Analysis of chemical abundances in planetary nebulae with [WC] central stars

I. Line intensities and physical conditions^⋆,⋆⋆

¹ Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
e-mail: jogarcia@iac.es; amd@iac.es; chris.morisset@gmail.com;
² Departamento de Astrofísica. Universidad de La Laguna, 38205 La Laguna, Tenerife, Spain
³ Instituto de Astronomía, Universidad Nacional Autónoma de México, Apdo. Postal 70264, 04510 Méx. D. F., Mexico
e-mail: miriam@astro.unam.mx
⁴ Departamento de Astronomía, Universidad de Chile, Casilla 36 D, Las Condes, Santiago, Chile
e-mail: mtruiz@das.uchile.cl

Received: 6 October 2011
Accepted: 16 November 2011

Abstract

Context. Planetary nebulae (PNe) around Wolf-Rayet [WR] central stars ([WR]PNe) constitute a particular photoionized nebula class that represents about 10% of the PNe with classified central stars.

Aims. We analyse deep high-resolution spectrophotometric data of 12 [WR] PNe. This sample of [WR]PNe represents the most extensive analysed so far, at such high spectral resolution. We aim to select the optimal physical conditions in the nebulae to be used in ionic abundance calculations that will be presented in a forthcoming paper.

Methods. We acquired spectra at Las Campanas Observatory with the 6.5-m telescope and the Magellan Inamori Kyocera (MIKE) spectrograph, covering a wavelength range from 3350 Å to 9400 Å. The spectra were exposed deep enough to detect, with signal-to-noise ratio higher than three, the weak optical recombination lines (ORLs) of O ii, C ii, and other species. We detect and identify about 2980 emission lines, which, to date, is the most complete set of spectrophotometric data published for this type of objects. From our deep data, numerous diagnostic line ratios for T_e and n_e are determined from collisionally excited lines (CELs), ORLs, and continuum measurements (H i Paschen continuum in particular).

Results. Densities are closely described by the average of all determined values for objects with n_e  <  10⁴ cm^-3, and by n_e([Cl iii]) for the densest objects. For some objects, n_e([Ar iv]) is adopted as the characteristic density of the high ionization zone. For T_e, we adopt a three-zone ionization scheme, where the low ionization zone is characterised by T_e([N ii]), the medium ionization zone by T_e([O iii]), and the highest ionization one by T_e([Ar iv]) when available. We compute T_e from the H i Paschen discontinuity and from He i lines. For each object, T_e(H i) is, in general, consistent with T_e derived from CELs, although it has a very large error. Values of T_e(He i) are systematically lower than the T_e derived from CELs. When comparing T_e(H i) and T_e(He i) it is unclear whether the behaviour of both temperatures agrees with the predictions of the temperature fluctuations paradigm, owing to the large errors in T_e(H i). We do not find any evidence of low-temperature, high-density clumps in our [WR]PNe from the analysis of faint O ii and N ii plasma diagnostics, although uncertainties dominate the observed line ratios in most objects. The behaviour of T_e([O iii])/T_e([N ii]), which is smaller for high ionization degrees, can be reproduced by a set of combined matter-bounded and radiation-bounded models, although, for the smallest temperature ratios, a too high metallicity seem to be required.