arXiv subjects

R. D. Hoffman

Publications and source records attributed to R. D. Hoffman.

22 records · Page 2Linked to original sources

arXiv1999

The Reaction Rate Sensitivity of Nucleosynthesis in Type II Supernovae

We explore the sensitivity of the nucleosynthesis of intermediate mass elements (28 < A < 80) in supernovae derived from massive stars to the nuclear reaction rates employed in the model. Two standard sources of reaction rate data (Woosley et al. 1978; and Thielemann et al. 1987) are employed in pairs of calculations that are otherwise identical. Both include as a common backbone the experimental reactions rates of Caughlan & Fowler (1988). Two stellar models are calculated for each of two main sequence masses: 15 and 25 solar masses. Each star is evolved from core hydrogen burning to a presupernova state carrying an appropriately large reaction network, then exploded using a piston near the edge of the iron core as described by Woosley & Weaver (1995). The final stellar yields from the models calculated with the two rate sets are compared and found to differ in most cases by less than a factor of two over the entire range of nuclei studied. Reasons for the major discrepancies are discussed in detail along with the physics underlying the two reaction rate sets employed. The nucleosynthesis results are relatively robust and less sensitive than might be expected to uncertainties in nuclear reaction rates, though they are sensitive to the stellar model employed.

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arXiv1996

Nucleosynthesis in neutrino-driven winds: II. Implications for heavy element synthesis

During the first 20 seconds of its life, the enormous neutrino luminosity of a neutron star drives appreciable mass loss from its surface. This neutrino-driven wind has been previously identified as a likely site for the r-process. Qian & Woosley (1996) have derived, both analytically and numerically, the physical conditions relevant for heavy element synthesis in the wind. These conditions include the entropy (S), the electron fraction (Ye), the dynamic time scale, and the mass loss rate. Here we explore the implications of these conditions for nucleosynthesis. We find that the standard wind models derived in that paper are inadequate to make the r-process, though they do produce some rare species above the iron group. We further determine the general restrictions on the entropy, the electron fraction, and the dynamic time scale that are required to make the r-process. In particular, we derive from nuclear reaction network calculations the conditions required to give a sufficient neutron-to-seed ratio for production of the platinum peak. These conditions range from Ye = 0.2 and S < 100 per baryon for reasonable dynamic time scales of 0.001-0.1 s, to Ye = 0.4-0.495 and S > 400 per baryon for a dynamic time scale of 0.1 s. These conditions are also derived analytically to illustrate the physics determining the neutron-to-seed ratio.

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arXiv1995

The Production of Ti44 and Co60 in Supernova

The production of the radioactive isotopes $^{44}$Ti and $^{60}$Co in all types of supernovae is examined and compared to observational constraints including Galactic $γ$--ray surveys, measurements of the diffuse 511 keV radiation, $γ$--ray observations of Cas A, the late time light curve of SN 1987A, and isotopic anomalies found in silicon carbide grains in meteorites. The (revised) line flux from $^{44}$Ti decay in the Cas A supernova remnant reported by COMPTEL on the Compton Gamma-Ray Observatory is near the upper bound expected from our models. The necessary concurrent ejection of $^{56}$Ni would also imply that Cas A was a brighter supernova than previously thought unless extinction in the intervening matter was very large. Thus, if confirmed, the reported amount of $^{44}$Ti in Cas A provides very interesting constraints on both the supernova environment and its mechanism. The abundances of $^{44}$Ti and $^{60}$Co ejected by Type II supernovae are such that gamma-radiation from $^{44}$Ti decay SN 1987A could be detected by a future generation of gamma-ray telescopes and that the decay of $^{60}$Co might provide an interesting contribution to the late time light curve of SN 1987A and other Type II supernovae. To produce the solar $^{44}$Ca abundance and satisfy all the observational constraints, nature may prefer at least the occasional explosion of sub-Chandrasekhar mass white dwarfs as Type Ia supernovae. Depending on the escape fraction of positrons due to $^{56}$Co made in all kinds of Type Ia supernovae, a significant fraction of the steady state diffuse 511 keV emission may arise from the annihilation of positrons produced during the decay of $^{44}$Ti to $^{44}$Ca. The Ca and Ti isotopic anomalies in pre-solar grains confirm the production of $^{44}$Ti in supernovae and that

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arXiv1995

26Al and 60Fe From Supernova Explosions

Using recently calculated yields for Type II supernovae, along with models for chemical evolution and the distribution of mass in the interstellar medium, the current abundances and spatial distributions of two key gamma-ray radioactivities, $^{26}$Al and $^{60}$Fe, are determined. The estimated steady state production rates are 2.0 $\pm$ 1.0 M\sun \ Myr$^{-1}$ for $^{26}$Al and 0.75 $\pm$ 0.4 M\sun \ Myr$^{-1}$ for $^{60}$Fe. This corresponds to 2.2 $\pm$ 1.1 M\sun \ of $^{26}$Al and 1.7 $\pm$ 0.9 M\sun \ of $^{60}$Fe in the present interstellar medium. Sources of uncertainty are discussed, one of the more important being the current rate of core collapse supernovae in the Galaxy. Our simple model gives three per century, but reasonable changes in the star formation rate could easily accommodate a core collapse rate one-half as large, and thus one-half the yields. When these stellar and chemical evolution results are mapped into a three dimensional model of the Galaxy, the calculated 1809 keV gamma-ray flux map is consistent with the {\it Compton Gamma Ray Observatory} observations of a steep decline in the flux outside a longitude of $\pm$ 50$^\circ$ from the Galactic center, and the slight flux enhancements observed in the vicinity of spiral arms. Other potential stellar sources of $^{26}$Al and $^{60}$Fe are mentioned, especially the possibility of $^{60}$Fe synthesis in Type Ia supernovae. Predictions for the $^{60}$Fe mass distribution, total mass, and flux map are given.

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