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This thesis describes the search for brown dwarfs in open clusters using optical and infrared photometry, optical spectroscopy and surveys using the Hubble Space Telescope (HST). It also includes an observational study of very low-mass stars (VLMS) using a filter combination (I and Z), unused previously in this field enabling the calibration of the colour (I--Z), for future survey work. Following a short introduction explaining the reasons for studying brown dwarfs this thesis reviews the theory of their formation and evolution and describes recent searches for brown dwarfs and their results. Modern CCD technology has led to larger and larger area cluster surveys and so chapter 3 reviews this technology and the data reduction software and techniques necessary for analysis. Chapters 4, 5, 6 and 7 present the observational results. Chapter 4 presents an essentially null result of a deep HST survey in the Pleiades. Chapter 5 presents the d ata used to calibrate the I--Z colour and chapter 6 describes the discovery of PIZ 1, a brown dwarf in the Pleiades of mass 0.048 M⊙ and effective temperature ~2200K. Chapter 7 presents the infrared follow-up results of a number of recent large area optical surveys. Chapters 8 and 9 use the results from the latest surveys to determine the mass and luminosity functions of both Praesepe and the Pleiades. In Praesepe the mass function is calculated down to the brown dwarf limit in the cluster and shows no sign of turning down. In the Pleiades the mass function appears to be flattening across the stellar sub-stellar boundary. Finally in Chapter 10 I summarise the important conclusions from each chapter and identify areas of future work.

This thesis presents a multiwavelength study of two X-ray selected samples of star-forming galaxies. The first sample is defined by the cross-correlation of ROSAT source catalogues with the IRAS Faint Source Catalogue, the Markarian and the Kiso catalogues. The main objective of this project is to investigate whether X-ray luminous star-forming galaxies exist, and if so to quantify their properties. A number of star-forming galaxies with luminosities > 1041erg s--1 are found. Their X-ray luminosity can be explained in terms of a superwind and an X-ray binary component. Moreover, their multi wavelength properties are very similar to the properties of their lower luminosity counterparts. The second sample studied is drawn from the optical spectroscopic atlas of Ho, Filippenko & Sargent (1997a). It consists of 44 bona-fide star-forming galaxies which are observed either as targets or serendipitously with the ROSAT PSPC. The correlation between their X-ray emission and their emission in the optical, far-infrared and radio bands are studied. These correlations are used in order to probe the origin of their X-ray emission. Also a correlation between the star-formation rate and X-ray luminosity for star-forming galaxies is calculated. This is convolved with the evolution of star-formation rate with redshift in order to estimate the contribution of star-forming galaxies to the XRB. It is found that they can produce between 10% -- 50% of the soft extragalactic (0.1-2.5keV) X-ray background. Finally, two examples of X-ray luminous starburst galaxies, namely Arp299 and NGC3310 are studied in detail using data from the ROSAT and ASCA. Their X-ray properties are similar to the X-ray properties of lower luminosity star-forming galaxies, suggesting that the same mechanisms are responsible for the X-ray emission of star-forming galaxies spanning three orders of magnitude in luminosity. Thirdly Holmberg-II which is an X-ray luminous dwarf star-forming galaxy is also studied. It is found that all of its X-ray emission arises from a single variable source. The fact that its X-ray luminosity is ~ 1040erg s--1 suggests that it could be a black-hole X-ray binary with a mass of ~ 200 M⊙ . This result together, with other recent studies, suggests that there might be a new class of X-ray binary systems with extremely high mass blackholes. If so, this raises intriguing questions concerning their formation and evolution. Finally, the potential of the new major X-ray observatories, XMM-Newton and Chandra is discussed in the light of the studies described in this thesis.

This thesis discusses the interaction of stellar magnetic fields with circumstellar material in a variety of situations. Chapter 2 examines models of young stellar objects where accretion from the surrounding disc is controlled by magnetic loop structures on the star. The models produce outflows at early times which are consistent with those observed for young objects and the resultant torque on the star spins it down on a timescale also consistent with observations. Chapter 3 presents models examining the consequences of the magnetic field of the secondary star in Cataclysmic Variables (CVs) for the outburst behaviour of dwarf novae. Resonance phenomena are identified and observational tests suggested along with upper limits for the magnetic fields of the secondaries in SU UMa and U Gem systems. Chapters 4 and 5 examine the general angular momentum evolution of CVs. Simulations of magnetic braking are produced which describe the evolution in different situations. The proposal of reduced magnetic braking in polar systems is examined. Simulations are carried out which suggest that the addition of a strongly magnetic primary actually increases the rate of angular momentum loss from the binary orbit.

