Chemical abundances of planetary nebulae from optical recombination lines - III. The galactic bulge PN M 1-42 and M 2-36

Abstract

We present deep, high-resolution optical spectra of two Galactic bulge planetary nebulae (PN), M 1-42 and M 2-36. The spectra show very prominent and rich optical recombination lines (ORLs) from C, N, O and Ne ions. Infrared spectra from 2.4-197 μm were also obtained using the Short and Long Wavelength Spectrometer (SWS and LWS) on board ISO. The optical and infrared spectra, together with archival IUE spectra, are used to study their density and thermal characteristics and to determine elemental abundances. We determine the optical and UV extinction curve towards these two bulge PN using observed Hi and Hen recombination line fluxes and the radio free-free continuum flux density. In the optical, the reddening curve is found to be consistent with the standard Galactic extinction law, with a total to selective extinction ratio R ≡ A(V)/E(B - V) = 3.1. However, the extinction in the UV is found to be much steeper, consistent with the earlier finding of Walton, Barlow & Clegg. The rich ORL spectra from C, N, O and Ne ions detected from the two nebulae have been used to determine the abundances of these elements relative to hydrogen. In all cases, the resultant ORL abundances are found to be significantly higher than the corresponding values deduced from collisionally excited lines (CELs). In M 2-36, the discrepancies are about a factor of 5 for all four elements studied. In M 1-42, the discrepancies reach a factor of about 20, the largest ever observed in a PN. M 1-42 also has the lowest Balmer jump temperature ever determined for a PN, Te(BJ) = 3560 K, 5660 K lower than its [O III] forbidden line temperature. We compare the observed intensities of the strongest O II ORLs from different electronic configurations, including λ4649 from 3s-3p, λ4072 from 3p-3d, λ4089 from 3d-4f, and λ4590 and λ4190 from the doubly excited 3s′-3p′ and 3p′-3d′ configurations, respectively. In all cases, in spite of the fact that the ratios of the ORL to CEL ionic abundances span a wide range from ∼5-20, the intensity ratios of λ4649, λ4072, λ4590 and λ4190 relative to λ4089 are found to be nearly constant, apart from some small monotonic increase of these ratios as a function of electron temperature. Over a range of Balmer jump temperature from 3500-8100 K, the variations amount to about 20 per cent for the 3s-3p and 3p-3d transitions and a factor of 2 for the primed transitions, and are consistent with the predictions of the current recombination theory. Our results do not support the claim by Dinerstein, Lafon & Garnett that the relative intensities of O II ORLs vary from nebula to nebula and that the scatter is largest in objects where the discrepancies between ORL and CEL abundances are also the largest. We find that the ORL to CEL abundance ratio is highly correlated with the difference between the temperatures yielded by the [O III] forbidden line ratio and by the Hi Balmer jump, providing the strongest evidence so far that the two phenomena, i.e. the disparity between ORL and CEL temperature and abundance determinations, are closely related. However, temperature fluctuations of the type envisaged by Peimbert are unable to explain the low ionic abundances yielded by IR fine-structure lines. The very low Balmer jump temperature of M 1-42. coupled with its very low Balmer decrement density, may also be difficult to explain with a chemically inhomogeneous composite model of the type proposed by Liu et al. for NGC 6153.