  The four directly imaged planets orbiting the star HR 8799 are an ideal
laboratory to probe atmospheric physics and formation models. We present more
than a decade's worth of Keck/OSIRIS observations of these planets, which
represent the most detailed look at their atmospheres to-date by its resolution
and signal to noise ratio. We present the first direct detection of HR 8799 d,
the second-closest known planet to the star, at moderate spectral resolution
with Keck/OSIRIS (K-band; R~4,000). Additionally, we uniformly analyze new and
archival OSIRIS data (H and K band) of HR 8799 b, c, and d. First, we show
detections of water (H2O) and carbon monoxide (CO) in the three planets and
discuss the ambiguous case of methane (CH4) in the atmosphere of HR 8799b.
Then, we report radial velocity (RV) measurements for each of the three
planets. The RV measurement of HR 8799 d is consistent with predictions made
assuming coplanarity and orbital stability of the HR 8799 planetary system.
Finally, we perform a uniform atmospheric analysis on the OSIRIS data,
published photometric points, and low resolution spectra. We do not infer any
significant deviation from to the stellar value of the carbon to oxygen ratio
(C/O) of the three planets, which therefore does not yet yield definitive
information about the location or method of formation. However, constraining
the C/O ratio for all the HR 8799 planets is a milestone for any multiplanet
system, and particularly important for large, widely separated gas giants with
uncertain formation processes.
