  Two searches for new phenomena involving top quarks are presented: a search
for a top partner in ttbar events with large missing transverse momentum, and a
search for ttbar resonances in proton-proton collisions at a center-of-mass
energy of 7 TeV. The measurements are based on 35 pb^-1 and 200 pb^-1 of data
collected with the ATLAS detector at the LHC in 2010 and 2011, respectively. No
evidence for a signal is observed. The first limits from the LHC are
established on the mass of a top partner, excluding a mass of 275 GeV for a
neutral particle mass less than 50 GeV and a mass of 300 GeV for a neutral
particle mass less than 10 GeV. Using the reconstructed ttbar mass spectrum,
limits are set on the production cross-section times branching ratio to ttbar
for narrow and wide resonances. For narrow Z' models, the observed 95% C.L.
limits range from approximately 38 pb to 3.2 pb for masses going from m_Z' =
500 GeV to m_Z' = 1300 GeV. In Randall-Sundrum models, Kaluza-Klein gluons with
masses below 650 GeV are excluded at 95% C.L.
