Problems for the WELS classification of planetary nebula central stars: self-consistent nebular modelling of four candidates

MNRAS 458, 2694–2709 (2016) doi:10.1093/mnras/stw468 Advance Access publication 2016 March 1 Problems for the WELS classification of planetary nebula central stars: self-consistent nebular modelling of four candidates Hassan M. Basurah,1 Alaa Ali,1,2‹ Michael A. Dopita,1,3 R. Alsulami,1 Morsi A. Amer1,2 and A. Alruhaili1 1 Astronomy Department, Faculty of Science, King Abdulaziz University, 21589 Jeddah, Saudi Arabia of Astronomy, Faculty of Science, Cairo University, 12613, Egypt 3 Research School of Astronomy and Astrophysics, Australian National University, Cotter Rd., Weston, ACT 2611, Australia 2 Department ABSTRACT We present integral field unit (IFU) spectroscopy and self-consistent photoionization modelling for a sample of four southern Galactic planetary nebulae (PNe) with supposed weak emissionline central stars. The Wide Field Spectrograph on the ANU 2.3 m telescope has been used to provide IFU spectroscopy for NGC 3211, NGC 5979, My 60, and M 4-2 covering the spectral range of 3400–7000 Å. All objects are high-excitation non-Type I PNe, with strong He II emission, strong [Ne V] emission, and weak low-excitation lines. They all appear to be predominantly optically thin nebulae excited by central stars with Teff > 105 K. Three PNe of the sample have central stars which have been previously classified as weak emission-line stars (WELS), and the fourth also shows the characteristic recombination lines of a WELS. However, the spatially resolved spectroscopy shows that rather than arising in the central star, the C IV and N III recombination line emission is distributed in the nebula, and in some cases concentrated in discrete nebular knots. This may suggest that the WELS classification is spurious, and that, rather, these lines arise from (possibly chemically enriched) pockets of nebular gas. Indeed, from careful background subtraction we were able to identify three of the sample as being hydrogen rich O(H)-Type. We have constructed fully self-consistent photoionization models for each object. This allows us to independently determine the chemical abundances in the nebulae, to provide new model-dependent distance estimates, and to place the central stars on the Hertzsprung–Russell diagram. All four PNe have similar initial mass (1.5 < M/M < 2.0) and are at a similar evolutionary stage. Key words: plasmas – ISM: abundances – planetary nebulae: individual: NGC 3211 – planetary nebulae: individual: NGC 5979 – planetary nebulae: individual: My 60 – planetary nebulae: individual: M 4-2. 1 I N T RO D U C T I O N Planetary nebulae (PNe) represent an advanced stage in the evolution of low- and intermediate-mass stars as they make the transition between the asymptotic giant branch (AGB) and the white dwarf (WD) stages. The gaseous nebula which appears now as a PN is the remnant of the deep convective envelope that surrounded the central core of AGB. This core is now revealed as the central star (CS) of the PN. Thus, the PNe provide fundamental data on the mass-loss processes during the AGB stage, the chemical enrichment of the envelope by dredge-up processes, as well as information about the mass, and effective temperature of the remaining stellar core.  E-mail: Up to now, most studies of the emission-line spectra of PNe have been derived from long-slit spectroscopic work. However, an accurate derivation of the physical conditions and chemical abundances in the nebular shell relies upon knowledge of the integrated spectra. For Galactic PNe, this can only be determined using integral field spectroscopic instruments. This field was pioneered by Monreal-Ibero et al. (2005) and Tsamis et al. (2007), although it is only recently that detailed physical studies using optical integral field data have been undertaken, amongst which we can cite Monteiro et al. (2013, NGC 3242), Danehkar, Parker & Ercolano (2013, SuWt 2), Danehkar et al. (2014, Abell 48), Danehkar & Parker (2015, Hen 3-1333 and Hen 2-113), and Ali et al. (2015b, PN G342.0−01.7). In this paper, we will analyse integral field data obtained with the Wide Field Spectrograph (WiFeS) integral field spectrograph (Dopita et al. 2007, 2010) to derive plasma  C 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society Accepted 2016 February 25. Received 2016 February 25; in original form 2015 October 15 Is the WELS class of PN central star real? The WELS denomination proposed by spectral characteristics of this class was described in detail by Marcolino & de Araújo (2003), who argue that there seems to be a general evolutionary sequence connecting the H-deficient CSs and the PG 1159 stars which link the PN stars to WDs. The proposed evolutionary sequence originally developed by Parthasarathy, Acker & Stenholm (1998) is [WCL] → [WCE] → WELS → PG 1159. Fogel, De Marco & Jacoby (2003) were unsuccessful in finding an evolutionary sequence for WELS similar to what had been established for the [WR] CS. However, they found that WELS have intermediate stellar temperature (30–80 kK). They found no WELS associated with Type I PNe – all studied objects having N/O ratios lower than 0.8, indicating lower mass precursors. However, we should note here that the lower limit of the N/O ratio in Type I PNe is 0.5 as defined by Peimbert & Torres-Peimbert (1983, 1987). Adopting this criterion of Type I PNe to the analysis of Fogel et al. (2003) would imply that ∼20 per cent (5 objects) of the sample (26 objects) are of Type I (fig. 2, Fogel et al. 2003). Girard, Köppen & Acker (2007) affirm, on average, WELS have slightly lower helium and nitrogen abundances compared to [WR] and non-WR PNe. They find somewhat enhanced helium and nitrogen abundances in [WR] PNe with an N/O ratio ∼4 times solar value, while WELS have N/O ratios which are nearly solar value. From the IRAS twocolour diagram, they find that WELS are shifted to bluer colours than the other [WR] PNe. Frew & Parker (2012) show that WELS have larger scaleheight compared to other many CS classes and consequently they rise from low-mass progenitor stars. The emission lines which characterize the WELS spectral class are the recombination lines of C and N and consist of the following: N III λλ4634, 4641, C III λ4650, C IV λ4658, and C IV λλ5801, 5812. These lines are indistinguishable in width from the nebular lines, although on low-dispersion spectra the group of lines around 4650 Å and the C IV doublet around 5805 Å may appear to be broad features. Frew & Parker (2012) noted that the characteristic WELS recombination lines such as C III, C IV, and N III were also observed in some massive O-type stars, as well as in low-mass X-ray binaries and cataclysmic variables. Corradi et al. (2011) observed these C III, C IV, and N III emission lines in the spectrum of the close binary CS of the highly excited PN IPHASX J194359.5+170901. Also, Miszalski et al. (2011) explained that many of the characteristic WELS emission lines have been (...truncated)