1995A&A...303..175B

Deep CCD spectra of the planetary nebula NGC 7027, taken over the range 654-1046nm at a spectral resolution of order 0.1nm, are presented. About 465 different lines are detected and intensities are provided for 405 features. A total of 680 emission lines is considered. In the best conditions, namely shortward of 900nm and outside the telluric absorption bands, emission lines can be detected and measured down to a de-reddened intensity of (1.5-2.5)x10^–5 relative to Hβ. The rate of line detection and identification achieved is a one-order-of-magnitude improvement over previous far-red spectroscopic surveys of nebulae, now superseding available optical surveys. Many of these lines have not been seen before. Altogether, the permitted and forbidden lines encompass 47 ions from 22 elements in this spectral range. Forbidden lines are produced by 31 ions from 18 elements. Iron-peak elements, notably chromium, manganese, iron, nickel, copper, and possibly vanadium, as well as heavy elements, notably krypton, xenon, selenium, bromine and strontium are detected. Identification of many permitted lines from high- and low-ionization stages of carbon, nitrogen, and oxygen is reported. Extrapolation of known energy levels is required in some cases. C II, C III, and C IV series discovered in the far-red can be followed up to large principal quantum numbers, providing a clue to explain optical lines, left unidentified for tens of years. Available compilations of high-nl atomic energy levels may not be sufficiently comprehensive to fully exploit the nebular spectroscopic data now accessible to common astrophysical devices. The quadrupolar line [S III]882.98nm is identified. The natural broadening of the upper autoionizing level of C II 879.4 is disclosed by the peculiar profile of the line. The importance of dielectronic recombination of C II through quartet terms is confirmed by the presence of the strong multiplet C II(14). The intensity and decrement of the Paschen lines suggest a markedly uneven distribution of dust absorption across the nebula. A diffuse interstellar band and K I lines are detected in absorption. About 13% of all lines are not identified, a majority of them being weak and affected by large errors. Only 4 of them exceed a de-reddened intensity of 3x10^–4I_β. Some 15 lines are left unidentified, which either exceed a de-reddened intensity of 10^–4I_β or are measured with an uncertainty smaller than 20%. Half these unidentified lines are affected by telluric absorption and only 6 fulfill both criteria.