Chemical abundances of planetary nebulae from optical recombination lines - I. Observations and plasma diagnostics

Abstract

We have obtained deep optical spectra of medium resolution for a sample of 12 Galactic planetary nebulae (PNe). Optical recombination lines (ORLs) from carbon, nitrogen and oxygen have been detected in 11 of them and neon ORLs in nine of them. All spectra were obtained by scanning a long slit across the nebular surface, yielding relative line intensities for the entire nebula that are suitable for comparison with integrated line fluxes measured in other wavelength regions using space‐borne facilities, such as the Infrared Space Observatory (ISO) and the International Ultraviolet Explorer (IUE). For 11 PNe, ISO infrared spectra between 2.4 and 197 μm are available, most of them taken by ourselves, plus a Kuiper Airborne Observatory (KAO) infrared spectrum of NGC 6210. IUE ultraviolet (UV) spectra are available for all nebulae except one in our sample. The UV, optical and infrared spectra have been combined to study nebular thermal and density structures and to determine elemental abundances. We have determined UV to optical extinction curves towards these PNe by examining observed fluxes of H i and He ii recombination lines, radio free–free continuum flux density, and UV to optical nebular continua. For 11 PNe in our sample, the derived optical reddening curves are found to be consistent with the standard Galactic extinction law for a total‐to‐selective extinction ratio, R≡A(V)/EB–V= 3.1. However, the optical extinction curve towards Hu 1–2 yields R= 2.0. The UV extinction towards Hu 1–2 and NGC 6572 is also found to be much steeper than the standard Galactic reddening law. In contrast, the UV extinction curve along the sight lines towards NGC 6210 is found to be much shallower, although in the latter case the uncertainties involved are quite large. Electron temperatures and densities have been derived using a variety of diagnostic ratios of collisionally excited lines (CELs) in the UV, optical and infrared. The results show clear stratifications, both in temperature and density. Lines emitted by ions formed in regions of higher ionization degree yield higher temperatures than lines arising from regions of lower ionization degree, while densities deduced from ratios of infrared diagnostic CELs of low critical densities, such as the [O iii] 88‐μm/52‐μm ratio, are systematically lower than those derived from UV and optical diagnostic lines, which in general have much higher critical densities than the infrared fine‐structure lines. Electron temperatures have also been derived from the ratio of the nebular continuum Balmer discontinuity to H 11 for 11 PNe. For four of these, the Balmer jump temperatures are more than 1000 K lower than values derived from the [O iii] optical collisionally excited diagnostic line ratio. With a difference of 3580 K, NGC 40 has the lowest Balmer jump temperature relative to the [O iii] optical forbidden‐line temperature. High‐order Balmer line decrements have been used to determine electron densities. The results are consistent with values derived from forbidden‐line density‐dia...

Abundances in the planetary nebula NGC 6210

“…It can be seen that the electron temperature is roughly constant as a function of ionization potential. The electron temperature has also been determined from the Balmer jump to be 9300 K. (Liu et al 2004b). Some PNe show a gradient of electron temperature but if any gradient exists in this nebula, is can only be seen in the slightly higher temperatures for the lowest ionization potentials (presumably formed in the outer regions of the nebula).…”

Herschelimages of NGC 6720: H2formation on dust grains

“…We also used two radio continuum flux densities at 4850 and 1400 MHz of 360 and 440 mJy respectively, which are averages of the data collected by Vollmer et al (2010). For the angular diameter we used 76 (Liu et al 2004). We adopted a distance of 740 pc (O'Dell et al 2009).…”

“…The method was slightly modified in that we used H-Ni model atmospheres from Rauch (2003). As input we used the UV and optical spectrum listed in Liu et al (2004). We rejected the SWS spectrum as it turned out that the aperture was mainly pointed at the central "cavity" and was therefore strongly biased towards the high excitation region of the nebula.…”

“…This made aperture correction factors highly dependent on the ionization stage and hence very uncertain. We did use a re-reduced version of the LWS spectrum of NGC 6720, adopting the aperture correction factor listed in Liu et al (2004).…”

Atomic data for neutron-capture elements

“…In spite of the low cosmic abundances of n-capture elements (Asplund et al 2009), deep ultraviolet (Sterling et al 2002;Sterling & Dinerstein 2003;Williams et al 2008), optical (e.g., Hyung et al 2001;Sharpee et al 2003;Liu et al 2004;Zhang et al 2005;Sharpee et al 2007;Sterling et al 2009;Otsuka et al 2010Otsuka et al , 2011 and near-infrared spectroscopy (Likkel et al 2006;Sterling et al 2007;Sterling & Dinerstein 2008) of several Galactic and Local Group PNe revealed spectral features of these species. Abundances of these elements have now been estimated in approximately 100 PNe, allowing for an analysis of s-process elemental enrichments in a statistically meaningful sample of objects.…”