Secondary maximum in the near-infrared light curves of Type Ia supernovae

We undertake a theoretical study of the near-infrared (NIR) light curves of Type Ia supernovae (SNe Ia). In these bands, the light curves are distinguished by a secondary maximum occurring roughly 20-30 days after the initial one. Using time-dependent multigroup radiative transfer calculations, we calculate the UBVRIJHK-band light curves of model SN Ia ejecta structures. Our synthetic NIR light curves show distinct secondary maxima and provide favorable fits to observed SNe Ia. We offer a detailed explanation of the origin of the NIR secondary maximum, which is shown to relate directly to the ionization evolution of iron group elements in the ejecta. This understanding provides immediate insight into the dependence of the NIR light curves on the physical properties of the ejecta and in particular explains why brighter supernovae have a later and more prominent secondary maximum. We demonstrate the dependence of the NIR light curves on the mass of Ni-56, the degree of Ni-56 mixing, the mass of electron capture elements, the progenitor metallicity, and the abundance of intermediate-mass elements (especially calcium). The secondary maximum is shown to be a valuable diagnostic of these important physical parameters. The models further confirm that SNe Ia should be excellent standard candles in the NIR, with a dispersion of less than or similar to 0.2 mag even when the physical properties of the ejecta are varied widely. This study emphasizes the consummate value of NIR observations in probing the structure of SNe Ia and in furthering their cosmological utility.