Extreme Coronal Line Emitters: Tidal Disruption of Stars by Massive
  Black Holes in Galactic Nuclei?
  Tidal disruption of stars by supermassive black holes at the centers of
galaxies is expected to produce unique emission line signatures, which have not
yet been explored adequately. Here we report the discovery of extremely strong
coronal lines from [Fe X] up to [Fe XIV] in a sample of seven galaxies
(including two recently reported cases), that we interpret as such signatures.
This is the first systematic search for objects of this kind, by making use of
the immense database of the Sloan Digital Sky Survey. The galaxies, which are
non-active as evidenced by the narrow-line ratios, show broad emission lines of
complex profiles in more than half of the sample. Both the high ionization
coronal lines and the broad lines turn out to be fading on time scales of years
in objects observed with spectroscopic follow-ups, suggesting their transient
nature. Variations of inferred non-stellar continua, which have absolute
magnitudes of at least -16 to -18 mag in the g band, are also detected in more
than half of the sample. These extreme coronal line emitters reside in sub-L_*
disk galaxies (-21.3 < M_i < -18.5) with small stellar velocity dispersions.
The sample seems to form two distinct types based on the presence or absence of
the [Fe VII] lines, with the latter having relatively low luminosities of [O
III], [Fe XI], and the host galaxies. These characteristics can most naturally
be understood in the context of transient accretion onto intermediate mass
black holes at galactic centers following tidal disruption of stars in a
gas-rich environment. We estimate the incidence of such events to be around
10^-5 per year for a galaxy with -21.5 < M_i < -18.5.
