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Astron. Astrophys. 342, 464-473 (1999)

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3. Thermonuclear reaction rates, physical inputs and initial conditions

In the investigation by HSH, the nuclear reaction network included 274 nuclides from 1H to 84 Kr. The nuclear reactions are assumed to proceed through [FORMULA], and their reverse reactions, and [FORMULA]-decays. Considerable progress in nuclear physics near the proton drip line has been reported in these 10 years (e.g. see the review by Schatz et al. 1998). We have extended the nuclear reaction network to include 463 nucleides in which the reaction rates in the network developed by Hashimoto & Arai (1985) have been replaced by the current ones (see Table 1). Our previous network (hereafter, HA85) which corresponds to cases A and B in Table 2 has used the compilation of the reaction rates by Fowler et al. (1975), Woosley et al. (1975, 1978), Wallace & Woosley (1981), Harris et al. (1983), Caughlan et al. (1985). The elements included in our new reaction network are shown in Table 1 which corresponds to case C in Table 2. For completeness, we have also included the neutron channels of [FORMULA] and their reverse reactions. As for the weak interaction processes, we have added [FORMULA]-decays and electron captures by Fuller et al. (1980, 1982). These nuclear data are taken from the data base REACLIB 1. Some reaction rates have been also included in connection to the calculations of nucleosynthesis in novae (Wanajo et al. 1998): the reactions (p, [FORMULA]) for the nuclei of [FORMULA], 26Si, 27P, 30S, [FORMULA], [FORMULA], and 35 K are taken from Herndl et al. (1995). Those for 25Mg and 25Al are taken from Iliadis et al. (1996) and the isomeric state of 26Al is separated from the ground state at [FORMULA] K and treated as a different nucleus (Wanajo et al. 1998). We note that the [FORMULA]-decay half life of 68Se in REACLIB is replaced from 1.6 min to 35.5 s (e.g. Horiguchi et al. 1996). Furthermore, we have replaced the rate of [FORMULA] in the above REACLIB by the new one (Rayet 1998) which includes the new experimental results (e.g. Kiener et al. 1993); this reaction is crucial for the break out from the HCNO cycle. For simplicity, we have not included such a 2p-capture process as suggested by Schatz et al. (1998). To investigate the effects of the Q-values and the reaction rates on the thermal history, we have performed the calculations for three cases as shown in Table 2. For case B, they are taken from Schatz et al. (1998), and for case C, they are from REACLIB. With the above modifications of the nuclear data, numerical computations are carried out in case A and case B by HA85, and in case C we use the updated network of Table 1. Screening factors for the thermonuclear reaction rates are taken from Ogata et al. (1991), and Ichimaru & Ogata (1991). It should be noted that the screening effects play an important role for [FORMULA].


[TABLE]

Table 1. Elements included in the nuclear reaction network.



[TABLE]

Table 2. Half lives and Q-values of the [FORMULA] reactions in units of keV which determine the waiting point during the shell flash and the final products.


It has been known that the amounts of CNO elements in the burning shell affect the thermonuclear history of the flash (e.g. Fujimoto et al. 1987). For example, we have [FORMULA] at the ignition and the ignition temperature [FORMULA] (Bildsten 1998). [FORMULA] ranges from [FORMULA] to [FORMULA] and other heavier elements could be transferred from the companion star. Since these initial abundances are rather uncertain, we assume [FORMULA], [FORMULA] and [FORMULA] from the [FORMULA]-decay saturated HCNO cycle as was done by HSH: denoted as HCNO in Table 3. We also assume the solar system abundances as an alternative initial composition, since often solar seeds have been adopted to study the nuclear process (e.g. Rembges et al. 1997): they are denoted as Solar in Table 3.


[TABLE]

Table 3. Final mass fractions after the shell flashes of log P = 23, log [FORMULA] = 14.75 when [FORMULA] with the different initial compositions and Q-values. [FORMULA] is the peak temperature in [FORMULA] K and [FORMULA] is the elapsed time until the peak temperature is attained. HCNO means the initial abundances of the [FORMULA]-decay saturated HCNO cycle. Solar means the solar system abundances.


For all computations, the initial temperature is set to be 1.5 108 K; the initial density is obtained from the equation of state. This assumption does not affect the computational results, because [FORMULA] is attained just after the main nuclear energy is released.

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© European Southern Observatory (ESO) 1999

Online publication: February 22, 1999
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