Gamma-Ray Constraints on Supernova Nucleosynthesis

Gamma-ray spectroscopy holds great promise for probing nucleosynthesis in individual supernova explosions via short-lived radioactivity, and for measuring current global Galactic supernova nucleosynthesis with longer-lived radioactivity. It was somewhat surprising that the former case was realized first for a Type II supernova, when both Co-56 and Co-57 were detected in SN 1987A. These provide unprecedented constraints on models of Type II explosions and nucleosynthesis. Live Al-26 in the Galaxy might come from Type II supernovae, and if it is eventually shown to be so, can constrain massive star evolution, supernova nucleosynthesis, and the Galactic Type II supernova rate. Type Ia supernovae, thought to be thermonuclear explosions, have not yet been detected in gamma-rays. This is somewhat surprising given current models and recent Co-56 detection attempts. Ultimately, gamma-ray measurements can confirm their thermonuclear nature, probe the nuclear burning conditions, and help evaluate their contributions to Galactic nucleosynthesis. Type Ib/c supernovae are poorly understood. Whether they are core collapse or thermonuclear events might be ultimately settled by gamma-ray observations. Depending on details of the nuclear processing, any of these supernova types might contribute to a detectable diffuse glow of Fe-60 gamma-ray lines. Previous attempts at detection have come very close to expected emission levels. Remnants of any type of age less that a few centuries might be detectable as individual spots of Ti-44 gamma-ray line emission. It is in fact quite surprising that previous surveys have not discovered such spots, and the constraints on the combination of nucleosynthesis yields and supernova rates are very interesting. All of these interesting limits and possibilities mean that the next mission, International Gamma-Ray Astrophysics Laboratory (INTEGRAL), if it has sufficient sensitivity, is very likely to lead to the realization of much of the great potential of gamma-ray spectroscopy for understanding supernovae

Hard Emission from Classical Novae

The thermonuclear model of classical novae has been very successful in explaining many observed features of novae. It also predicts unique gamma-ray and X-ray signatures, a few of which possibly have already been observed. We describe recent developments in the expectations of these signatures and we discuss prospects for additional observations with current observatories

Angular Distribution of Interstellar Al-26

The angular distribution and local concentration of interstellar Al-26 are calculated for five theoretical models of its origin, as a guide for future observations. The models considered relate Al-26 to current star formation (with surface CO density as its indicator), treat Al-26 as a secondary-nucleosynthesis product on seed nuclei with the same metallicity gradient as O, assume the Al-26 ejection rate is proportional to present light output, make Al-26 synthesis depend on total H I + H2 gas density, or assume that Al-26 is formed mainly in novae. The local decay and production rates for the five models are found to lie within a factor of 2 of surface density 0.0003/sq cm s, corresponding to a present (Al-26)/(Al-27) ratio of 5 x 10 to the -6th averaged over H I and H2 phases

The Gamma-Ray Light Curves of SN 1987A

Observations of the SN 1987A ejecta in four Co-56-decay gamma-ray lines, obtained using the SMM gamma-ray spectrometer between February 1987 and May 1989, are reported and analyzed. The instrument characteristics and data-reduction procedures are described, and the results are presented in extensive tables and graphs and discussed with reference to theoretical models. Gamma-ray fluxes significantly above possible instrumental levels (as determined from analysis of pre-1987 data) were detected in the second half of 1987 and the first half of 1988. The data are found to favor a model with some Co-56 in regions of low gamma-ray optical depth by 200 d after the SN outburst over models with all Co-56 at one depth within a uniform expanding envelope. Also investigated are the gamma-ray contribution to the total bolometric luminosity and the escape (and potential observability) of Co-57 gamma rays

