  A search for weakly interacting massive particle dark matter produced in
association with bottom or top quarks is presented. Final states containing
third-generation quarks and missing transverse momentum are considered. The
analysis uses 36.1 $fb^{-1}$ of proton-proton collision data recorded by the
ATLAS experiment at $\sqrt{s}$ = 13 TeV in 2015 and 2016. No significant excess
of events above the estimated backgrounds is observed. The results are
interpreted in the framework of simplified models of spin-0 dark-matter
mediators. For colour-neutral spin-0 mediators produced in association with top
quarks and decaying into a pair of dark-matter particles, mediator masses below
50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary
couplings. For scalar and pseudoscalar mediators produced in association with
bottom quarks, the search sets limits on the production cross-section of 300
times the predicted rate for mediators with masses between 10 and 50 GeV and
assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on
colour-charged scalar simplified models are also presented. Assuming a
dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV
are excluded for couplings yielding a dark-matter relic density consistent with
measurements.
