  Identifying terrestrial planets in the habitable zones (HZs) of other stars
is one of the primary goals of ongoing radial velocity and transit exoplanet
surveys and proposed future space missions. Most current estimates of the
boundaries of the HZ are based on 1-D, cloud-free, climate model calculations
by Kasting et al.(1993). The inner edge of the HZ in Kasting et al.(1993) model
was determined by loss of water, and the outer edge was determined by the
maximum greenhouse provided by a CO2 atmosphere. A conservative estimate for
the width of the HZ from this model in our Solar system is 0.95-1.67 AU.
  Here, an updated 1-D radiative-convective, cloud-free climate model is used
to obtain new estimates for HZ widths around F, G, K and M stars. New H2O and
CO2 absorption coefficients, derived from the HITRAN 2008 and HITEMP 2010
line-by-line databases, are important improvements to the climate model.
According to the new model, the water loss (inner HZ) and maximum greenhouse
(outer HZ) limits for our Solar System are at 0.99 AU and 1.70 AU,
respectively, suggesting that the present Earth lies near the inner edge.
Additional calculations are performed for stars with effective temperatures
between 2600 K and 7200 K, and the results are presented in parametric form,
making them easy to apply to actual stars. The new model indicates that, near
the inner edge of the HZ, there is no clear distinction between runaway
greenhouse and water loss limits for stars with T_{eff} ~< 5000 K which has
implications for ongoing planet searches around K and M stars. To assess the
potential habitability of extrasolar terrestrial planets, we propose using
stellar flux incident on a planet rather than equilibrium temperature. Our
model does not include the radiative effects of clouds; thus, the actual HZ
boundaries may extend further in both directions than the estimates just given.
