Abundances in planetary nebulae

Abstract. We present recent advances in the determination of chemical abundances of galactic Planetary Nebulae and discuss implications resulting from the comparison with theoretical predictions. From the analysis of diagrams of abundances of N/O vs He/H, N/O vs N/H and N/O vs O/H we argue that very likely the often used solar photospheric abundance of oxygen of 8.9, in usual units, is overestimated by a factor of 2-3, as suggested by very recent work in the Sun. This would solve an astrophysical problem with the measured abundances in planetaries. 

Hu region abundances in gas-rich dwarfs. Richer, McCall, Stasinska (1998) compared dlrr H n region O abundances with O abundances of planetary nebulae (PNe) in dSphs. While the offset persisted, PNe have only been detected in the two most luminous dSphs and trace primarily intermediate-age populations as opposed to the present-day abundances in Hu regions. 

The most direct, model independent, way to test the validity of the mixing solution is to measure the 3He abundance in the ejecta of low-mass stars, i.e. in planetary nebulae (PNe). The search for 3He in the ejecta of PNe via the 8.667 GHz spin-flip transition of 3He+, painstakingly carried out by Rood and coworkers at the Green Bank radiotelescope since 1992 (see summary of results in Balser et al. 1997), has produced so far one solid detection (NGC 3242, see Rood, Bania, Wilson 1992 confirmed with the Effelsberg radiotelescope by... 

Carbon abundances are measured in planetary nebulae by observing from space the ultraviolet lines of doubly ionized C++. The observed C/O abundance ratio is large and variable, indicating C production in the intermediate-mass stars thatare the immediate precursors to the planetary nebulae. Evidence of different nuclei synthesized in different types of stars is established by such data. They also reveal the nucleosynthesis of carbon to be more complex than that of oxygen. 

As mentioned above, statistical methods for abundance determinations assume that the nebulae under study form a one parameter family. This is why they work reasonably well in giant H II regions. They are not expected to work in planetary nebulae, where the effective temperatures range between 20 000 K and 200000 K. Still, it has been shown empirically that there is an upper envelope in the [O ill] A5007/H/ vs. O/H relation (Richer 1993), probably corresponding to PNe with the hottest central stars. 

Fig. 1. Interstellar 3He/H abundances as a function of source metallicity [2], The [3He/H] abundances by number derived for the H n region sample are given with respect to the solar ratio. Also shown is the abundance derived for the planetary nebula NGC3242 (triangle). We note that there is no trend in the 3He/H abundance with source metallicity...

Abundance Variations in the Galactic Disk Planetary Nebulae, Open Clusters and Field Stars... 

Radioastronomers first learned of 3He in 1955 at the fourth I.A.U. Symposium in Jodrell Bank, when the frequency of the hyperfine 3He+ line at 8.666 GHz (3.46 cm) was included by Charles Townes in a list of radio-frequency lines of interest to astronomy (Townes 1957). The line was (probably) detected for the first time only twenty years later, by Rood, Wilson Steigman (1979) in W51, opening the way to the determination of the 3He abundance in the interstellar gas of our Galaxy via direct (although technically challenging) radioastronomical observations. In the last two decades, a considerable collection of 3He+ abundance determinations has been assembled in Hi I regions and planetary nebulae. The relevance of these results will be discussed in Sect. 4 and 5 respectively. 

Fig. 3.40. Abundances in Galactic stars, H n regions and planetary nebulae, as a function of Galactocentric distance, with the Sun shown for comparison. After Hou, Prantzos and Boissier (2000). The curves show a model calculation by the authors nitrogen is underproduced in the model because only massive stars were considered. 

Fig. 4.7. 3He/H in simple Galactic H n regions, i.e. those thought to be reasonably well represented by homogeneous spherical models (Balser et al. 1999), and one planetary nebula, as a function of their oxygen abundance. 3He/H is plotted on a logarithmic scale relative to the proto-solar value of 1.5 x 10-5. After Bania, Rood and Balser (2002). Reprinted by permission from Macmillan Publishers Ltd. Courtesy Tom Bania.

By studying radio and optical spectra from HII regions and planetary nebulas, to be discussed immediately below, we may establish the abundances of several elements, in particular, helium, absent from the solar spectrum, a point of great cosmological significance, but also nitrogen and oxygen. 

Observations of isotopic abundances provides information on the nucleosynthesis operating in the compact core of stars and supernova explosions and on the chemical evolution of the Galaxy. The CNO nuclides in late-type stars are affected by freshly synthesized core material brought up by dredge-up events. On the other hand, the Si isotopes are involved in later phases of nuclear burning, a narrow span of the red giant lifetime before planetary nebulae or supernovae. Therefore relative abundances of Si isotopes we observe remain unchanged from those of interstellar matter from which a star was formed. 

Fullerenes and graphite may originate abundantly in stellar atmospheres rich in carbon like those of some giant stars and some progenitors of planetary nebulae (Fig. 1.5). These objects display for important mass loss rates and are therefore able to greatly enrich the interstellar medium. 

The interstellar light extiction curve at 2,175 A can be modeled using fulleranes with different degree of hydrogenation (Webster 1995 Cataldo 2003 Cataldo et al. 2009). The fraction of interstellar carbon abundance needed to produce fulleranes is high, but not prohibitively and carbon stars and planetary nebulae could provide these molecules in sufficient quantity. The experimental determination of optical and infrared spectra for laboratory-isolated fulleranes would be very valuable in assessing their role as carriers of the DIBs and of the unidentified infrared emission features. 

Besides the problem of accounting for the chemical abundances of planetary noble gases, there are characteristic differences in isotopic composition between planetary noble gases in meteorites and the solar gases that presumably represent the nebula from which meteorites formed. For Ar and Kr the differences are modest or perhaps nonexistent or can ultimately be explained in terms of a reasonable degree of mass-dependent isotopic fractionation. For Ne (Figure 3.3) and Xe (Figure 7.6), the... 

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