In section 6. Interstellar and circumstellar matter
An imaging spectroscopic survey of the planetary nebula NGC 7009 with MUSE
This paper presents a comprehensive and remarkable application of integral field spectroscopic methods to the bright, O-rich iconic planetary nebula, NGC 7009 during the science verification observation with Multi Unit Spectroscopic Explorer (MUSE) ion VLT. The wide-field mode, with an FOV of about 1 arcmin^2, is an ideal match for this target. The authors present density, ionization, and abundance maps at a spatial resolution of about 0.7 arcsec (from seeing), although kinematic information cannot be obtained because the lines were unresolved in velocity (resolution of about 100 km/s). The spectral coverage was 4750-9300A, covering lines from three ionization stages of oxygen, two of sulfur (forbidden lines of S II and III), He I and II, Balmer and Paschen recombination lines), and a host of other forbidden lines, for instance [Mn V], [Cl III], [Ar II], and [N II], and even relatively weak recombination lines, for example, C II and N II. The authors present electron temperature and density maps using line ratio diagnostics and individual lines with different sensitivities; the Paschen jump is also used to obtain continuum measures of the plasma conditions (the appendix will be especially useful to future observers). Neutral helium line optical depth maps are also shown. Significant fine structure appears from comparisons among the different temperature and density maps. The authors show that the He abundance is constant to within about 2\% and that N/O varies considerably, although these still require ionization correction factors. They also compare integrated spectra with photoionization models. This pathfinder study demonstrates the unique and extensive capabilities of IFS observations to unravel complex regions and, extended to targets with larger velocity gradients, even to obtain three dimensional structures.