  We have revisited the search for carbon monoxide absorption features in
transmission during the transit of the extrasolar planet HD 209458b. We
acquired 1077 high resolution spectra at 2 microns using NIRSPEC on Keck II
during three transits. Our sensitivity is sufficient to test the degree of CO
absorption in the first overtone bands during transit, based on plausible
models of the planetary atmosphere. We compare to theoretical tangent geometry
absorption spectra, computed by adding height-invariant ad hoc temperature
pertubations to the model atmosphere of Sudarsky et al., and by treating cloud
height as an adjustable parameter. We do not detect CO absorption. Our analysis
indicates a weakening similar to the case of sodium, suggesting that a general
masking mechanism is at work in the planetary atmosphere. If this masking is
provided by high clouds, our analysis defines the maximum cloud top pressure
(i.e., minimum height) as a function of the model atmospheric temperature. For
the relatively hot model used by Charbonneau et al. to interpret their sodium
detection, our CO limit requires cloud tops at or above 3.3 mbar, and these
clouds must be opaque at a wavelength of 2 microns. High clouds comprised of
submicron-sized particles are already present in some models, but may not
provide sufficient opacity to account for our CO result. Cooler model
atmospheres, having smaller atmospheric scale heights and lower CO mixing
ratios, may alleviate this problem to some extent. However, even models 500K
cooler that the Sudarsky et al. model require clouds above the 100 mbar level
to be consistent with our observations. Our null result therefore requires that
clouds exist at an observable level in the atmosphere of HD 209458b, unless
this planet is dramatically colder than current belief.
