  Context. Carbon monoxide (CO) has been detected in all Giant Planets and its
origin is both internal and external in Jupiter and Neptune. Despite its first
detection in Uranus a decade ago, the magnitude of its internal and external
sources remains unconstrained.
  Aims. We targeted CO lines in Uranus in the submillimeter range to constrain
its origin.
  Methods. We recorded disk-averaged spectra of Uranus with a very high
spectral resolution at the frequencies of CO rotational lines in the
submillimeter range in 2011-2012. We used empirical and diffusion models of the
atmosphere of Uranus to constrain the origin of CO. We also used a
thermochemical model of its troposphere to derive an upper limit on the O/H
ratio in the deep atmosphere of Uranus.
  Results. We have detected the CO(8-7) rotational line for the first time,
with Herschel-HIFI. Both empirical and diffusion model results show that CO has
an external origin. An empirical profile in which CO is constant above the
100mbar level with a mole fraction of 7.1-9.0E-9 (depending on the adopted
stratospheric thermal structure) reproduces the data. Sporadic and steady
source models cannot be differentiated with our data. Taking Teanby & Irwin's
internal source model upper limit of a mole fraction of 2.1E-9 [Teanby & Irwin
2013. ApJ, 775, L49], the deep O/H ratio of Uranus is lower than 500 times
solar according to our thermochemical computations.
  Conclusions. Our work shows that the average mole fraction of CO decreases
from the stratosphere to the troposphere and thus strongly advocates for an
external source of CO in Uranus. Photochemical modeling of oxygen species in
the atmosphere of Uranus and more sensitive observations are needed to reveal
the nature of the external source.