An observational study of AM Herculis system is presented, incorporating photometry and spectroscopy at both X-ray and optical wavelengths. Much of this thesis focuses upon one system, HU Aqr, a bright eclipsing system discovered by ROSAT. HU Aqr is unique amongst AM Hers because it has eclipses and possesses an accretion stream with significant optical flux, thus allowing detailed study of the nature of the accretion flow for the first time. Strong constraints on the accretion stream in HU Aqr have been obtained from optical photometry and spectroscopy. Changes to the mass accretion rate lead not only to variations in the mean flux from the system, but also to changes in the number of accreting poles and to alterations to the accretion geometry. A correlation analysis of the short timescale flickering in the data reveals a strong association between the photometry and polarimetry during the absorption dip. This is interpreted as arising from density structure within the accretion stream. The spectroscopic data reveals a highly complex emission pattern arising within the stream. A model has been developed which is used to measure the brightness distribution along the stream using both eclipse and emission line data. This has demonstrated that the line and continuum emission regions are located in well-defined yet separate regions within the stream. Further evidence for complex density structure in the accretion stream has been obtained from short timescale variations in the X-ray flux during the absorption dip of both HU Aqr and UZ For. In Hu Aqr, the first detection has been made of coronal emission from the secondary star in an AM Her system. Both systems have also provided clear evidence for spectral structure within the accretion region, comprising a hard core of emission with a softer region surrounding it. This X-ray analysis was extended to include a total of 30 systems in a study of the soft X-ray light curves and phase-resolved spectral behaviour of AM Her systems. In addition to confirming that the overall morphology of AM Her light curves is broadly consistent with most of our expectations, evidence for structured accretion regions, similar to those found in HU Aqr and UZ For, were discovered in six other AM Her systems.

White dwarfs in unresolved pairs with normal stars (spectral type K or earlier) are invisible at optical wavelengths, due to the close proximity of the much more luminous main sequence companion. ROSAT has provided evidence for the existence of a growing sample of these hidden white dwarfs through the detection of EUV and soft X-ray emission. For companions of spectral type ~A5 or earlier, the white dwarf can be spectroscopically identified at far-ultraviolet wavelengths by IUE. Eleven such systems had previously been found in this way from ROSAT, EUVE and IUE observations. A search for fainter, less obvious samples of these binaries is presented, and five new systems have been discovered. Three new close, pre-CV WD+dM binaries have also been found in the ROSAT WFC survey. Intriguingly, all three degenerates are rare mixed hydrogen/helium atmosphere DAO white dwarfs. The EUVE spectrum of one of these new systems, RE J0720-318, is analysed in detail. In particular, it is found that, while the optical spectrum can only be reproduced with a homogeneously mixed atmosphere, the EUVE spectrum can only be matched by a layered model, implying that the underlying structure of the white dwarf is stratified. The hydrogen layer mass of 3x10-14 M is the lowest measured for any white dwarf from EUVE spectra. In addition, an unprecedented HeI/HI ratio of ~1 is detected for the absorbing column along the line of sight, implying a hydrogen ionisation fraction of >90%, if all of this material resides in local interstellar medium. It is suggested that most of the helium lies in the vicinity of the star, possible in the form of a circumbinary disk left over from the common envelope phase. These results have important implications for our understanding of the evolutionary status of DAO white dwarfs in particular, and for post-common envelope systems in general. A catalogue of all the detected white dwarf binaries found in the ROSAT survey is presented, with an analysis of the white dwarf mass distribution. Compared with optically selected samples, a significant excess of hot, massive objects is detected. This excess probably arises from the slower cooling rates of massive (>0.9M) white dwarfs in comparison to normal mass (0.6M) stars.

This thesis describes work undertaken using data from the ROSAT and ASCA missions, in order to study the coronal x-ray emission of cool, late type stars. Chapters 1 and 2 summarise the emission processes involved and the methods by which the x-rays are detected and analysed, including a discussion of the ROSAT and ASCA missions. Chapter 3 details a deep soft x-ray survey of the Galactic Plane using the ROSAT Position Sensitive Proportional Counter. The data consist of 9 fields, each of 10ks exposure, pointed at positions on or very close to the plane. Presented are the source list, number-flux relation and the x-ray to optical flux ratios. I present the results of source count modelling which suggest 50% stellar and 8% extra-galactic contributions. The nature of the unidentified population is discussed. Chapter 1 discusses the temporal variability of the x-ray emission of stars in the Hyades cluster. The data consist of 13 PSPC fields, each of 20ks exposure, targeted in the central regions of the cluster. Little evidence is present to suggest long term variability, though 14 do exhibit short term variability. Time resolved spectra of detected flares are presented. Flare modelling results suggest flare loop heights of 1-2 solar radii. Chapters 5 details observations, obtained using the ASCA satellite, of 3 sources, VW Cep. SW Lac and 44i Boo. These observations form part of an ongoing survey of W UMa systems. Spectral and temporal results are compared to a previous ROSAT survey. I show that the observed systems have a high temperature component comparable with that seen in RS CVN and BY Dra systems. I also provide evidence for non-solar metal abundances in these systems.

