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Tidal disruption of stars by supermassive black holes at the centers ofgalaxies is expected to produce unique emission line signatures, which have notyet been explored adequately. Here we report the discovery of extremely strongcoronal 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 ofthe immense database of the Sloan Digital Sky Survey. The galaxies, which arenon-active as evidenced by the narrow-line ratios, show broad emission lines ofcomplex profiles in more than half of the sample. Both the high ionizationcoronal lines and the broad lines turn out to be fading on time scales of yearsin objects observed with spectroscopic follow-ups, suggesting their transientnature. Variations of inferred non-stellar continua, which have absolutemagnitudes of at least -16 to -18 mag in the g band, are also detected in morethan 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 ofthe [Fe VII] lines, with the latter having relatively low luminosities of [OIII], [Fe XI], and the host galaxies. These characteristics can most naturallybe understood in the context of transient accretion onto intermediate massblack holes at galactic centers following tidal disruption of stars in agas-rich environment. We estimate the incidence of such events to be around10^-5 per year for a galaxy with -21.5 < M_i < -18.5.

Extreme Coronal Line Emitters: Tidal Disruption of Stars by Massive Black Holes in Galactic Nuclei?