How does this paper define the habitable zone?
Chapter 10 of the paper, which can be downloaded though the link above,
explains their concept of the habitable zone.
The core sentences are:
There is general agreement that the HZ is defined as the planet-star distance range
within which liquid water can exist on a planet’s surface. The surface temperature of
a planet is a function of stellar luminosity, albedo, greenhouse effect, eccentricity,
obliquity, rotation rate, and geologic age. Of these, only the first parameter can be
estimated for the Kepler planets. To encompass the effect of the remaining parame-
ters, this paper adopts three ranges that have been proposed to date ...
They are followed by examples and formulas for wide, average and narrow
habitable zone models.