  Molecular species in planetary atmospheres provide key insights into their
atmospheric processes and formation conditions. In recent years,
high-resolution Doppler spectroscopy in the near-infrared has allowed
detections of H$_2$O and CO in the atmospheres of several hot Jupiters. This
method involves monitoring the spectral lines of the planetary thermal emission
Doppler-shifted due to the radial velocity of the planet over its orbit.
However, aside from CO and H$_2$O, which are the primary oxygen- and
carbon-bearing species in hot H$_2$-rich atmospheres, little else is known
about molecular compositions of hot Jupiters. Several recent studies have
suggested the importance and detectability of nitrogen-bearing species in such
atmospheres. In this Letter, we confirm potential detections of CO and H$_2$O
in the hot Jupiter HD 209458b using high-resolution spectroscopy. We also
report a cross-correlation peak with a signal-to-noise ratio of $4.7$ from a
search for HCN. The results are obtained using high-resolution phase-resolved
spectroscopy with the Very Large telescope CRyogenic high-resolution InfraRed
Echelle Spectrograph (VLT CRIRES) and standard analysis methods reported in the
literature. A more robust treatment of telluric contamination and other
residuals would improve confidence and enable unambiguous molecular detections.
The presence of HCN could provide constraints on the C/O ratio of HD~209458b
and its potential origins.
