  We use signal enhancement techniques and a matched filter analysis to search
for the K band spectroscopic absorption signature of the close orbiting
extrasolar giant planet, HD 189733b. With timeseries observations taken with
NIRSPEC at the Keck II telescope, we investigate the relative abundances of H2O
and carbon bearing molecules, which have now been identified in the dayside
spectrum of HD 189733b. We detect a candidate planet signature with a low level
of significance, close to the ~153 km/s velocity amplitude of HD 189733b.
However, some systematic variations, mainly due to imperfect telluric line
removal, remain in the residual spectral timeseries in which we search for the
planetary signal. The robustness of our candidate signature is assessed,
enabling us to conclude that it is not possible to confirm the presence of any
planetary signal which appears at Fp/F* contrasts deeper than the 95.4 per cent
confidence level. Our search does not enable us to detect the planet at a
contrast ratio of Fp/F* = 1/1920 with 99.9 per cent confidence. We also
investigate the effect of model uncertainties on our ability to reliably
recover a planetary signal. The use of incorrect temperature, model opacity
wavelengths and model temperature-pressure profiles have important consequences
for the least squares deconvolution procedure that we use to boost the S/N
ratio in our spectral timeseries observations. We find that mismatches between
the empirical and model planetary spectrum may weaken the significance of a
detection by ~30-60 per cent, thereby potentially impairing our ability to
recover a planetary signal with high confidence.
