through convection, and starspots are the most visible manifestation of the interplay between stellar rotation rate and magnetic fields. It is revealed through high end observations of evolution of magnetic fields and rotation rate of the Sun and other solar type stars that they exhibit a wide range of variation among their rotation rates yet there are some common ingredients such as rotational shear, turbulent transport and various nonlinear transport mechanisms which contribute towards the evolution and maintenance of the magnetic activity displayed by them. Also, these observations provide us with valuable information about the dependence of differential rotation and magnetic activity on rotation rate of stars with different ages and different rotation rates. Thus, the main challenge in dynamo theory is to explain these observations which is in fact a very strenuous problem and is challenging to do with full MHD simulations due to the various constraints such as expensive computations in terms of time and resolution. Therefore, it is useful to construct a simple parameterized model in order to understand the evolution of rotation rate and magnetic fields which can provide valuable insight into the various observations. This thesis discusses the modelling of solar dynamo and spindown of solar-type stars by using ODE and the effect of shear in kinematic dynamo in full MHD. We propose a simple parameterized model to understand the effect of nonlinear transport coefficients as well as mean/fluctuating differential rotation in the generation and destruction of magnetic fields and their capability in the working of dynamo near marginal stability. This model is then utilised to discuss detailed dynamics to understand the self-regulation of magnetic fields in solar/stellar dynamo. This work is further extended to understand the spindown of solar-type stars where the angular momentum loss is dynamically prescribed via equation of evolution of rotation rate and magnetic fields. The results obtained from this model are consistent with observations. Furthermore, regulatory behaviour of a kinematic dynamo by shear flow is investigated. Specifically, we study the induction equation by prescribing small scale velocity field to which a large scale radial/latitudinal shear is added in the direction of zonal flow. The results from numerical simulations are analysed and we conclude that the presence of large scale shear suppresses the small scale flows and results in quenching of a kinematic dynamo.

This thesis contains a study of molecules within circumstellar discs around young stars. Firstly, the chemistry of a disc around a young, Class 0 protostar is modelled. Such discs are thought to be massive, and thus experience gravitational instabilities, which produce spiral density waves. These affect the chemistry in three ways; by desorbing molecules from dust grains, by providing extra energy for new reactions to take place, and by mixing the internal structure of the disc to provide a rich chemistry near the midplane. Secondly, high resolution near-infrared spectra of 20 massive young stellar objects are presented. The objects display CO first overtone bandhead emission, which is excited in the conditions expected within circumstellar discs. The emission is modelled using a simple analytic model of a Keplerian disc, and good fits are found to all spectra. On average, the discs correspond to being geometrically thin, spread across a wide range of inclinations. The discs are located within the dust sublimation radius, providing strong evidence that the CO emission originates in small gaseous discs, supporting the scenario in which massive stars form via disc accretion. Finally, medium resolution near-infrared spectra of 5 Herbig Ae & Be stars are presented. The spectra cover both CO bandhead and Br γ emission. Accretion rates are derived from the measuring the Br γ emission and through modelling the CO emission, however these accretion rates are found to be inconsistent. High resolution archival data of one of the targets is presented, and it is shown that this CO disc model is unable to fit the high resolution data. Therefore, it is concluded that to properly fit CO spectra, high resolution data are needed, and that previously published information determined from low resolution spectra should be treated with caution.

The aim of this research project has been to investigate eclipsing binaries using archival data from the SuperWASP (Wide Angle Search for Planets) survey, and specifically to search for evidence of orbital period variations. The short cadence and long time base of SuperWASP light curves make them highly suitable for variable star research of many kinds, and the size of the database (~30 million objects) allows for the discovery and study of rare objects and stellar types. Techniques optimized for analyzing SuperWASP data were developed over the course of the research, using custom IDL codes, and proved successful in measuring orbital periods and detecting period changes in real and synthetic test data. Employing these, original results were achieved in a range of areas. A sample of 143 eclipsing binary candidates were found with periods close to the short-period limit of the period distribution for main sequence binaries, which shed light on the likely cause of the limit. Significant period increases and decreases were detected in many of these, and several individual objects were followed up with spectroscopic and multicolour photometric observations, by ourselves and others, to allow confirmation of their binary nature and modelling to determine system and component parameters. Amongst these were a probable close triple system containing a low-mass contact binary, and a doubly-eclipsing quintuple system, only the sixth known of its kind. Eleven post-common-envelope eclipsing binaries were also studied for evidence of period changes, potentially associated with circumbinary planets; our findings add to the ongoing debate on the reality and stability of planetary systems in such environments. Finally, we used our findings on period changes in ~14000 candidate SuperWASP eclipsing binaries of all types to estimate the higher-order multiplicity fraction of such systems.

