The Mrk 231 molecular outflow as seen in OH
  We report on the Herschel/PACS observations of OH in Mrk 231, with detections
in 9 doublets observed within the PACS range, and present radiative transfer
models for the outflowing OH. Signatures of outflowing gas are found in up to 6
OH doublets with different excitation requirements. At least two outflowing
components are identified, one with OH radiatively excited, and the other with
low excitation, presumably spatially extended. Particularly prominent, the blue
wing of the absorption detected in the in-ladder 2Pi_{3/2} J=9/2-7/2 OH doublet
at 65 um, with E_lower=290 K, indicates that the excited outflowing gas is
generated in a compact and warm (circum)nuclear region. Because the excited,
outflowing OH gas in Mrk 231 is associated with the warm, far-IR continuum
source, it is likely more compact (diameter of 200-300 pc) than that probed by
CO and HCN. Nevertheless, its mass-outflow rate per unit of solid angle as
inferred from OH is similar to that previously derived from CO,
>~70x(2.5x10^{-6}/X_{OH}) Msun yr^{-1} sr^{-1}, where X_{OH} is the OH
abundance relative to H nuclei. In spherical symmetry, this would correspond to
>~850x(2.5x10^{-6}/X_{OH}) Msun yr^{-1}, though significant collimation is
inferred from the line profiles. The momentum flux of the excited component
attains ~15 L_{AGN}/c, with an OH column density of (1.5-3)x10^{17} cm^-2 and a
mechanical luminosity of ~10^{11} Lsun. The detection of very excited OH
peaking at central velocities indicates the presence of a nuclear reservoir of
gas rich in OH, plausibly the 130-pc scale circumnuclear torus previously
detected in OH megamaser emission, that may be feeding the outflow. An
exceptional ^{18}OH enhancement, with OH/^{18}OH<~30 at both central and
blueshifted velocities, is likely the result of interstellar-medium processing
by recent starburst/SNe activity.
