2019ApJ...870...42R

We present spatial- and velocity-resolved spectroscopy of NGC 7009 acquired with the UVES spectrograph at the VLT UT2/Kueyen. We use these data to determine the structure of the electron temperature and electron density based upon O II permitted (recombination) lines. We find a strong gradient in the O II-based electron temperature. It agrees with the electron temperature determined by forbidden (collisionally excited) lines in part of the nebular volume, but also differs by more than 6000 K in other parts of the nebular volume. This result supports the hypothesis that NGC 7009 contains two plasma components, one of which emits both forbidden and permitted lines and the other that emits only permitted lines. For the component that emits only permitted lines, we find a lower limit to the electron density of 10⁴ cm^–3 from the O II permitted lines, which is higher than derived from forbidden lines. We are unable to determine whether the two plasma components are in pressure equilibrium from our data, but there exist temperature and density combinations that allow this equilibrium for temperatures between 600 and 6000 K. For most of the temperature and density conditions allowed for the component that emits only permitted lines, its mass of O²⁺ is less than that of the plasma component that emits forbidden lines.