Faint recombination lines in Galactic PNe with a [WC] nucleus

Abstract

Aims. We present spatially resolved high-resolution spectrophotometric data for the planetary nebulae PB 8, NGC 2867, and PB 6. We have analyzed two knots in NGC 2867 and PB 6 and one in PB 8. The three nebulae are ionized by [WC] type nuclei: early [WO] for PB 6 and NGC 2867 and [WC 5-6] in the case of PB 8. Our aim is to study the behavior of the abundance discrepancy problem (ADF) in this type of planetary nebula. Methods. We measured a large number of optical recombination (ORL) and collisionally excited lines (CEL), from different ionization stages (many more than in any previous work), thus, we were able to derive physical conditions from many different diagnostic procedures. We determined ionic abundances from the available collisionally excited and recombination lines. Based on both sets of ionic abundances, we derived total chemical abundances in the nebulae using suitable ionization correction factors. Results. From CELs, we have found abundances typical of Galactic disk planetary nebulae. Moderate ADF(O ++ ) were found for PB 8 (2.57) and NGC 2867 (1.63). For NGC 2867, abundances from ORLs are higher but still consistent with Galactic disk planetary nebulae. On the contrary, PB 8 presents a very high O/H ratio from ORLs. A high C/O was obtained from ORLs for NGC 2867; this ratio is similar to C/O obtained from CELs and with the chemical composition of the wind of the central star, indicating that there was no further C-enrichment in the star, relative to O, after the nebular material ejection. On the contrary, we found C/O < 1 in PB 8. Interestingly, we obtain (C/O) ORLs /(C/O) CELs < 1 in PB 8 and NGC 2867; this added to the similarity between the heliocentric velocities measured in [O iii] and O ii lines for our three objects argue against the presence of H-deficient metal-rich knots coming from a late thermal pulse event.

The central star of the planetary nebula PB 8: a Wolf-Rayet-type wind of an unusual WN/WC chemical composition

“…Moreover, the electron density in PB 8 measured by García-Rojas et al (2009), n e = 2550 ± 550 cm −3 is typical for young planetary nebulae, but several times higher than found in ring nebulae (Mathis et al 1992).…”

Chemical abundances in Galactic planetary nebulae withSpitzerspectra

“…If these discrepancies are produced by temperature fluctuations (Peimbert 1967) and they are moderate, then they would affect most of the elements in a similar way (see e.g. García-Rojas et al 2009. On the other hand, if the discrepancy is due to the presence of chemical inhomogeneities, our analysis would not be affected, since the bulk of the emission of the PN is coming from CELs (Liu et al 2006).…”

s-process enrichment in the planetary nebula NGC 3918. Results from deep echelle spectrophotometry

“…We also detect several forbidden and semi-forbidden lines from ions such as [N i] The depth of our spectra allows us to detect lines as fainter as 10 −5 ×I(Hβ). The identification and adopted laboratory wavelengths of the lines are based on several previous identifications in the literature (see Baluteau et al 1995;Clegg et al 1987;García-Rojas et al 2009Péquignot & Baluteau 1994;Sharpee et al 2007;Zhang et al 2005, and references therein). We also made use of Peter van Hoof's atomic line list v2.05B18 2 .…”

“…Owing to the high ionization of NGC 3918, we detect lines of C ii, C iii and C iv that are excited by pure recombination and therefore are suitable for abundance determinations (see García-Rojas et al 2009, and references therein). In Table 10 we show the ionic abundances from recombination lines of C. The results obtained for the different multiplets of each given ion clearly show the excellent agreement between the different lines, strengthening the hypothesis of the recombination origin of these lines.…”