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[Abridged] We classify AT 2020zso as a TDE based on the blackbody evolutioninferred from UV/optical photometric observations, and spectral line contentand evolution. We identify transient, double-peaked Bowen (N III), He I, He IIand Halpha emission lines. We model medium resolution optical spectroscopy ofthe He II (after careful deblending of the N III contribution) and Halpha linesduring the rise, peak and early decline of the light curve using relativistic,elliptical accretion disk models. We find that the spectral evolution beforepeak can be explained by optical depth effects consistent with an outflowing,optically thick Eddington envelope. Around peak the envelope reaches itsmaximum extent (approximately 10^15 or 3000-6000 gravitational radii for aninferred black hole mass of 5-10 10^5) and becomes optically thin. The Halphaand He II emission lines at and after peak can be reproduced with a highlyinclined (i=85+-5 degrees), highly elliptical (e=0.97+-0.01) and relativelycompact (Rin = several 100 Rg and Rout = several 1000 Rg ) accretion disk.Overall, the line profiles suggest a highly elliptical geometry for the newaccretion flow, consistent with theoretical expectations of newly formed TDEdisks. We quantitatively confirm, for the first time, the high inclinationnature of a Bowen (and X-ray dim) TDE, consistent with the unification pictureof TDEs where the inclination largely determines the observational appearance.Rapid line profile variations rule out the binary SMBH hypothesis as the originof the eccentricity; these results thus provide a direct link between a TDE inan AGN and the eccentric accretion disk. We illustrate for the first time howoptical spectroscopy can be used to constrain the black hole spin, through (thelack of) disk precession signatures (changes in inferred inclination) - andrule out high black hole spin values (a < 0.8).

An elliptical accretion disk following the tidal disruption event AT 2020zso