Astronomy and Astrophysics – Astrophysics
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.385l&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 385
Astronomy and Astrophysics
Astrophysics
Galaxy, Gamma-Rays, Nucleosynthesis, Radioactivity, Supernova
Scientific paper
Gamma ray spectroscopy holds great promise for probing nucleosynthesis in individual nucleosynthesis events, via observations of short-lived radioactivity, and for measuring global galactic nucleosynthesis today with detections of longer-lived radioactivity. Many of the astrophysical issues addressed by these observations are precisely those that must be understood in order to interpret observations of extinct radioactivity in meteorites. It was somewhat surprising that the former case was realized first for a Type II supernova, when both 56Co [1] and 57Co [2] were detected in SN 1987A. These provide unprecedented constraints on models of Type II explosions. Live 26Al in the galaxy might come from Type II supernovae and their progenitors, and if this is eventually shown to be the case, can constrain massive star evolution, supernova nucleosynthesis, the galactic Type II supernova rate, and even models of the chemical evolution of the galaxy [3]. Titanium-44 is produced primarily in the alpha-rich freezeout from nuclear statistical equilibrium, possibly in Type Ia [4] and almost certainly in Type II supernovae [5]. The galactic recurrence time of these events is comparable to the 44Ti lifetime, so we expect to be able to see at most a few otherwise unseen 44Ti remnants at any given time. No such remnants have been detected yet [6]. Very simple arguments lead to the expectation that about 4 x 10^-4 M(sub)solar mass of 44Ca are produced per century. The product of the supernova frequency times the 44Ti yield per event must equal this number. Even assuming that only the latest event would be seen, rates in excess of 2 century^-1 are ruled out at >=99% confidence by the gamma ray limits. Only rates less than 0.3 century^-1 are acceptable at >5% confidence, and this means that the yield per event must be >10^-3 M(sub)solar mass to produce the requisite 44Ca. Rates this low are incompatible with current estimates for Type II supernovae and yields this high are also very difficult to understand for any standard supernova models. This situation is puzzling. Searches for 60Fe gamma rays have also produced only upper limits, corresponding to a limit of 1.7 M(sub)solar mass in the present interstellar medium. Given the usual assumption of steady state between production and decay, the current rate of synthesis of 60Fe is less than 1.7 M(sub)solar mass/2.2 m.y. It has been suggested that a neutron-rich NSE occurs in small regions in both Type Ia supernovae supernovae and in core-collapse supernovae [7]. Either type might eject significant quantities of 60Fe. If we know the frequency of a particular type of 60Fe-producing event in the past few million years, then we can limit the mean 60Fe mass ejected per event. We have M(sub)ej (60Fe) <= 8 x 10^-5/R(SN) M(sub)solar mass where R(sub)SN is the frequency of the supernovae that eject 60Fe, in number per century. Type Ia supernovae might eject roughly 10^-4 M(sub)solar mass of 60Fe [8], which is very close to this limit. References: [1] Leising M. D. and Share G. H. (1990) Astrophys. J., 357, 638. [2] Kurfess J. D. et al. (1992) Astrophys. J. Lett., 399, L137. [3] Clayton D. D. et al. (1993) Astrophys. J. Lett., submitted. [4] Nomoto K. et al. (1984) Astrophys. J., 286, 644. [5] Woosley S. E. (1988) Proc. Astron. Soc. Aust., 7, 355. [6] Leising M. D. and Share G. H. (1993) Astrophys. J., submitted. [7] Hartmann D. H. et al. (1985) Astrophys. J., 297, 837. [8] Woosley S. E. (1991) In Gamma-Ray Line Astrophysics (P. Durouchoux and N. Prantzos, eds.), 270-290, AIP Conf. Proc. No. 232, New York.
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