We present deep optical spectra of the archetypal young planetary nebula ( PN ) NGC 7027 , covering a wavelength range from 3310 to 9160 Å .
The observations were carried out by uniformly scanning a long slit across the entire nebular surface , thus yielding average optical spectra for the whole nebula .
A total of 937 emission features are detected .
The extensive line list presented here should prove valuable for future spectroscopic analyses of emission line nebulae .
The optical data , together with the archival IUE and ISO spectra , are used to probe the temperature and density structures and to determine the elemental abundances from lines produced by different excitation mechanisms .
Electron temperatures have been derived from the hydrogen recombination Balmer jump ( BJ ) , from ratios of He i optical recombination lines ( ORLs ) and from a variety of diagnostic ratios of collisionally excited lines ( CELs ) .
Electron densities have been determined from the intensities of high-order H i Balmer lines and of He ii Pfund lines , as well as from a host of CEL diagnostic ratios .
CEL and ORL diagnostics are found to yield compatible results .
Adopting respectively electron temperatures of T _ { e } = 12 600 and 15 500 K for ions with ionization potentials lower or higher than 50 eV and a constant density of N _ { e } = 47 000 cm ^ { -3 } , elemental abundances have been determined from a large number of CELs and ORLs .
The C ^ { 2 + } /H ^ { + } , N ^ { 2 + } /H ^ { + } , O ^ { 2 + } /H ^ { + } and Ne ^ { 2 + } /H ^ { + } ionic abundance ratios derived from ORLs are found to be only slightly higher than the corresponding CEL values .
We conclude that whatever mechanism is causing the BJ/CEL temperature discrepanies and the ORL/CEL abundance discrepancies that have been observed in many PNe , it has an insignificant effect on this bright young compact PN .
The properties of the central star are also discussed .
Based on the integrated spectrum and using the energy-balance method , we have derived an effective temperature of 219 000 K for the ionizing star .
Finally , we report the first detection in the spectrum of this bright young PN of Raman-scattered O vi features at 6830 and 7088 Å , pointing to the existence of abundant neutral hydrogen around the ionized regions .