Astron. Astrophys. 358, L67-L70 (2000)

3. Results and discussion

The various seed abundances of Fig. 2 are used to compute the production of the p-nuclides in the PPLs of the 25 star considered here. As in RAHPN, the abundance of a p-nuclide i is characterized by its mean overproduction factor , where is its solar mass fraction (Anders & Grevesse 1989), and

where is the mass fraction of isotope i at the mass coordinate , is the total mass of the PPLs, the sum running over all the PPLs ( corresponds to the bottom layer). An overproduction factor averaged over all 35 p-nuclei is calculated as , and is a measure of the global p-nuclide enrichment in the PPLs. So, if the computed p-nuclei abundance distribution were exactly solar, the normalized mean overproduction factor would be equal to unity for all i.

Fig. 3 shows the normalised p-nuclide overproduction factors derived from the seed abundance distributions calculated with the rates R₁, R₃ and R₅. Changes in the shape of the p-nuclide abundance distribution are clearly noticeable, at least for . The use of R₁ leads to a more or less substantial underproduction of not only , , and , a `classical' result in p-process studies (see RAHPN), but also of and , which was not predicted in previous calculations. This new feature directly relates to the larger abundances around used by RAHPN (dashed curve in Fig. 2), in contrast to the much flatter seed distribution obtained with R₁. This Kr-Sr-Mo-Ru trough is gradually reduced, and in fact essentially disappears, for rates of the order or in excess of R₃. This situation is most clearly illustrated by Fig. 4. In these very same conditions, for and comes much closer to unity as well. It has to be noticed that this situation does not result from a stronger production of these two nuclides by the p-process, but instead from their increased initial abundances associated with a more efficient s-process when going from R₁ to R₅. In contrast, the pattern does not depend on the adopted rate for . This is expected from a mere inspection of the s-nuclide seed distributions displayed in Fig. 2. In particular, and remain underproduced. This cannot be considered as an embarassment as these two nuclides can emerge from the s-process in low- or intermediate-mass stars.

Fig. 3. Values of and of derived from the seed abundances calculated with the rate R1 (upper panel), R3 (middle panel) and R5 (lower panel). Lines connect different isotopes of the same element. The dotted horizontal lines delineate the range

Fig. 4. Values of the normalized overproduction factors of the Kr, Sr, Mo and Ru p-isotopes as a function of the rate

In addition, the overall efficiency of the p-nuclide production substantially increases with increasing burning rates. More specifically, is multiplied by a factor of about 15 when going from R₁ to R₅. This could largely ease, and even solve, the problem of the relative underproduction of the p-nuclides with respect to oxygen identified by RAHPN. For their considered 25 model star calculated with the rate from Caughlan et al. (1985), they obtain and report a value of 4.4 for the ratio of the oxygen to p-process yields. This value would come close to unity for rates in the vicinity of R₃-R₄, as the p-nuclides would be about 3 to 6 times more produced than in RAHPN.

© European Southern Observatory (ESO) 2000

Online publication: June 20, 2000
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