Very deep spectroscopy of the bright Saturn nebula NGC 7009 - I. Observations and plasma diagnostics

We present very deep CCD spectrum of the bright, medium-excitation planetary nebula NGC 7009, with a wavelength coverage from 3040 to 11 000 Å. Traditional emission line identification is carried out to identify all the emission features in the spectra, based on the available laboratory atomic transition data. Since the spectra are of medium resolution, we use multi-Gaussian line profile fitting to deblend faint blended lines, most of which are optical recombination lines (ORLs) emitted by singly ionized ions of abundant second-row elements such as C, N, O and Ne. Computer-aided emission-line identification, using the code EMILI developed by Sharpee et al., is then employed to further identify all the emission lines thus obtained. In total about 1200 emission features are identified, with the faintest ones down to fluxes 10^-4 of Hβ. The flux errors for all emission lines, estimated from multi-Gaussian fitting, are presented. Plots of the whole optical spectrum, with identified emission lines labelled, are presented along with the results of multi-Gaussian fits.

Of all the properly identified emission lines, permitted lines contribute 81 per cent to the total line number. More than 200 O II permitted lines are presented, as well as many others from N II and Ne II. Due to its relatively simple atomic structure, C II presents few lines. Within the flux range 10^-2-10^-4 Hβ where most permitted lines of C II, N II, O II and Ne II fall, the average flux measurement uncertainties are about 10-20 per cent. Comparison is also made of the number of emission lines identified in the current work of NGC 7009 and those of several other planetary nebulae (PNe) that have been extensively studied in the recent literature, and it shows that our line-deblending procedure increases the total line number significantly, especially for emission lines with fluxes lower than 10^-3 of Hβ. Higher resolution is still needed to obtain more reliable fluxes for those extremely faint emission lines, lines of fluxes of the order of 10^-5-10^-6 of Hβ.

Plasma diagnostics using optical forbidden line ratios give an average electron temperature of 10 020 K, which agrees well with previous results of the same object. The average electron density of NGC 7009 derived from optical forbidden line ratios is 4290 cm^-3. The [O III] λ4959/λ4363 nebular-to-auroral line ratio yields an electron temperature of 9800 K. The ratio of the nebular continuum Balmer discontinuity at 3646 Å to H II reveals an electron temperature of 6500 K, about 600 K lower than the measurements published in the literature. The Balmer decrement reveals a density of about 3000 cm^-3. Also derived are electron temperatures from the He I line ratios, and a value of 5100 K from the λ7281/λ6678 ratio is adopted. Utilizing the effective recombination coefficients newly available, we find an electron temperature around 1000 K from the O II ORL spectrum. Thus the general pattern of electron temperatures, T_e([O III]) ≳T_e(H I BJ) ≳T_e(He I) ≳T_e(O II), which is seen in many PNe, is repeated in NGC 7009. Far-infrared fine-structure lines, with observed fluxes adopted from the literature, are also used to derive T_e and N_e. The [O III] (52 + 88 μm)/λ4959 line ratio gives an electron temperature of 9260 K, and the 52 μm/88 μm ratio yields an electron density of 1260 cm^-3.