The multi-wavelength evolution of luminous red nova AT 2021aess.

Luminous Red Novae (LRNe) are a phenomenon yet to be fully understood. They are currently interpreted as the coalescence of two non-degenerate stars in a close interacting binary system after the ejection of their common envelope. In this context, the study of LRNe is a hot topic, as it has important implications for the formation and evolution of close compact binaries. Nevertheless, until now, only a handful of LRNe are available in the literature with high-quality datasets allowing for a comprehensive transient study. This thesis project aims to characterize the LRN AT 2021aess through an extensive analysis of its photometric and spectroscopic evolution in the optical and near-infrared domains. The follow-up campaign of AT 2021aess was conducted using several instruments available to the Padova Supernova Group, the ePESSTO, CSP, and NUTS2 collaborations, and data from public surveys. The LRN outburst was extensively monitored in the photometric regime, allowing us to construct a well-sampled light curve, which shows a double peak with a prominent initial maximum ($M_V = -15.5\pm 0.2$ mag), followed by a redder broad peak ($M_V = -14.2 \pm 0.2$ mag) and later a steep luminosity decline. Based on these observations, AT 2021aess is the brightest LRN ever observed and among the most long-lasting. Thanks to the available multi-band coverage, the bolometric luminosity, temperature and radius of the emitting source are inferred from a black-body (BB) fit to the spectral energy distribution of the transient. The spectra cover the evolution of AT 2021aess from soon after the discovery up to the second peak. The very early spectra are characterised by a blue continuum with superimposed prominent Balmer emission lines displaying a P~Cygni profile. At later phases, the continuum becomes rapidly redder with the Balmer emission features becoming less prominent as a forest of metal absorption lines arises. This metamorphosis is consistent with the typical evolution observed in other LRNe spectra. The study of archival images reveals the presence of a source at the transient location years before the outburst. The analysis of the data allowed us to constrain the mass of the progenitor system up to 60$\pm 10 $ M$_\odot$. This is consistent with what we found from empirical relations found in the literature. The information obtained in this analysis expands our understanding of LRNe arising from high-mass progenitor systems, which are rare and still understudied.