We report high-resolution (0."3-2") interferometric IRAM Plateau de Bure
Interferometer observations of HCN, HCO+, and HNC emission in the
high-velocity molecular outflow of the nearby quasar Mrk 231. We find
that the three species have distinct, and differing, velocity structure
consistent with chemical differentiation in the outflow. The most
significant clump of each species appears in the same order in the red
and the blue outflow (HCO+ at 600 and -450 km/s); (HCN at 500 and -400
km/s); (HNC at 350 km/s and -340 km/s). As the outflow appears to slow
down with increasing radius, it suggests that the HCO+ gas is enhanced
close to the nucleus and HNC farthest out in the outflow. A possible
interpretation is that HCN emission is tracing the recently ejected gas
which is interacting with the surrounding gas in a shock, HNC is
primarily found in the slower post-shock gas, while HCO+ is found in
faster gas closer to the nucleus before the shock. It is interesting to
note that (again under the assumption of a decelerating outflow) the
HCO+ enhancement is closest to the AGN - in contrast to more common
interpretations of the HCN/HCO+ line ratio. In other outflows
HCN-enhancements have been found (e.g. M51, NGC1068) and the enhancement
is attributed to the interaction with a radio jet. If jet interaction
occurs in Mrk 231 and is causing the HCN peak - it appears to happen a
distance away from the nucleus.