Abstract
We explore the physics of Type Ia supernova (SN Ia) light curves and spectra using the 1D non-local thermodynamic equilibrium (non-LTE) time-dependent radiative-transfer code cmfgen. Rather than adjusting ejecta properties to match observations, we select as input one ‘standard’ 1D Chandrasekhar-mass delayed-detonation hydrodynamical model, and then explore the sensitivity of radiation and gas properties of the ejecta on radiative-transfer modelling assumptions. The correct computation of SN Ia radiation is not exclusively a solution to an ‘opacity problem’, characterized by the treatment of a large number of lines. We demonstrate that the key is to identify and treat important atomic processes consistently. This is not limited to treating line blanketing in non-LTE. We show that including forbidden-line transitions of metals, and in particular Co, is increasingly important for the temperature and ionization of the gas beyond maximum light. Non-thermal ionization and excitation are also critical since they affect the colour evolution and the ΔM15 decline rate of our model. While impacting little the bolometric luminosity, a more complete treatment of decay routes leads to enhanced line blanketing, e.g. associated with 48Ti in the U and B bands. Overall, we find that SN Ia radiation properties are influenced in a complicated way by the atomic data we employ, so that obtaining converged results is a real challenge. Nonetheless, with our fully fledged cmfgen model, we obtain good agreement with the golden standard Type Ia SN 2005cf in the optical and near-IR, from 5 to 60 d after explosion, suggesting that assuming spherical symmetry is not detrimental to SN Ia radiative-transfer modelling at these times. Multi-D effects no doubt matter, but they are perhaps less important than accurately treating the non-LTE processes that are crucial to obtain reliable temperature and ionization structures.
Highlights
Over the last two decades Type Ia supernovae (SNe Ia) have become important tools for measuring basic cosmological parameters and the energy content of the Universe (Riess et al 1998; Perlmutter et al 1999)
Having covered the various ingredients controlling SN Ia radiation, we study the origin of the secondary maximum observed in near-IR SN Ia light curves (Section 6)
It is important to realize that, in this approach, the entire ejecta is modelled at all times by CMFGEN, i.e. that the radiative transfer is solved at all depths with merely a change in inner boundary condition at V0 when the ejecta turns nebular
Summary
Over the last two decades Type Ia supernovae (SNe Ia) have become important tools for measuring basic cosmological parameters and the energy content of the Universe (Riess et al 1998; Perlmutter et al 1999). One major disadvantage of the method is that the infrared spectral range is optically thin even before the B-band maximum, and the concept of a well-defined photosphere becomes meaningless Another approach is to perform time-dependent radiation transport and model the entire SN ejecta. An important message from our work on SNe Ia is that with detailed non-LTE radiative transfer, we can reproduce the fundamental SN Ia light curve and spectral properties with the basic delayed-detonation scenario, even with the assumption of spherical symmetry. This represents a very important result, it has limited value if we do not understand why or how it works.
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