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We analyze JWST/NIRSpec observations of the CO rovibrational v=1-0 band at4.67um around the dust-embedded southern active galactic nucleus (AGN) ofNGC3256 (d=40Mpc; L(IR)=10^11.6 Lsun). We classify the CO v=1-0 spectra intothree categories based on the behavior of P- and R-branches of the band: (a)both branches in absorption toward the nucleus; (b) P-R asymmetry (P-branch inemission and R-branch in absorption) along the disk of the galaxy; and (c) bothbranches in emission in the outflow region above and below the disk. In thispaper, we focus on the outflow. The CO v=1-0 emission can be explained by thevibrational excitation of CO in the molecular outflow by the bright mid-IR~4.7um continuum from the AGN up to r~250pc. We model the ratios between theP(J+2) and R(J) transitions of the band to derive the physical properties(column density, kinetic temperature, and CO-to-H2 conversion factor, alpha_CO)of the outflowing gas. We find that the 12CO v=1-0 emission is optically thickfor J<4, while the 13CO v=1-0 emission remains optically thin. From theP(2)/R(0) ratio, we identify a temperature gradient in the outflow from >40K inthe central 100pc to <15K at 250pc sampling the cooling of the molecular gas inthe outflow. We used three methods to derive alpha_CO in eight 100pc (0.5")apertures in the outflow by fitting the P(J+2)/R(J) ratios with non-LTE models.We obtain low median alpha_CO factors (0.40-0.61) x 3.2e-4/[CO/H2] Msun (Kkm/s/pc2)^-1 in the outflow regions. This implies that outflow rates andenergetics might be overestimated if a ULIRG-like alpha_CO, which is 1.3-2times larger, is assumed. The reduced alpha_CO can be explained if theoutflowing molecular clouds are not virialized. We also report the firstextragalactic detection of a broad (sigma=0.0091um) spectral feature at 4.645umassociated with aliphatic deuterium on polycyclic aromatic hydrocarbons(D_n-PAH).

The CO-to-H2 conversion factor of molecular outflows. Rovibrational CO emission in NGC 3256-S resolved by JWST/NIRSpec