  The orbital, which represents the shape of the electron cloud, very often
strongly influences the manifestation of various exotic phenomena, e.g.,
magnetism, metal-insulator transition, colossal magnetoresistance,
unconventional superconductivity etc. in solid-state systems. The observation
of the antiferromagnetism in $RE$TiO$_3$ ($RE$=rare earth) series has been
puzzling since the celebrated Kugel-Khomskii model of spin-orbital super
exchange predicts ferromagnetism in an orbitally degenerate $d^1$ systems.
Further, the existence of the orbitally ordered vs. orbital liquid phase in
both antiferromagnetic and paramagnetic phase have been unsettled issues thus
far. To address these long-standing questions, we investigate single
crystalline film of PrTiO$_3$. Our synchrotron X-ray diffraction measurements
confirm the retention of bulk-like orthorhombic ($D_{2h}$) symmetry in the thin
film geometry. We observe similar X-ray linear dichroism signal in both
paramagnetic and antiferromagnetic phase, which can be accounted by ferro
orbital ordering (FOO). While the presence of $D_{2h}$ crystal field does not
guarantee lifting of orbital degeneracy always, we find it to be strong enough
in these rare-earth titanates, leading to the FOO state. Thus, our work
demonstrates the orthorhombic distortion is the driving force for the orbital
ordering of antiferromagnetic $RE$TiO$_3$.
