Context : In recent years , several potentially habitable , probably terrestrial exoplanets and exoplanet candidates have been discovered .
The amount of CO _ { 2 } in their atmosphere is of great importance for surface conditions and habitability .
In the absence of detailed information on the geochemistry of the planet , this amount could be considered as a free parameter .

Aims : Up to now , CO _ { 2 } partial pressures for terrestrial planets have been obtained assuming an available volatile reservoir and outgassing scenarios .
This study aims at calculating the allowed maximum CO _ { 2 } pressure at the surface of terrestrial exoplanets orbiting near the outer boundary of the habitable zone by coupling the radiative effects of the CO _ { 2 } and its condensation at the surface .
These constraints might limit the permitted amount of atmospheric CO _ { 2 } , independent of the planetary reservoir .

Methods : A 1D radiative-convective cloud-free atmospheric model was used to calculate surface conditions for hypothetical terrestrial exoplanets .
CO _ { 2 } partial pressures are fixed according to surface temperature and vapor pressure curve .
Considered scenarios cover a wide range of parameters , such as gravity , central star type and orbital distance , atmospheric N _ { 2 } content and surface albedo .

Results : Results show that for planets in the habitable zone around K- , G- , and F-type stars the allowed CO _ { 2 } pressure is limited by the vapor pressure curve and not by the planetary reservoir .
The maximum CO _ { 2 } pressure lies below the CO _ { 2 } vapor pressure at the critical point of p _ { crit } = 73.8 bar .
For M-type stars , due to the stellar spectrum being shifted to the near-IR , CO _ { 2 } pressures above p _ { crit } are possible for almost all scenarios considered across the habitable zone .
This implies that determining CO _ { 2 } partial pressures for terrestrial planets by using only geological models is probably too simplified and might over-estimate atmospheric CO _ { 2 } towards the outer edge of the habitable zone .

Conclusions :