5.2. What do PN abundances tell us?
The chemical composition of PNe envelopes results from a mixing of elements produced by the central star and dredged up to the surface with the original material out of which the star was made. Basically, the evolution of the central star can be described as follows (Blöcker 1999, Lattanzio & Forestini 1999). After completion of central hydrogen burning through the CNO bicycle, hydrogen burns in a shell around the He core. Due to core contraction the envelope expands. The star evolves towards larger radii and lower effective temperatures and ascends the red giant branch (RGB). During evolution on the RGB, the envelope convection moves downward reaching layers which have previously experienced H-burning (first dredge up), and brings up processed material to the surface. This material is mainly 14N, 13C, 12C, and 4He (Renzini & Voli 1981).
The ascent on the giant branch is terminated by ignition of the central helium. The subsequent evolution is characterized by helium burning in a convective core and a steadily advancing hydrogen shell. The fusion of helium produces 12C by the triple process, and this carbon is in turn subject to capture to form 16O. Eventually the helium supply is totally consumed, leaving a core of carbon and oxygen. The star begins to ascend the giant branch again, now called the asymptotic giant branch (AGB). When a star reaches the AGB, it has the following structure: a CO core, a He burning shell, a He intershell, a H burning shell, and a convective envelope. In stars more massive than 4 M, the envelope penetrates the region where H burning has occured, dredging up some of its material to the stellar surface (second dredge up). During this episode, 14N and 4He increase, while 12C and 13C decrease with 12C / 13C staying around one, and 16O slightly decreases.
While on the AGB, the star experiences further nucleosynthesis. Thermal pulses of the He shell induce a flash-driven convection zone, which extends from the helium shell almosto the H shell and deposits there some 12C made in the He shell. As the helium flash dies away, the energy deposited causes expansion and cooling, and the external convective region reaches down the carbon-rich region left after the flash, bringing 12C and 4He to the star surface (third dredge up). During thermal pulses, elements beyond iron are produced by slow neutron capture (s-process). This requires partial mixing of hydrogen into the carbon rich intershell (Lattanzio & Forestini 1999): these protons are captured by 12C to produce 13C which later releases neutrons via the 13C(, n)16O reaction. For stars above 5 M(at solar metallicity) a second important phenomenon is hot bottom burning. The convective envelope penetrates into the top of the H-burning shell. Temperatures can reach as high as 108 K. This results in the activation of the CN cycle within the envelope, and the consequent processing of 12C into 13C and 14N, with the result that 12C/16O is smaller than one.
In summary, nucleosynthesis in PNe progenitors mainly affects the abundances of He, N and C in the envelope. The He abundance increases during the first, second and third dredge up. The 14N abundance increases during the first, second and third dredge up. In the case of hot bottom burning, primary N is produced out of C synthesized in the He shell and brought to the H shell after the flash. The 12C abundance decreases during first and second dredge up but increases during third dredge up, and decreases during hot bottom burning. From the synthetic evolutionary models of Marigo (2001), the resulting enrichment in PNe envelopes with respect to the ISM may be as large as a factor of 10 or more for 12C and 14N.
The abundance of 16O is slightly reduced as a consequence of hot bottom burning while, as pointed out by Marigo (2001), low mass stars may produce positive yields of 16O, which is brought to the surface by third dredge up. Globally, the oxygen abundance is expected to be little affected by nucleosynthesis in PN progenitors (Renzini & Voli 1981, Forestini & Charbonnel 1997, van den Hoek & Groenewegen 1997, Marigo 2001). From the synthetic evolutionary models of Marigo (2001), the PN progenitors modify the PN oxygen abundance by at most a factor of 2, the effect being strongest at low metallicities (1/4 solar). At solar and half solar metallicity, the effect is practically negligible. As a consequence, the abundance of oxygen should be representative of the chemical composition of the matter out of which the progenitor star was made. The same holds for the abundances of elements such as Ne, Ar, S. On the other hand, the abundances of He, C, N and the s-process elements tell about the nuclear and mixing processes in the PN progenitors.
When using PNe as indicators of the chemical evolution of galaxies, one should be aware that PNe with different central star masses probe different epochs and are subject to different selection effects. The mere existence of the PN phenomenon requires that the star must have reached a temperature sufficient to ionize the surrounding gas before the ejected envelope has vanished into the interstellar space. Now, the evolution of the central star is more rapid for higher masses. PNe ionized by more massive nuclei reach higher luminosities, and they will be the ones for which abundances will be preferentially measured in distant galaxies. In nearby galaxies and in the Milky Way, observations are feasible for lower luminosity PNe. The observability of a PN depends on the detection threshold, but if it is low enough, PNe with less massive nuclei will be visible for a considerably longer time than PNe with massive nuclei. This results from the post-AGB evolution time being a strongly decreasing function of core mass (see e. g. the models of Blöcker 1995). Another point is that, because of the existence of an initial-final mass relation (e.g. Weidemann 1987), PNe with less massive nuclei correspond to stars with lower initial masses, which are far more numerous according to the Salpeter initial mass function. For these two reasons, samples of nearby PNe will not contain a large proportion of objects with high mass progenitors. They will not contain many PNe with central star masses below 1-1.5 M either, because such stars are believed to turn into very slowly evolving post-AGB stars and the ejected envelope will have dispersed into the interstellar medium before being ionized. This is why the distribution of central star masses is so strongly peaked around 0.6 M (Stasinska et al. 1997). PNe of different central star masses probe different epochs of galaxy history. Schematically, they can be classified as shown in Table 7 (which however must be taken only as a rough guideline). The subdivision of PNe into four types by Peimbert (1978) was motivated by this kind of considerations (but several revisions to his initial scheme were proposed later, as will be discussed in Sect. 5.4.1). All the above considerations need confirmation from observational data on PNe samples.
|progenitor mass||central star mass||progenitor's birth||PN typea|
|2.4 - 8 M||> 0.64 M||1 Gyr ago||Type I|
|1.2 - 2.4 M||0.58 - 0.64 M||3 Gyr ago||Type II|
|1.0 - 1.2 M||~ 0.56 M||6 Gyr ago||Type III|
|0.8 - 1.0 M||~ 0.555 M||10 Gyr ago||Type IV|
|a PN types according to Peimbert (1978, 1990)|