We examined the effects of the reverse shock on the 44Ti -decay rate in young supernova remnants. For this purpose, we employed the analytic remnant model of McKee & Truelove (1995), which was used to describe the hydrodynamic evolution. We assumed that 44Ti is carried by 56Fe-dominated, dense clumps which are mixed into the otherwise homogeneous supernova ejecta and account for a minor fraction of the ejecta mass. Strong ionization of 44Ti due to the heating by the reverse shock and a sizable influence on the 44Ti decay is obtained if the clumps are assumed to be located at intermediate values of the radial mass coordinate q. The observationally important change of the 44Ti decay activity, however, depends also on the values of the remnant parameters, namely the explosion energy, the ejecta mass, the density of the ambient medium and the assumed overdensity factor of the clumps relative to the embedding homogeneous ejecta.
We applied our model to the case of Cas A, a young supernova remnant with an age of about 320 years. Observational data are considered to constrain the parameter space which was explored. We found that under certain conditions the ionization of 44Ti and the corresponding delay of its decay can yield an up to three times higher 44Ti activity at the present time than predicted on the grounds of the laboratory decay rate. This effect is large enough to reduce the apparent discrepancies between the 44Ti production in the explosion as inferred from the COMPTEL -line measurements and the theoretical expectations from the current supernova nucleosynthesis models.
We emphasize again that to get an enhanced 44Ti decay activity we had to assume 44Ti to be associated with inhomogeneities in the ejecta which contain the Fe-group elements. Hydrodynamical models have so far not been able to predict the fraction of Fe and 44Ti in such clumpy structures, the overdensity of the clumps relative to the surrounding material at the considered age of a supernova remnant, the dynamics of the clumps, and their size and distribution. Multi-dimensional supernova models are called for, which connect the very early phase of the explosion with the remnant evolution a few hundred years later. Self-consistent multi-dimensional simulations have yet to be carried out for an improved theoretical picture of the explosive 44Ti production in supernovae. All published theoretical yields have so far only been obtained with ad-hoc assumptions, most often by spherically symmetric models and/or simulations of the supernova explosions which were started from artificial initial conditions ("piston" models) rather than self-consistent situations. We add that such calculations also suffer from uncertainties concerning the nuclear reaction rates which are important to determine the 44Ti yield (The et al. 1998).
On the other hand, more detailed X-ray and -ray observations with a better spatial (and energy) resolution are desirable to confirm or reject the implications of the described model, e.g., the existence of Fe-containing knots with temperatures in the approximate range of keV (cf. Tsunemi 1997). The current angular resolution of the ASCA/SIS observations, about one arcmin, is insufficient to measure the thermodynamic properties of individual clumps in the ejecta of Cas A. Only information is available for the average electron temperature in the matter that is heated by the forward and reverse shocks. Comparing the observations with predictions from the remnant model used in this paper would therefore require monitoring of the thermodynamic history of the homogeneous ejecta. For this, one would need to compute the thermal state of the electrons, and would have to specify the unknown composition of the ejecta, which is very uncertain because it depends on the type of progenitor and explosion. The described investigations of 44Ti decay in supernova remnants could be done without such additional assumptions, but the models do not provide a description of the thermodynamic state of the remnant which allows for a direct comparison with current observational data.
Finally, we mention that we applied our analysis also to the new
supernova remnant, RX J0852.0-4622, discovered in the direction
of the Vela remnant by Aschenbach (1998), in which a source of
44Ti line emission has been detected (Iyudin et al. 1998).
However, for the estimated low density of the ambient medium of less
than cm-3 with a
distance d to the remnant (Aschenbach 1998), we found that the
reverse shock does not heat the ejecta to sufficiently high
temperatures to ionize 44Ti.
© European Southern Observatory (ESO) 1999
Online publication: June 17, 1999