We present deep optical spectra of 23 galactic planetary nebulae, which are analysed in conjunction with archival infrared and ultraviolet spectra. We derive nebular electron temperatures based on standard collisionally excited line (CEL) diagnostics as well as the hydrogen Balmer jump and find that, as expected, the Balmer jump almost always yields a lower temperature than the [O III] nebular-to-auroral line ratio. We also make use of the weak temperature dependence of helium and O II recombination line ratios to further investigate the temperature structure of the sample nebulae. We find that, in almost every case, the derived temperatures follow the relation T e (CEL)T e (BJ) T e (He I) T e (O II), which is the relation predicted by two-component nebular models in which one component is cold and hydrogen-deficient. T e (O II) may be as low as a few hundred Kelvin, in line with the low temperatures found for the hydrogen-deficient knots of Abell 30 by Wesson, Liu and Barlow.Elemental abundances are derived for the sample nebulae from both CELs and optical recombination lines (ORLs). ORL abundances are higher than CEL abundances in every case, by factors ranging from 1.5 to 12. Five objects with O 2+ abundance discrepancy factors greater than 5 are found. DdDm 1 and Vy 2-2 are both found to have a very large abundance discrepancy factor of 11.8.We consider the possible explanations for the observed discrepancies. From the observed differences between T e (O III) and T e (BJ), we find that temperature fluctuations cannot resolve the abundance discrepancies in 22 of the 23 sample nebulae, implying some additional mechanism for enhancing ORL emission. In the one ambiguous case, the good agreement between abundances derived from temperature-insensitive infrared lines and temperature-sensitive optical lines also points away from temperature fluctuations being present. The observed recombination line temperatures, the large abundance discrepancies and the generally good agreement between infrared and optical CEL abundances all suggest instead the existence of a cold hydrogen-deficient component within the 'normal' nebular gas. The origin of this component is as yet unknown.

We report the discovery of the rotational spectrum of CH

We have analysed deep optical spectra of the ‘born‐again’ planetary nebula Abell 58 and its hydrogen‐deficient knot, surrounding V605 Aql, which underwent a nova‐like eruption in 1919. Our analysis shows that the extinction towards the central knot is much higher than previously thought, with c(Hβ) = 2.0. The outer nebula is less reddened, with c(Hβ) = 1.04. We find that the outer nebula has a Ne/O ratio higher than the average PN value. The electron temperature we derive for the central knot varies widely depending on the diagnostic used. The [O iii] nebular‐to‐auroral transition ratio gives a temperature of 20 800 K, while the ratio of the [N ii] nebular and auroral lines gives Te= 15 200 K. The helium line ratios λ5876/λ4471 and λ6678/λ4471 imply temperatures of 350 and 550 K, respectively. Weakly temperature‐sensitive O ii recombination line ratios imply similarly low electron temperatures. Abundances derived from recombination lines are vastly higher than those found from collisionally excited lines, with the abundance discrepancy factor (ADF) for O2+ reaching 89 – the second highest known value after that found for the hydrogen‐deficient knots in Abell 30. The observed temperature diagnostics and abundances support the idea that, like Abell 30, the knot of Abell 58 contains some very cold ionized material. Although the central star is carbon‐rich (C/O > 1), the knot is found to be oxygen‐rich, a situation not predicted by the single‐star ‘born‐again’ theory of its formation. We compare the known properties of Abell 58 to those of Abell 30, Sakurai's Object and several novae and nova remnants. We argue that the abundances in the ejecta observed in A 30 and A 58 have more in common with neon novae than with Sakurai's Object, which is believed to have undergone a final helium flash. In particular, the C/O ratio of less than unity and the presence of substantial quantities of neon in the ejecta of both Abell 30 and Abell 58 are not predicted by very late thermal pulse models.

No abstract

Abstract. We present the first 3D Monte Carlo photoionisation code to include a fully self-consistent treatment of dust radiative transfer (RT) within the photoionised region. This is the latest development of the recently published pure photoionisation code mocassin (Ercolano et al., 2003a) and it is currently undergoing several benchmarking tests. The preliminary results of these tests are presented in these conference proceedings. The new code provides the ideal tool for the analysis of dusty ionised regions showing asymmetries and/or density and chemical inhomogeneities