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

Radioactive isotopes such as (44) Ti offer probably the most direct probe into 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 straightforward, at least in the case of (44) Ti, an orbital-electron capture decay isotope. Using the analytic model of McKee & Truelove (1995) for young supernova remnants, and assuming the existence of overdense (56) Fe-dominated clumps that contain also (44) Ti, we show that a high degree of ionization may be caused by the reverse shock so that the electron-capture rate of (44) Ti could be significantly reduced from its laboratory value. When applied to Cas A, this increases under certain conditions the current (44) Ti-activity by a factor 1.5 ~ 2.5, which yields a better compatibility between the COMPTEL observation of the 1.16 MeV line activity associated with the (44) Ti decay and the SN model predictions of the initial (44) Ti abundance. This possibility is, however, subject to various uncertainties, and in particular to the unknown properties and radial distribution of the clumps in the ejecta.