  We report on a measurement of astrophysical tau neutrinos with 9.7 years of
IceCube data. Using convolutional neural networks trained on images derived
from simulated events, seven candidate $\nu_\tau$ events were found with
visible energies ranging from roughly 20 TeV to 1 PeV and a median expected
parent $\nu_\tau$ energy of about 200 TeV. Considering backgrounds from
astrophysical and atmospheric neutrinos, and muons from $\pi^\pm/K^\pm$ decays
in atmospheric air showers, we obtain a total estimated background of about 0.5
events, dominated by non-$\nu_\tau$ astrophysical neutrinos. Thus, we rule out
the absence of astrophysical $\nu_\tau$ at the $5\sigma$ level. The measured
astrophysical $\nu_\tau$ flux is consistent with expectations based on
previously published IceCube astrophysical neutrino flux measurements and
neutrino oscillations.
