Some tidal disruption events (TDEs) exhibit blueshifted broad absorption lines (BALs) in their rest-frame ultraviolet (UV) spectra, while others display broad emission lines (BELs). Similar phenomenology is observed in quasars and accreting white dwarfs, where it can be interpreted as an orientation effect associated with line formation in an accretion disc wind. We propose and explore a similar unification scheme for TDEs. We present synthetic UV spectra for disc and wind-hosting TDEs, produced by a state-of-the-art Monte Carlo ionization and radiative transfer code. Our models cover a wide range of disc wind geometries and kinematics. Such winds naturally reproduce both BALs and BELs. In general, sightlines looking into the wind cone preferentially produce BALs, while other orientations preferentially produce BELs. We also study the effect of wind clumping and CNO-processed abundances on the observed spectra. Clumpy winds tend to produce stronger UV emission and absorption lines, because clumping increases both the emission measure and the abundances of the relevant ionic species, the latter by reducing the ionization state of the outflow. The main effect of adopting CNO-processed abundances is a weakening of C IV 1550 Å and an enhancement of N V 1240 Å in the spectra. We conclude that line formation in an accretion disc wind is a promising mechanism for explaining the diverse UV spectra of TDEs. If this is correct, the relative number of BAL and BEL TDEs can be used to estimate the covering factor of the outflow. The models in this work are publicly available online and upon request.

A significant number of tidal disruption events (TDEs) radiate primarily at optical and ultraviolet (UV) wavelengths, with only weak soft X-ray components. One model for this optical excess proposes that thermal X-ray emission from a compact accretion disc is reprocessed to longer wavelengths by an optically thick envelope. Here, we explore this reprocessing scenario in the context of an optically thick accretion disc wind. Using a state-of-the-art Monte Carlo radiative transfer and ionization code, we produce synthetic UV and optical spectra for wind and disc-hosting TDEs. Our models are inspired by observations, spanning a realistic range of accretion rates and wind kinematics. We find that such outflows can efficiently reprocess the disc emission and produce the broad Balmer and helium recombination features commonly seen in TDEs. The emission lines feature an asymmetric red wing, with electron scattering having a dramatic influence on their profiles. Moreover, the characteristic colour temperature of the reprocessed spectral energy distribution (SED) is much lower than that of the accretion disc. We show explicitly how changes in black hole mass, accretion rate and wind properties affect the observed broadband SED and line spectrum. In general, slower, denser winds tend to reprocess more radiation and produce stronger Balmer emission. Most of the outflows we consider are too highly ionized to produce UV absorption features, but this is sensitive to the input SED. For example, truncating the inner disc at just 4 ~ ISCO lowers the wind ionization state sufficiently to produce UV absorption features for sight lines looking into the wind.

When an unlucky star wanders too close to a supermassive black hole, the self-gravity keeping that star together is completely overwhelmed by the tidal forces of the interaction and is torn asunder in something known as a tidal disruption event (TDE). The fallback of the stellar debris, and subsequent accretion, onto the black hole drives a powerful, but transient, luminous flare visible across the electromagnetic spectrum: a shining light, even in death. Within the past decade, the census of TDEs has rapidly grown to a dizzying 56 events. Theoretical progress, on the other hand, has somewhat lagged behind and our understanding of these events is held back by our ignorance of the emission mechanisms fuelling the luminous flare.

Given their extreme luminosities, TDEs are expected to generate significant mass loss in the form of radiation driven winds, but their properties remain poorly understood even though their importance is widely acknowledged. In this thesis, I use state-of-the-art Monte Carlo radiative transfer and ionization software to model the profound impact such powerful outflows have on the optical and ultraviolet (UV) spectra of these events. Focusing exclusively on the sub-Eddington accretion phase, I first show how the diverse family of UV spectra, showcasing broad absorption and emission lines, can be unified as an orientation effect associated with line formation in an accretion disc wind. And if true, I suggest that the relative number of broad absorption to emission line TDEs could be used to estimate the outflow covering fraction.

I also demonstrate how such outflows can efficiently reprocess the accretion disc emission, producing the broad Balmer and helium optical recombination features characteristic of TDEs, as well as a spectral energy distribution (SED) with a much lower characteristic colour temperature. I next apply the exact same numerical techniques to post-process a snapshot of a realistic hydrodynamic model of the super-Eddington accretion phase, illustrating how different reprocessing regimes can modify the SED and influence the observable signatures of TDEs.

In summary, then, this thesis demonstrates how optically thick outflows impact and shape the observational properties of TDEs, and why, in the future, detailed radiative transfer calculations are required to understand the complexities of TDE emission.