Ti-44: its effective decay rate in young supernova remnants, and its abundance in Cassiopeia A

Radioactive isotopes such as Ti-44 offer probably the most direct probe int o nucleosynthesis environments in the interior of exploding stars, when the associated gamma-ray activities in the explosion remnant are detected and translated back to the isotopic abundances at the time of the explosion. In this paper, we assert that the procedure may not necessarily be straightfo rward, at least in the case of Ti-44, an orbital-electron capture decay iso tope. Using the analytic model of McKee & Truelove (1995) for young superno va remnants, and assuming the existence of overdense Fe-56-dominated clumps that contain also Ti-44, we show that a high degree of ionization may be c aused by the reverse shock so that the electron-capture rate of Ti-44 could be significantly reduced from its laboratory value. When applied to Cas A, this increases under certain conditions the current Ti-44-activity by a fa ctor 1.5 similar to 2.5, which yields a better compatibility between the CO MPTEL observation of the 1.16 MeV line activity associated with the Ti-44 d ecay and the SN model predictions of the initial Ti-44 abundance. This poss ibility is, however, subject to various uncertainties, and in particular to the unknown properties and radial distribution of the clumps in the ejecta .