A 1D hydrodynamical code is used to model the viscous evolution of VY Scl stars, which are a subclass of Cataclysmic Variable. Low states arise as a result of occasional drops in the mass transfer rate, which probably result from the passage of starspots across the inner Lagrangian point on the secondary star. The model includes the heating of the accretion disc by irradiation from the white dwarf and shows that outbursts from the low state can be suppressed if the temperature of the white dwarf is sufficiently high (Twd 40 000 K). A magnetic propeller model is used to show that the quiescent value of the viscosity parameter of the accretion disc within WZ Sge is likely to be occo d 0.02, in agreement with estimates of ojcold f r other dwarf novae. Assuming the white dwarf in WZ Sge to be weakly magnetic it is shown that, in quiescence, material close to the white dwarf can be propelled to larger radii, depleting the inner accretion disc. This has the effect of stabilizing the inner disc and allowing the outer disc to accumulate mass. Numerical models yield an estimated recurrence time of rec 30 10 yr, in agreement with the observed recurrence time of trec 33 yr. The model is also used to follow WZ Sge through outburst, producing lightcurves that are in good agreement with observation. Finally, high-speed K-band photometry of WZ Sge is presented. Analysis of the data reveals a strong oscillation at 27.88 0.01 s, along with weaker oscillations at slightly longer periods. The principal oscillation is attributed to the presence of a rapidly rotating weakly magnetic white dwarf, and possible explanations for the weaker oscillations are discussed. The long term brightness variation in the K-band lightcurves is analysed, providing tentative evidence of a precessing, elliptical disc. The observational properties can be explained if the white dwarf possesses a weak magnetic field.

This thesis presents the techniques and procedures that enable a new Galactic plane survey for 6668-MHz methanol masers, exclusive signposts of high-mass star formation. The statistics and properties of the largest ever sample of 6668MHz methanol masers arc detailed, including completeness and total population estimates. Through the application of rotation curves the distances are found, which then facilitates establishing the Galactic distribution of these masers, and thus of high-mass star formation regions. The relationship between these and the Spiral arms is examined. The most accurate luminosity distribution of 6668MHz methanol masers in the Galaxy and the Large Magellanic Cloud are given, together with new detections of 6668-MHz methanol and 6035-MHz hydroxyl masers in the Large Magellanic Cloud. A high spatial resolution study of an individual massive star formation region, aNI, is also presented, which not only details the precise structure and relation of methanol and excited-state hydroxyl to other species of masers for the first time, but also presents the first tentative detection of Zeeman splitting in a 6668-MHz methanol maser.

A novel approach to cylindrical wire array z-pinches has been developed in order to create a rotating plasma flow analogous to astrophysical accretion discs. The method involves subjecting the wire array to a cusp magnetic field (B_r) to create converging off axis ablation streams to form a rotating flow. The rotation is sustained by the ram pressure of the ablation streams in a quasi-equilibrium state for approximately 150 ns. This corresponds to one full rotation of the plasma about the axis. The rotating plasma is supersonic with Mach number ~2 and a radially constant rotation velocity between 60 and 75 km/s; the angular velocity therefore has an r^-1 dependence and the flow is differential. A Thomson scattering diagnostic is used to measure the electron and ion temperatures as Te ~30 eV and Ti >55 eV and the ionisation of the plasma (Z) between 6 and 8. These parameters are used to calculate the Reynolds number (10^5 to 10^6) and magnetic Reynolds numbers (20 to 100) which are large enough for viscous and resistive effects to be negligible on the large scale of the flow. These are of sufficient magnitude for the experiment to be scalable to astrophysical accretion discs. Further more the Reynolds number for the experiment is large enough for shear instabilities to manifest in the plasma. Some evidence for this can be seen in XUV images and Thomson spectra which indicate the development of perturbations and vorticity within the flow. Predictions for the growth rate of the Kelvin Helmholtz instability, 12 to 40 ns, agree reasonably well with the observed perturbation growth of ~30 ns. It is also possible that shear instabilities are driving hydrodynamic turbulence. Turbulent heating of the plasma could explain the approximately 500 eV increase in the ion temperature observed from some Thomson spectra. Further work is required however to prove the existence of shear flows and turbulence within the experiments.