Gamma-Ray Limits on Galactic Fe-60 and Ti-44 Nucleosynthesis

We have searched nearly 10 years of data from NASA\u27s Solar Maximum Mission (SMM) Gamma-Ray Spectrometer for evidence of gamma-ray line emission from the decay of the shorter lived daughters, Co-60 and Sc-44, of nucleosynthetic Fe-60 and Ti-44. The data are compared with models of the expected signals from the annual scan of the ecliptic by SMM. These models include (1) the extended diffuse emision from the many supernovae which should contribute Fe-60 over its 2.2 Myr lifetime, and (2) point sources at various locations in the Galactic plane which could be previously undiscovered remnants of supernovae which ejected Ti-44. We find no evidence of Galactic emission from either nucleus; upper limits (99% confidence) are near 8 x 10-5 photons/sq cm/s, for both the 1.17 MeV line from Co-60 decay integrated over the central radian of Galactic longitude and for the 1.16 MeV line from Sc-44 from points near the Galactic center. The limits on 1.16 MeV flux from longitudes near + or - 90 deg rise to approximately 2 x 10-4 photons/sq cm/s because the large angular distance to the ecliptic reduces the sensitivity in those directions. The mass of Fe-60 in the interstellar medium today is constrained to be less than 1.7 solar mass. This sets a limit on the current Galactic production rate of Fe-60 and of other isotopes coproduced with it, for example, Ca-48 and Ti-50. Estimating the current production of these stable isotopes from their solar abundances suggests that there should be about 0.9 solar mass of Fe-60 in the interstellar medium and indicates that Fe-60 could soon be detected with a slightly more sensitive instrument. Comparing the estimated production rates of stable isotopes with the gamma-ray limits on those of radioactive isotopes allows us to constrain some models of Galactic chemical evolution. The mass of Ti-44 at the Galactic center, for example, is (99% confidence) less than 8 x 10-5 solar mass. This is a quite improbable result viewed in either of two ways. Employing plausible models of Galactic chemical evolution constrained to produce the solar concentration of Ca-44 in the Galaxy 4.5 Gyr ago suggests that Ca-44 is produced today at the rate (3-4) x 10-4 solar mass per century. This production rate is consistent with our measurement at 5% confidence only for supernova rates less than 1.5 per century, depending slightly on the actual Ti-44 lifetime, and assuming all Ca-44 is ejected as Ti-44. Lower rates are consistent with our data, because the implied interval with no supernovae is not so unlikely, but the required higher yields of Ti-44 begin to strain current supernova nucleosynthesis calculations. Apart from the solar abundance requirement, we can check the consistency of any combination of supernova rate and Ti-44 yield. A Galactic supernova rate of three per century and a yield 10-4 solar mass of Ti-44 per event, both very reasonable estimates, are consistent with our data at only 5% confidence. Perhaps the typical yield of frequent supernovae is significantly smaller than this, and the source of most Ca-44 is a rare type of high-yield of frequent supernovae is significantly smaller than this, and the source of most Ca-44 is a rare type of high-yield event which has not occurred recently. The isotope Ti-44 is probably not a major contributor to interstellar positions

A Search for Gamma-Ray Lines from the Decay of Fe-59 in Supernova 1987A

We have searched spectra of Supernova (SN) 1987A, accumulated during several 35-day intervals after the explosion by the Solar Maximum Mission (SMM) Gamma Ray Spectrometer (GRS), for gamma-ray lines at 1.099 and 1.292 MeV from the decay of Fe-59 which may have been produced in the progenitor\u27s helium shell. We find no evidence for these lines, down to 3-sigma upper limits approximately = 7 x 10-4 gamma/sq cm/s for the 1.099 MeV line, or approximately = 4.5 x 10-4 gamma/sq cm/s for the 1.292 MeV line, in any 35-day interval. We derive a conservative 3-sigma upper limit on the mass fraction of Fe-59 in the helium shell of 2.9 x 10-3

On Al-26 and Other Short-Lived Interstellar Radioactivity

Several authors have shown that massive stars exploding at a rate of about three per century can account for a large portion, if not all, of the observed interstellar Al-26. In a separate argument using models of Galactic chemical evolution, Clayton (1984) showed that the Al-26/Al-27 production ratio was not large enough to maintain enough Al-26 in the Galactic disk gas of about 10 exp 10 solar masses having solar composition. We present a resolution of those conflicting arguments. A past history of Galactic infall growing the Galactic disk so dilutes the stable Al-27 concentration that the two approaches can be brought into near agreement. If massive stars dominate the production of Al-26, we suggest that the apparent shortfall of their Al-26/Al-27 yield ratio is to be interpreted as evidence for significant growth of the Galactic disk. We also discuss the implications of these arguments for other extinct radioactivities in meteorites, using I-129 and Sm-146 as examples

On 26Al and other Short-lived Interstellar Radioactivity

Several authors have shown that massive stars exploding at a rate of about three per century can account for a large portion, if not all, of the observed interstellar 26Al. In a separate argument using models of Galatic chemical evolution, Clayton (1984) showed that the 26Al/27Al production ratio was not large enough to maintain enought 26Al in the Galactic disk gas of ~ 10^10 M⊙ having solar composition. We present a resolution of those conflicting arguments. A past history of Galactic infall growing the Galatic disk so dilutes the stable 27Al concentration that the two approaches can be brought into near agreement. If massive stars dominate the production of 26Al, we suggest that the apparent shortfall of their 26Al/27Al yield ratio is to be interpreted as evidence for signigicant growth of the Galactic disk. We also discuss the implications of these arguments for other extinct radioactivites in meteories, using 129I and 146Sm as examples

The Cosmic Gamma-Ray Background from Type Ia Supernovae

We present an improved calculation of the cumulative gamma-ray spectrum of Type Ia supernovae during the history of the universe. We follow Clayton & Ward (1975) in using a few Friedmann models and two simple histories of the average galaxian nucleosynthesis rate, but we improve their calculation by modeling the gamma-ray scattering in detailed numerical models of SN Ia\u27s. The results confirm that near 1 MeV the SN Ia background may dominate, and that it is potentially observable, with high scientific importance. A very accurate measurement of the cosmic background spectrum between 0.1 and 1.0 MeV may reveal the turn-on time and the evolution of the rate of Type Ia supernova nucleosynthesis in the universe