  Comets have been invoked in numerous studies as a potentially important
source of dust and gas around stars, but none has studied the thermo-physical
evolution, out-gassing rate, and dust ejection of these objects in such stellar
systems. We investigate the thermo-physical evolution of comets in
exo-planetary systems in order to provide valuable theoretical data required to
interpret observations of gas and dust. We use a quasi 3D model of cometary
nucleus to study the thermo-physical evolution of comets evolving around a
single star from 0.1 to 50 AU, whose homogeneous luminosity varies from 0.1 to
70 solar luminosities. This paper provides mass ejection, lifetimes, and the
rate of dust and water gas mass productions for comets as a function of the
distance to the star and stellar luminosity. Results show significant physical
changes to comets at high stellar luminosities. The models are presented in
such a manner that they can be readily applied to any planetary system. By
considering the examples of the Solar System, Vega and HD 69830, we show that
dust grains released from sublimating comets have the potential to create the
observed (exo)zodiacal emission. We show that observations can be reproduced by
1 to 2 massive comets or by a large number of comets whose orbits approach
close to the star. Our conclusions depend on the stellar luminosity and the
uncertain lifetime of the dust grains. We find, as in previous studies, that
exozodiacal dust disks can only survive if replenished by a population of
typically sized comets renewed from a large and cold reservoir of cometary
bodies beyond the water ice line. These comets could reach the inner regions of
the planetary system following scattering by a (giant) planet.
