The Astrophysical Journal
We have searched nearly 10 years of data from NASA's 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.
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