  Systems of close-in super-Earths display striking diversity in planetary bulk
density and composition. Giant impacts are expected to play a role in the
formation of many of these worlds. Previous works, focused on the mechanical
shock caused by a giant impact, have shown that these impacts can eject large
fractions of the planetary envelope, offering a partial explanation for the
observed spread in exoplanet compositions. Here, we examine the thermal
consequences of giant impacts, and show that the atmospheric loss caused by
these effects can significantly exceed that caused by mechanical shocks for
hydrogen-helium (H/He) envelopes. When a giant impact occurs, part of the
impact energy is converted into thermal energy, heating the rocky core and the
envelope. We find that the ensuing thermal expansion of the envelope can lead
to a period of sustained, rapid mass loss through a Parker wind, resulting in
the partial or complete erosion of the H/He envelope. The fraction of the
envelope lost depends on the planet's orbital distance from its host star and
its initial thermal state, and hence age. Planets closer to their host stars
are more susceptible to thermal atmospheric loss triggered by impacts than ones
on wider orbits. Similarly, younger planets, with rocky cores which are still
hot and molten from formation, suffer greater atmospheric loss. This is
especially interesting because giant impacts are expected to occur
$10{-}100~\mathrm{Myr}$ after formation. For planets where the thermal energy
of the core is much greater than the envelope energy, the impactor mass
required for significant atmospheric removal is $M_\mathrm{imp} / M_p \sim \mu
/ \mu_c \sim 0.1$, approximately the ratio of the heat capacities of the
envelope and core. When the envelope energy dominates the total energy budget,
complete loss can occur when the impactor mass is comparable to the envelope
mass.
