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Tidal disruption events (TDEs) provide a unique opportunity to study the environments and accretion mechanisms of otherwise quiescent supermassive black holes (SMBHs). This dissertation explores the physical properties of the circumnuclear dust, gas, and stellar populations in TDE host galaxies, connecting small-scale processes around SMBHs to galaxy-wide characteristics. Using high-cadence photometry and spectroscopy, we probe the dynamics and composition of material near SMBHs during and after TDE flares. In Chapter 2, we analyze the TDE AT 2020mot, discovering a near-infrared dust echo produced by concentric rings of dust within the smallest scales yet inferred near a SMBH, highlighting the potential of TDEs for uncovering subparsec dust structures when near-infrared observations are included in transient studies. In Chapter 3, we present AT 2022upj, the first confirmed TDE with simultaneous-at-peak extreme coronal line emission. This discovery links highionization gas to TDE flares and reveals a stratified distribution of circumnuclear material, with coronal gas within 0.1 pc and dust at ∼0.4 pc. Finally, in Chapter 4, we utilize HST/STIS spectroscopy of the post-starburst TDE host ASASSN-14li to uncover a stellar age gradient, with a younger starburst near the nucleus (∼230 Myr) and an older population at 87 pc (∼550 Myr). Together, these studies use TDEs to uncover the complex environments of SMBHs, from subparsec dust and gas structures to the nuclear stellar populations that govern TDE rates. By combining observations across wavelengths and spatial scales, this work advances our understanding of SMBH-host galaxy interactions in the unique environments of TDE hosts.

Stars strongly impact their environment, and shape structures on all scales throughout the universe, in a process known as “feedback.” Due to the complexity of both stellar evolution and the physics of larger astrophysical structures, there remain many unanswered questions about how feedback operates and what we can learn about stars by studying their imprint on the wider universe. In this white paper, we summarize discussions from the Lorentz Center meeting “Bringing Stellar Evolution and Feedback Together” in 2022 April and identify key areas where further dialog can bring about radical changes in how we view the relationship between stars and the universe they live in.

Stars strongly impact their environment, and shape structures on all scales throughout the universe, in a process known as “feedback.” Due to the complexity of both stellar evolution and the physics of larger astrophysical structures, there remain many unanswered questions about how feedback operates and what we can learn about stars by studying their imprint on the wider universe. In this white paper, we summarize discussions from the Lorentz Center meeting “Bringing Stellar Evolution and Feedback Together” in 2022 April and identify key areas where further dialog can bring about radical changes in how we view the relationship between stars and the universe they live in.

Using HST/STIS observations, we present the highest-spatial-resolution spectroscopic study to date of four tidal disruption event (TDE) host galaxies, with the best observed being the post-starburst (PSB) host of ASASSN-14li. The stellar population of ASASSN-14li's host, within 44 pc of the nucleus, reveals a younger recent starburst (\\(\\)340 Myr) compared to the population at an offset radius of 88 pc that excludes the nucleus (\\(\\)550 Myr), a radial age gradient suggesting gas inflows from a minor merger. We estimate a stellar density of \\(5900 800 \\, M_ / pc^3\\) within 30 pc of the nucleus of ASASSN-14li's host, exceeding densities expected for nuclear star clusters. High-ionization ``coronal\" emission lines, [Fe VI] \\( 5677\\), [Fe VII] \\( 6087\\), and [Fe X] \\( 6375\\), are also detected within the nuclear spectra of the hosts of ASASSN-14li and PTF09ge, importantly alongside the non-detection of [O III] at the same scale. We similarly do not detect [O III] in the nuclear region of ASASSN-14ae's host despite its presence in the SDSS spectrum. The different ionization radiation levels detected at various radii from TDE host nuclei may indicate echoes of earlier accretion episodes, including, potentially, a prior TDE. We posit that a minor merger driving gas inflow to the nucleus could drive the enhanced TDE rates in post-starburst galaxies, inducing variation in nuclear gas properties and star formation history on \\(<\\)150 pc scales in TDE hosts.

We present an analysis of the optical and UV properties of AT 2020wey, a faint and fast tidal disruption event (TDE) at 124.3 Mpc. The light curve of the object peaked at an absolute magnitude of \\(M_{g} = -17.45\\) mag and a maximum bolometric luminosity of \\(L_{\\rm peak}=(8.74\\pm0.69)\\times10^{42}\\) erg s\\(^{-1}\\), making it comparably faint with iPTF16fnl, the faintest TDE to date. The time from the last non-detection to the \\(g\\)-band peak is 22.94 \\(\\pm\\) 2.03 days and the rise is well described by \\(L\\propto t^{1.8}\\). The decline of the bolometric light curve is described by a sharp exponential decay steeper than the canonical \\(t^{-5/3}\\) power law, making AT 2020wey the fastest declining TDE to date. Multi-wavelength fits to the light curve indicate a complete disruption of a star of \\(M_*=0.11M_{\\odot}\\) by a black hole of \\(M_{\\rm BH}=10^{6.46}M_{\\odot}\\). Our spectroscopic dataset reveals broad (\\(\\sim10^{4}\\) km s\\(^{-1}\\)) Balmer and He II \\(\\lambda\\)4686 lines, with H\\(\\alpha\\) reaching its peak with a lag of \\(\\sim8.2\\) days compared to the continuum. In contrast to previous faint and fast TDEs, there are no obvious Bowen fluorescence lines in the spectra of AT 2020wey. There is a strong correlation between the MOSFIT-derived black hole masses of TDEs and their decline rate. However, AT 2020wey is an outlier in this correlation, which could indicate that its fast early decline may be dictated by a different physical mechanism than fallback. After performing a volumetric correction to a sample of 30 TDEs observed between 2018 and 2020, we conclude that faint TDEs are not rare by nature and that they should constitute up to \\(\\sim\\) 50 - 60 % of the entire population and their numbers could alleviate some of the tension between the observed and theoretical TDE rate estimates. We calculate the optical TDE luminosity function and we find a steep power-law relation \\(dN/dL_{g} \\propto {L_{g}}^{-2.36}\\).

