  Ultraviolet (UV) photodissociation of CO controls the abundances and
distribution of CO and its photodissociation products. This significantly
influences the gas-phase chemistry in the circumstellar envelope (CSE) around
evolved stars. A better understanding of CO photodissociation in outflows also
provides a more precise estimate of mass-loss rates. We aim to update the CO
photodissociation rate in an expanding spherical envelope assuming that the
interstellar radiation field (ISRF) photons penetrate through the envelope.
This will allow us to precisely estimate the CO abundance distributions in CSEs
around evolved stars. We used the most recent CO spectroscopic data to
precisely calculate the depth dependency of the photodissociation rate of each
CO dissociating line. We calculated the CO self- and mutual-shielding functions
in an expanding envelope. We investigated the dependence of the CO profile on
the five fundamental parameters mass-loss rate, the expansion velocity, the CO
initial abundance, the CO excitation temperature, and the strength of the ISRF.
Our derived CO envelope size is smaller than the commonly used radius derived
by Mamon et al. 1988. The difference between results varies from 1% to 39% and
depends on the H2 and CO densities of the envelope. We list two fitting
parameters for a large grid of models to estimate the CO abundance
distribution. We demonstrate that the CO envelope size can differ between
outflows with the same effective content of CO, but different CO abundance,
mass-loss rate, and the expansion velocity as a consequence of differing
amounts of shielding by H2 and CO. We find that the abundance of CO close to
the star and the outflow density both can have a significant effect on the size
of the molecular envelope. We also demonstrate that modest variations in the
ISRF can cause measurable differences in the envelope extent.
