Evidence for the preferential disruption of moderately massive stars by
  supermassive black holes
  Tidal disruption events (TDEs) provide a unique opportunity to probe the
stellar populations around supermassive black holes (SMBHs). By combining light
curve modeling with spectral line information and knowledge about the stellar
populations in the host galaxies, we are able to constrain the properties of
the disrupted star for three TDEs. The TDEs in our sample have UV spectra, and
measurements of the UV N III to C III line ratios enabled estimates of the
nitrogen-to-carbon abundance ratios for these events. We show that the measured
nitrogen line widths are consistent with originating from the disrupted stellar
material dispersed by the central SMBH. We find that these nitrogen-to-carbon
abundance ratios necessitate the disruption of moderately massive stars
($\gtrsim 1 - 2 M_\odot$). We determine that these moderately massive
disruptions are over-represented by a factor of $\gtrsim 10^2$ when compared to
the overall stellar population of the post-starburst galaxy hosts. This implies
that SMBHs are preferentially disrupting higher mass stars, possibly due to
ongoing top-heavy star formation in nuclear star clusters or to dynamical
mechanisms that preferentially transport higher mass stars to their tidal
radii.