We report the results of a rapid follow-up campaign on the Type IIb Supernova (SN) 2022hnt. We present a daily, multi-band, photometric follow-up using the Las Cumbres Observatory, the Zwicky Transient Facility, the orbiting \\textit{Swift} observatory, and the Asteroid Terrestrial-impact Last Alert System (ATLAS). A distinctive feature in the light curve of SN 2022hnt and other IIb SNe is an early narrow peak prior to the \\({}^{56}\\)Ni peak caused by rapid shock cooling of the hydrogen envelope, which can serve as an important probe of the properties of the massive progenitor star in the moments before explosion. Using SN 2022hnt as a case study, we demonstrate a framework of considerations for the application of shock cooling models to type IIb SNe, outlining a consistent procedure for future surveys of Type IIb SNe progenitor and explosion properties. \\hll{We fit several recent models of shock-cooling emission and obtain progenitor radii between \\(\\sim50\\) and \\(\\sim100\\) \\(R_\\odot\\), as well as hydrogen-enriched envelope masses between \\(\\sim0.01\\) and \\(\\sim0.1\\) \\(M_\\odot\\), both consistent with values for other IIb SNe. One of these models is the model of \\cite{Morag2023}, marking the first time this model has been applied to a Type IIb SN.} We evaluate contrasting predictions between shock-cooling models to construct a fiducial parameter set which can be used for comparison to other SNe. Finally, we investigate the possibility of extended wind breakout or precursor emission captured in the earliest detections.

Stars in galactic centers are occasionally scattered so close to the central supermassive black hole that they are completely disrupted by tidal forces, initiating a transient accretion event. The aftermath of such a tidal disruption event (TDE) produces a bright-and-blue accretion flow which is known to persist for at least a decade (observationally) and can in principle produce ionizing radiation for hundreds of years. Tidal disruption events are known (observationally) to be overrepresented in galaxies which show extended emission line regions (EELRs), with no pre-TDE classical AGN activity, and to produce transient ``coronal lines'', such as [FeX] and [FeXIV]. Using coupled CLOUDY-TDE disk simulations we show that tidal disruption event disks produce a sufficient ionizing radiation flux over their lifetimes to power both EELR of radial extents of \\(r \\sim 10^4\\) light years, and coronal lines. EELRs are produced when the ionizing radiation interacts with low density \\(n_H \\sim 10^1 - 10^3 \\, {\\rm cm}^{-3}\\) clouds on galactic scales, while coronal lines are produced by high density \\(n_H \\sim 10^6 - 10^8 \\, {\\rm cm}^{-3}\\) clouds near the galactic center. High density gas in galactic centers will also result in the rapid switching on of narrow line features in post-TDE galaxies, and also various high-ionization lines which may be observed throughout the infrared with JWST. Galaxies with a higher intrinsic rate of tidal disruption events will be more likely to show macroscopic EELRs, which can be traced to originate from the previous tidal disruption event in that galaxy, which naturally explains why TDEs are more likely to be discovered in galaxies with EELRs. We further argue that a non-negligible fraction of so-called optically selected ``AGN'' are tidal disruption events.

Core-collapse supernovae (CCSNe) may contribute a significant amount of dust in the early universe. Freshly formed coolant molecules (e.g., CO) and warm dust can be found in CCSNe as early as ~100 d after the explosion, allowing the study of their evolution with time series observations. In the Type II SN 2023ixf, we aim to investigate the temporal evolution of the temperature, velocity, and mass of CO and compare them with other CCSNe, exploring their implications for the dust formation in CCSNe. From observations of velocity profiles of lines of other species (e.g., H and He), we also aim to characterize and understand the interaction of the SN ejecta with preexisting circumstellar material (CSM). We present a time series of 16 near-infrared spectra of SN 2023ixf from 9 to 307 d, taken with multiple instruments: Gemini/GNIRS, Keck/NIRES, IRTF/SpeX, and MMT/MMIRS. The early (t<70 d) spectra indicate interaction between the expanding ejecta and nearby CSM. At t<20 d, intermediate-width line profiles corresponding to the ejecta-wind interaction are superposed on evolving broad P Cygni profiles. We find intermediate-width and narrow lines in the spectra until t<70 d, which suggest continued CSM interaction. We also observe and discuss high-velocity absorption features in H \\(\\alpha\\) and H \\(\\beta\\) line profiles formed by CSM interaction. The spectra contain CO first overtone emission between 199 and 307 d after the explosion. We model the CO emission and find the CO to have a higher velocity (3000-3500 km/s) than that in Type II-pec SN 1987A (1800-2000 km/s) during similar phases (t=199-307 d) and a comparable CO temperature to SN 1987A. A flattened continuum at wavelengths greater than 1.5 \\(\\mu\\)m accompanies the CO emission, suggesting that the warm dust is likely formed in the ejecta. The warm dust masses are estimated to be on the order of ~10\\(^{-5} M_{\\odot}\\).}