We present results from spectroscopic observations of AT 2018hyz , a transient discovered by the ASAS-SN survey at an absolute magnitude of M _ { V } \sim - 20.2 mag , in the nucleus of a quiescent galaxy with strong Balmer absorption lines .
AT 2018hyz shows a blue spectral continuum and broad emission lines , consistent with previous TDE candidates .
Modelling of the light curve ( presented in our companion paper , Gomez et al .
2020 ) , indicates AT2018hyz originated from the partial disruption of a low mass star .
High cadence follow-up spectra show broad Balmer lines and He I in early spectra , with He II making an appearance after \sim 70 - 100 days .
The Balmer lines evolve from a smooth broad profile , through a boxy , asymmetric double-peaked phase consistent with accretion disk emission , and back to smooth at late times .
The Balmer lines are unlike typical AGN in that they show a flat Balmer decrement ( H \alpha /H \beta \sim 1.5 ) , suggesting the lines are collisionally excited rather than being produced via photo-ionisation .
The flat Balmer decrement together with the complex profiles suggest that the emission lines originate in a disk chromosphere , analogous to those seen in cataclysmic variables .
The low optical depth of material from the partial disruption may be what allows us to observe these double-peaked , collisionally excited lines .
The late appearance of He II may be due to an expanding photosphere or outflow , or late-time shocks in debris collisions .