Secondary Maximum in the Near-infrared Lightcurves of Type Ia Supernovae

We undertake a theoretical study of the near-infrared (NIR) lightcurves of Type Ia supernovae (SNe Ia). In these bands, the lightcurves are distinguished by a secondary maximum occurring roughly 20 to 30 days after the initial one. Using time-dependent multi-group radiative transfer calculations, we calculate the UBVRIJHK-band lightcurves of model SN Ia ejecta structures. Our synthetic NIR lightcurves 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 intuition into the dependence of the NIR lightcurves 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 lightcurves on the mass of Ni, the degree of Ni mixing, the mass of electron caputre 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 . 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. Subject headings: radiative transfer – supernovae: general