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We present light curves and spectra of the tidal disruption event (TDE)ASASSN-18pg / AT 2018dyb spanning a period of one year. The event shows aplethora of strong emission lines, including the Balmer series, He II, He I andmetal lines of O III $\lambda$3760 and N III $\lambda\lambda$ 4100, 4640(blended with He II). The latter lines are consistent with originating from theBowen fluorescence mechanism. By analyzing literature spectra of past events,we conclude that these lines are common in TDEs. The spectral diversity ofoptical TDEs is thus larger than previously thought and includes N-rich eventsbesides H- and He-rich events. We study how the spectral lines evolve withtime, by means of their width, relative strength, and velocity offsets. Thevelocity width of the lines starts at $\sim$ 13000 km s$^{-1}$ and decreaseswith time. The ratio of He II to N III increases with time. The same is truefor ASASSN-14li, which has a very similar spectrum to AT 2018dyb but its linesare narrower by a factor of $>$2. We estimate a black hole mass of $M_{\rm BH}$= $3.3^{+5.0}_{-2.0}\times 10^6$ $M_{\odot}$ by using the $M$-$\sigma$relation. This is consistent with the black hole mass derived using the MOSFiTtransient fitting code. The detection of strong Bowen lines in the opticalspectrum is an indirect proof for extreme ultraviolet and (reprocessed) X-rayradiation and favors an accretion origin for the TDE optical luminosity. Amodel where photons escape after multiple scatterings through a super-Eddingtonthick disk and its optically thick wind, viewed at an angle close to the diskplane, is consistent with the observations.

The spectral evolution of AT 2018dyb and the presence of metal lines in tidal disruption events