  A star crossing the tidal radius of a supermassive black hole will be
spectacularly ripped apart with an accompanying burst of radiation. A few tens
of such tidal disruption events (TDEs) have now been identified in the optical
wavelengths, but the exact origin of the strong optical emission remains
inconclusive. Here we report polarimetric observations of three TDEs. The
continuum polarization is independent of wavelength, while emission lines are
partially depolarized. These signatures are consistent with optical photons
being scattered and polarized in an envelope of free electrons. An almost
axisymmetric photosphere viewed from different angles is in broad agreement
with the data, but there is also evidence for deviations from axial symmetry
before the peak of the flare and significant time evolution at early times,
compatible with the rapid formation of an accretion disk. By combining a
super-Eddington accretion model with a radiative transfer code we generate
predictions for the degree of polarization as a function of disk mass and
viewing angle, and we show that the predicted levels are compatible with the
observations, for extended reprocessing envelopes of $\sim$1000 gravitational
radii. Spectropolarimetry therefore constitutes a new observational test for
TDE models, and opens an important new line of exploration in the study of
TDEs.
