The tidal disruption event AT2017eqx: spectroscopic evolution from
  hydrogen rich to poor suggests an atmosphere and outflow
  We present and analyse a new tidal disruption event (TDE), AT2017eqx at
redshift z=0.1089, discovered by Pan-STARRS and ATLAS. The position of the
transient is consistent with the nucleus of its host galaxy; it peaks at a
luminosity of $L \approx 10^{44}$ erg s$^{-1}$; and the spectrum shows a
persistent blackbody temperature $T \gtrsim 20,000$ K with broad H I and He II
emission. The lines are initially centered at zero velocity, but by 100 days
the H I lines disappear while the He II develops a blueshift of $\gtrsim 5,000$
km s$^{-1}$. Both the early- and late-time morphologies have been seen in other
TDEs, but the complete transition between them is unprecedented. The evolution
can be explained by combining an extended atmosphere, undergoing slow
contraction, with a wind in the polar direction becoming visible at late times.
Our observations confirm that a lack of hydrogen a TDE spectrum does not
indicate a stripped star, while the proposed model implies that much of the
diversity in TDEs may be due to the observer viewing angle. Modelling the light
curve suggests AT2017eqx resulted from the complete disruption of a solar-mass
star by a black hole of $\sim 10^{6.3} M_\odot$. The host is another quiescent,
Balmer-strong galaxy, though fainter and less centrally concentrated than most
TDE hosts. Radio limits rule out a relativistic jet, while X-ray limits at 500
days are among the deepest for a TDE at this phase.
