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The finite time, $\tau_{\rm dep}$, over which positrons from $\beta^{+}$decays of $^{56}$Co deposit energy in type Ia supernovae ejecta lead, in casethe positrons are trapped, to a slower decay of the bolometric luminositycompared to an exponential decline. Significant light-curve flattening isobtained when the ejecta density drops below the value for which $\tau_{\rmdep}$ equals the $^{56}$Co life-time. We provide a simple method to accuratelydescribe this "delayed deposition" effect, which is straightforward to use foranalysis of observed light curves. We find that the ejecta heating is dominatedby delayed deposition typically from 600 to 1200~day, and only later by longerlived isotopes $^{57}$Co and $^{55}$Fe decay (assuming solar abundance). Forthe relatively narrow $^{56}$Ni velocity distributions of commonly studiedexplosion models, the modification of the light curve depends mainly on the$^{56}$Ni mass-weighted average density, $\langle \rho \rangle t^{3}$. Accuratelate-time bolometric light curves, which may be obtained with JWST far-infrared(far-IR) measurements, will thus enable to discriminate between explosionmodels by determining $\langle \rho \rangle t^3$ (and the $^{57}$Co and$^{55}$Fe abundances). The flattening of light curves inferred from recentobservations, which is uncertain due to the lack of far-IR data, is readilyexplained by delayed deposition in models with $\langle \rho\rangle t^{3}\approx 0.2\,M_{\odot}\,(10^{4}\, \textrm{km}\,\textrm{s}^{-1})^{-3}$, and doesnot imply supersolar $^{57}$Co and $^{55}$Fe abundances.

Constraints on the density distribution of type Ia supernovae ejecta inferred from late-time light-curve flattening