Low-mass stars with masses below about 1.3 solar masses exhibit intriguing trends in the evolution of their spins. These stars also exist in tidally locked binaries with a white dwarf (WD) companion, the cataclysmic variables (CVs). Their orbital evolution is driven by angular momentum loss due to magnetic braking (MB). Although the same fundamental mechanism likely drives MB in all these systems, a holistic study of such a mechanism has not been carried out before. Here I aim to investigate whether observations of CV evolution and the spins of single stars can be explained by the same fundamental mechanism driving MB in all low-mass stars. In Chapter 1, I illustrate the salient features of the evolution of low-mass stars with convective exteriors and radiative cores (if any), describe the physics of MB and provide a background on CVs and their evolution. I also touch upon the working of the Cambridge stellar evolution code with which I evolve and model these stellar systems. In Chapter 2, I model the secular evolution of CVs with a double dynamo model wherein there is an interplay between two α-Ω dynamos, one in the convective envelope and the other at the boundary of the radiative core and the convective envelope. I confirm that this model provides a physical formalism for the interrupted magnetic braking paradigm. I construct the relative probability distribution of orbital periods from the mass distribution of white dwarfs in CVs and find that this model reproduces the period gap and the observed period minimum spike in CV distribution. In Chapter 3, I explore the implications of the hypothesis that MB, which may be an angular momentum loss mechanism working in addition to gravitational wave radiation for CVs below the period gap, weakens around their period minimum. This explains the strong disagreement between theory and observations of the relative fraction of period bouncer CVs. I find that, with such a mechanism, the CV virtually stalls around the period minimum and then vanishes from observations, so reducing the number of observable systems. In Chapter 4, I model CVs with evolved donors that eventually form AM CVn stars through the Evolved CV formation channel by extending the MB model of Chapter 2. I find that the MB timescale in my model is shorter than that of previously used empirical formulae. Owing to this, CVs evolving from a larger parameter space of initial conditions form AM CVn stars than with other models. Modelling these systems beyond their orbital period minima, I find that a significant number become extremely H-exhausted systems. This makes them indistinguishable from AM CVn systems evolved through the He-star and the White Dwarf formation channels in terms of the absence of H in their spectra. In Chapter 5, I investigate AM CVn formation through the He-star formation channel. I show that semi-degenerate, H-exhausted, He-rich donors can be formed after common envelope evolution (CEE) with a WD if either additional sources of energy are used to eject the common envelope or a different formalism of CEE is implemented. I follow the evolution of such binary systems after the CEE to the AM CVn phase by implementing the same MB mechanism as in Chapter 2 and find that these models can explain unusual AM CVn systems such as Gaia14aae and ZTFJ1637+49. In Chapter 6 and 7, I extend the MB model of Chapter 2 to address the spin evolution of isolated fully convective M-dwarf (FCMD) and partly convective dwarf (PCD) stars. As well as making observationally-motivated changes, I add to my model a parametrized reduction in the wind mass loss because of the entrapment of stellar material in dead zones. I find that several recent observational trends in the spins of open-cluster and field stars can be explained by this mechanism of MB and internal shear. My MB model relates physically motivated estimates of the magnetic field strength and stellar wind to properties of the stellar dynamo. There I also find general agreement with observations of winds, Alfvén radii and surface magnetic fields. I find that my model gives robust estimates of the core-envelope convergence timescale in PCDs that improve on those of previous studies. In Chapter 8, I highlight the key results from this dissertation and avenues to explore in the future, intending to further explore the mechanism that drives MB in all stars.
low-mass stars, magnetic braking, stellar evolution
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