The Tidal Disruption Event AT 2018hyz I: Double-peaked emission lines
  and a flat Balmer decrement
  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. High cadence follow-up spectra show broad Balmer
lines and He I in early spectra, with He II making an appearance after
$\sim70-100$ days. The Balmer lines evolve from a smooth broad profile, through
a boxy, asymmetric double-peaked phase consistent with accretion disc 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$\beta\sim1.5$), 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 disc chromosphere, analogous to
those seen in cataclysmic variables. The low optical depth of material due to a
possible 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.
