The candidate Thorne–Żytkow object (TŻO), HV2112, is becoming a well-studied if enigmatic object. A key point of its candidacy as a TŻO is whether or not it resides in the Small Magellanic Cloud (SMC). HV2112 has detections in a series of photometric catalogues which have resulted in contradictory estimates of its proper motion and, therefore, its membership within the SMC. This letter seeks to resolve the issue of the SMC membership of HV2112 through a reanalysis of extant photometric data. We also demonstrate the difficulties and downfalls inherent in considering a range of catalogue proper motions. We conclude that the proper motion, and associated ancillary radial velocity, positional and photometric properties, are fully consistent with HV2112 being within the SMC and thus it remains a candidate TŻO.

The very bright red star HV2112 in the Small Magellanic Cloud could be a massive Thorne-A >> ytkow object (TA >> O), a supergiant-like star with a degenerate neutron core. With a luminosity of over 10(5) L-aS (TM), it could also be a super asymptotic giant branch (SAGB) star, a star with an oxygen/neon core supported by electron degeneracy and undergoing thermal pulses with third dredge up. Both TA >> Os and SAGB stars are expected to be rare. Abundances of heavy elements in HV2112's atmosphere, as observed to date, do not allow us to distinguish between the two possibilities based on the latest models. Molybdenum and rubidium can be enhanced by both the irp-process in a TA >> O or by the s-process in SAGB stars. Lithium can be generated by hot bottom burning at the base of the convective envelope in either. HV2112's enhanced calcium could thus be the key determinant. Neither SAGB stars nor TA >> Os are known to be able to synthesize their own calcium but it may be possible to produce it in the final stages of the process that forms a TA >> O, when the degenerate electron core of a giant star is tidally disrupted by a neutron star. Hence, it is more likely, on a fine balance, that HV2112 is indeed a genuine TA >> O.

Thorne-Zytkow objects (TZOs) are a theoretical class of star in which a compact neutron star is surrounded by a large, diffuse envelope. Supergiant TZOs are predicted to be almost identical in appearance to red supergiants (RSGs). The best features that can be used at present to distinguish TZOs from the general RSG population are the unusually strong heavy-element and Li lines present in their spectra, products of the star's fully convective envelope linking the photosphere with the extraordinarily hot burning region in the vicinity of the neutron star core. Here we present our discovery of a TZO candidate in the Small Magellanic Cloud. It is the first star to display the distinctive chemical profile of anomalous element enhancements thought to be unique to TZOs. The positive detection of a TZO will provide the first direct evidence for a completely new model of stellar interiors, a theoretically predicted fate for massive binary systems, and never-before-seen nucleosynthesis processes that would offer a new channel for Li and heavy-element production in our universe.

The mechanism by which the supermassive black holes that power bright quasars at high redshift form remains unknown. One possibility is that ... the monolithic collapse of a massive protogalactic disc ... leads to the formation of a quasi-star: a growing black hole, initially of typical stellar-mass, embedded in a hydrostatic giant-like envelope. Quasi-stars are the main object of study in this dissertation. ... In Chapter 1, I introduce the problem posed by the supermassive black holes that power high-redshift quasars. ... In Chapter 2, I outline the Cambridge STARS code and the modifications that are made to model quasi-star envelopes. In Chapter 3, I present models of quasi-stars where the base of the envelope is located at the Bondi radius of the black hole. The black holes in these models are subject to a robust upper fractional mass limit of about one tenth. In addition, the final black hole mass is sensitive to the choice of the inner boundary radius of the envelope. In Chapter 4, I construct alternative models of quasi-stars by drawing from work on convection- and advection-dominated accretion flows ... The evolution of these quasi-stars is qualitatively different from those described in Chapter 3. ... [T]he core black holes are no longer subject to a fractional mass limit and ultimately accrete all of the material in their envelopes. In Chapter 5, I demonstrate that the fractional mass limit found in Chapter 3 ... is in essence the same as the Schönberg-Chandrasekhar limit. The analysis demonstrates ... that limits exist under a wider range of circumstances than previously thought. A test is provided that determines whether a composite polytrope is at a fractional mass limit. In Chapter 6, I apply this test to realistic stellar models and find evidence that the existence of fractional mass limits is connected to the evolution of stars into the red giants.

The existence of bright quasars at high redshifts implies that supermassive black holes were able to form in the early Universe. Though a number of mechanisms to achieve this have been proposed, none yet stands out. A recent suggestion is the formation of quasi-stars, initially stellar-mass black holes accreting from hydrostatic giant-like envelopes of gas, formed from the monolithic collapse of pre-galactic gas clouds. In this work, we modify the Cambridge stars stellar evolution package to construct detailed models of the evolution of these objects. We find that, in all of our models, the black hole inside the envelope is able to reach slightly more than one-tenth of the total mass of the system before hydrostatic equilibrium breaks down. This breakdown occurs after a few million years of evolution. We show that the mechanism which causes the hydrostatic evolution to end is present in polytropic models. We also show that the solutions are highly sensitive to the size of the inner boundary radius and that no physical solutions exist if the inner boundary is chosen to be less than about 0.3 of the Bondi radius.

We propose an approach to explaining why naive large quantum fluctuations are not the right estimate for the cosmological constant. We argue that the universe is in a superposition of many vacua, in such a way that the resulting fluctuations are suppressed by level repulsion to a very small value. The approach combines several aspects of string theory and the early history of the universe, and is only valid if several assumptions hold true. The approach may also explain why the effective cosmological constant reamins small as the universe evolves though several phase transitions. It provides a non-anthropic mechansim leading to a small, non-zero cosmological constant.

Motivated by our earlier argument that the apparent large cosmological constant from quantum fluctuations is actually an artifact of not using a full quantum mechanical superposition to determine the ground state in which the universe lives in the de Sitter space at the beginning of inflation, we calculate the tunneling probability for the two-well potential for a scalar field in de Sitter space. We include ocupling of the potential to gravity, and the effective potential from quantum corrections. The results show the eigenstates are the sum and differences of the wavefunctions for the seperate wells, i.e. a full superposition, and the energy levels are split, with tunneling between them determined by the Hawking-Moss instanton and not supressed.

We argue that if string theory as an approach to the fundamental laws of nature is correct, then there is almost no room for anthropic arguments in cosmology. The quark and lepton masses and interaction strengths are determined.

The general relativistic equations of stellar structure and evolution are reformulated in a notation which makes easy contact with Newtonian theory. Also, a general relativistic version of the mixing-length formalism for convection is presented. Finally, it is argued that in previous work on spherical systems general relativity theorists have identified the wrong quantity as "total mass-energy inside radius r."