Kinematics of the high-excitation HH 890 jet in the Rosette Nebula

Photoionized jets immersed in HII regions display special properties, which made them a distinctive category of Herbig-Haro (HH) flows. Detailed studies of such jet systems became one of the key issues in our understanding of jet production and evolution. HH 890, initially called the Rosette HH2 jet, is the second photoionized jet discovered in the spectacular HII region of the Rosette Nebula. Contrary to conventional impres-sions of a jet, its discrete components are found to be unexpectedly broad and spatially detached from the proposed energy source. The jet displays additional unusual features which point to the disputable nature of the system. Here, we investigate the kinematics of the jet through high-quality echelle spectrograms. It is distinctively resolved into a fast component with a mean approaching velocity of-39 km s-1 with respect to the systemic rest frame and a slow component with radial velocity centered at -9 km s-1. The slow component indicates an apparently larger dispersion in radial velocity in various emission lines and is likely dissolving at roughly the speed of sound, which favors a photoevap-orated origin. The SII] doublet ratios indicate an electron density of～1.1×103 cm-3in the collimated jet and ～9×102cm-3 in the HII region. This, along with the diffuse appearance of the extensive part of the jet, leads to a dissipation of the jet in the fully ionized medium of Rosette. In addition, time series of photometric observations provide evidence for remarkable light variations of the energy source. Its amplitudes of variation amount to1 mag in both R and I, which is commensurate with the young evolutionary status of the source as indicated by a red, late type optical spectrum.