We test state-of-the-art model atmospheres for young low-mass stars and brown dwarfs in the infrared, by comparing the predicted synthetic photometry over 1.2-24um to the observed photometry of M-type spectral templates in star-forming regions. In both early and late young M types, the model atmospheres imply effective temperatures (Teff) several hundred Kelvin lower than predicted by the standard Pre-Main Sequence spectral type-Teff conversion scale (where the latter is based on theoretical evolutionary models). We postulate that in the early M types this discrepancy arises from remaining uncertainties in the treatment of atmospheric convection, whereas in the late M types it is likely due to an underestimation of dust opacity in the atmospheric models and an attendant overestimation of H2O opacity, due to insufficient backwarming by dust. Using the synthetic spectra to estimate stellar properties leads to reasonably accurate bolometric luminosities (Lbol), but overestimates radii (due to underestimated Teff) for the early and late young M types compared to evolutionary theory; this then leads to underestimations of age and mass, which we demonstrate for a large sample of young Cha I and Taurus sources. By selecting the model atmospheres which best represent the true stellar continuum at each spectral type, we then go on to infer the disk parameters for a selection of young, low-mass disk-bearing objects in Cha I by modelling the IR SEDs (again over 1.2-24um) using the radiative transfer code ttsre, coupled to the machine learning code skynet and a Bayesian parameter estimation. Just over half of the sample are able to be accurately fit by the analysis. No noticeable level of correlation is found between the extent of settling and grain growth in the disk and the stellar mass, an interesting result given the lessened gravitational/luminosity effects that lower mass objects should have on their disks. No correlation was also found between the disk thickness and accretion rate (a tracer of the system age), which, when coupled to the fact that a wide range of disk flaring indices are found, suggests that grain growth/settling occurs over a rapid timescale in the disk's early evolutionary sequence.

One of the most fundamental unknowns about pulsars is the mechanism bywhich they radiate. Although many steps have been taken towards understandingthis in the 48 years since their discovery, there is still much to learn. The recentlaunch of the Fermi γ-ray satellite has led to an astonishing increase in thenumber of pulsars detected in γ-rays, making this an ideal time to consider theemission at both low energies (radio) and high energies (γ-rays).In this thesis we use radio polarisation data and a novel method to determine the viewing geometries of two samples of pulsars, one consisting of youngpulsars which have been detected in γ-rays and the other consisting of comparable young pulsars which are not detected in γ-rays. We find that the magneticinclination angle distribution of the γ-ray-detected pulsars does not match thecommonly-expected random distribution for such young pulsars as these, insteadbeing skewed towards lower values. We find two possible explanations for this. The first is that the expectation of a random distribution is erroneous, a situationwhich will have important consequences for pulsar population studies, considerations of the origins of magnetic fields in neutron stars and models of the supernovaprocess. The alternative is that the radio beams are larger than expected by afactor which depends on the magnetic inclination, which would have implicationsfor the magnetic field structure. We also find that the γ-ray- and non-γ-ray-detected pulsars differ in radioprofile width. This leads us to present a unified model of these young pulsarswhere the γ-ray detectability is highly dependent on magnetic inclination, whichdetermines if our line of sight samples the γ-ray emission. We report for the firsttime ever a significant correlation between the radio and γ-ray profile morphology. This is independent confirmation that the existing models of the radio andγ-ray emission regions are, at least qualitatively, realistic, although significantquantitative problems are reported.

11 of the Gogny forces available in literature have been studied in order to evaluate their isovector properties and to comment on their viability for beyond 1S0 pairing channels, and neutron star cooling calculations. I find that, even within a relatively narrow set of Gogny functionals, there is a large variation in isospin properties. In particular, I find that the density dependence of the symmetry energy provided by Gogny forces is too soft and lies outside of currently accepted values. This points to poor constraints in the isovector sector, that should be improved in future fitting protocols. In addition to this, the pairing properties of the Gogny forces have been examined for the 1S0, 3S1, 3P2 and 1P1 channels. Although most forces are in keeping with literature for the 1S0 gaps, the remaining channels are not particularly constrained and many of the Gogny forces produce unphysical results. The pairing gaps generated for the D1P parametrisation have been used to calculate neutron star cooling curves as a proof-of-concept for the Gogny force. Successful cooling curves incorporating superfluid effects have been produced using the NSCool software package. Although most Gogny forces perform poorly in both the isovector and pairing sectors, it is shown that consistent neutron star models cooling models can be generated with the Gogny force. Suggestions are made as to the fitting considerations for a future force.

Infrared dark clouds (IRDCs) are seen in absorption against the mid-infrared back-ground and are thought to represent likely sites of future massive star formation. We investigate these IRDCs to probe the conditions present when star formation begins. The Spitzer dark cloud (SDC) catalogue contains ∼11,000 IRDCs. We extend this catalogue to include the inner 20◦ of the Galactic plane, adding 4334 SDCs to the catalogue. Some of the objects in the catalogue are artefacts - ‘dips’ in the mid-infrared emission rather than regions where the emission is absorbed. With the advent of data from the Herschel satellite, we are able to construct column density maps of the objects in the SDC catalogue to identify which SDCs are true IRDCs and which are artefacts. We compare the properties of the IRDCs in the Galactic centre and in star forming regions with those in more quiescent regions. We find that the IRDCs towards star forming regions and the Galactic centre tend to have a higher column density and a slightly higher temperature, implying that the conditions within IRDCs are dependent on the environment in which they are found. Star formation has long been associated with filaments. Filaments containing longstrings of IRDCs have been observed in the Galactic plane. We apply a minimum spanning tree (MST) algorithm to the SDC catalogue to identify 88 filamentary candidates, 22 of which appear to be isolated, linear filaments similar to the Nessie nebula. Filaments tend to fragment into clumps regularly spaced along the length of the filaments. We compare theoretical predictions of this fragmentation with the clumps observed in the 22 linear filaments identified by the MST algorithm and find our results are consistent with those predicted by the sausage instability for filaments dominated by thermal pressure.

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