  We study the temporal evolution of the habitable zone (HZ) of low-mass stars
- only due to stellar evolution - and evaluate the related uncertainties. These
uncertainties are then compared with those due to the adoption of different
climate models. We computed stellar evolutionary tracks from the pre-main
sequence phase to the helium flash at the red-giant branch tip for stars with
masses in the range [0.70 - 1.10] Msun, metallicity Z in the range [0.005 -
0.04], and various initial helium contents. We evaluated several
characteristics of the HZ, such as the distance from the host star at which the
habitability is longest, the duration of this habitability, the width of the
zone for which the habitability lasts one half of the maximum, and the
boundaries of the continuously habitable zone (CHZ) for which the habitability
lasts at least 4 Gyr. We developed analytical models, accurate to the percent
level or lower, which allowed to obtain these characteristics in dependence on
the mass and the chemical composition of the host star. The metallicity of the
host star plays a relevant role in determining the HZ. The initial helium
content accounts for a variation of the CHZ boundaries as large as 30% and 10%
in the inner and outer border. The computed analytical models allow the first
systematic study of the variability of the CHZ boundaries that is caused by the
uncertainty in the estimated values of mass and metallicity of the host star.
An uncertainty range of about 30% in the inner boundary and 15% in the outer
one were found. We also verified that these uncertainties are larger than that
due to relying on recently revised climatic models, which leads to a CHZ
boundaries shift within 5% with respect to those of our reference scenario. We
made an on-line tool available that provides both HZ characteristics and
interpolated stellar tracks.
