ISO/SWS observations of SN 1987A: II. A refined upper limit on the mass of Ti-44 in the ejecta of SN 1987A

ISO/SWS observations of SN 1987A on day 3425 show no emission in [Fe I] 24.05 microns and [Fe II] 25.99 microns down to the limits of roughly 0.39 Jy and 0.64 Jy, respectively. Assuming a homogeneous distribution of Ti-44 inside 2000 km/s and negligible dust cooling, we have made time dependent theoretical models to estimate an upper limit on the mass of ejected Ti-44. Assessing various uncertainties of the model, and checking the late optical emission it predicts, we obtain an upper limit of 1.1EE-4 solar masses. This is lower than in our previous estimate using other ISO data, and we compare our new result with other models for the late emission, as well as with expected yields from explosion models. We also show that steady-state models for the optical emission are likely to overestimate the mass of ejected Ti-44. The low limit we find for the mass of ejected Ti-44 could be higher if dust cooling is important. A direct check on this is provided by the gamma-ray emission at 1.157 Mev as a result of the radioactive decay of Ti-44.