  [Abridged] We present 2.12-2.23 um high contrast integral field spectroscopy
of the extrasolar planet HR 8799 b. Our observations were obtained with OSIRIS
on the Keck II telescope and sample the 2.2 um CH4 feature, which is useful for
spectral classification and as a temperature diagnostic for ultracool objects.
The spectrum of HR 8799 b is relatively featureless, with little or no methane
absorption, and does not exhibit the strong CH4 seen in T dwarfs of similar
absolute magnitudes. Overall, we find that HR 8799 b has a spectral type
consistent with L5-T2, although its SED is atypical compared to most field
objects. We fit the 2.2 um spectrum and the infrared SED using the Hubeny &
Burrows, Burrows et al., and Ames-Dusty model atmosphere grids, which
incorporate nonequilibrium chemistry, non-solar metallicities, and clear and
cloudy variants. No models agree with all of the data, but those with
intermediate clouds produce significantly better fits. The largest discrepancy
occurs in the J-band, which is highly suppressed in HR 8799 b. The best-fitting
effective temperatures range from 1300-1700 K with radii between ~0.3-0.5 RJup.
These values are inconsistent with evolutionary model-derived values of 800-900
K and 1.1-1.3 RJup based on the luminosity of HR 8799 b and the age of HR 8799,
a discrepancy that probably results from imperfect atmospheric models or the
limited range of physical parameters covered by the models. The low temperature
inferred from evolutionary models indicates that HR 8799 b is ~400 K cooler
than field L/T transition objects, providing further evidence that the L/T
transition is gravity-dependent. With an unusually dusty photosphere, an
exceptionally low luminosity for its spectral type, and hints of extreme
secondary physical parameters, HR 8799 b appears to be unlike any class of
field brown dwarf currently known.
