Light curve models for type IA supernovae - Physical assumptions, their influence and validity
Abstract
A state of the art radiation transfer code is presented for computing bolometric and monochromatic light curves of Type Ia supernovae. The radiation transfer code, which is also applicable to Type II supernovae, consists of (i) a LTE radiation transfer scheme (including an energy equation for matter and radiation, and effects due to electron and line scattering) based on the time-dependent, frequency integrated moment equations which are solved implicitly, (ii) a detailed equation of state with an elaborate treatment of the ionization balance and the ionization energies, (iii) time-dependent expansion opacities which take into account the composition structure of the explosion model, and (iv) a Monte Carlo gamma-ray deposition scheme which takes into account all relevant gamma-ray transitions and interaction processes. concerning the evolution of the structure of the explosion model a homologous expansion of the ejecta is assumed. The opacities are calculated under the assumption that temperature, density, chemical composition and expansion rate are constant over the free mean path of a photon. The mean expansion opacity which is obtained from the calculated monochromatic expansion opacities both by a Rosseland and a Planck mean, is given in tabular form. We find that the Rosseland and Planck mean opacity can differ by more than an order of magnitude. Each opacity table contains the mean expansion opacity as a function of temperature density and expansion rate for a prescribed composition. In the light curve calculations interpolations have to be performed between different tables corresponding to chemically different layers of the ejecta. Using our radiation transfer code we have calculated the bolometric and monochromatic light curves of a particular delayed detonation model (N21). The calculations have also been performed assuming various levels of physical simplifications (e.g. constant opacity diffusion approximation- no scattering) in order to study the influence and validity of many commonly made physical assumptions in modelling Type Ia supernova light curves. According to our results the temperature structure of the envelope critically depends on the use of a time- dependent opacity, the inclusion of line scattering, the correct; distinction between the frequency averaging in the radiation energy (Planck mean) and radiation flux (Rosseland mean) moment equations, and on the approximations used in the radiation transfer. In particular, the coupling between temperature profile and optical depth cannot be neglected, e.g., as done when assuming a constant opacity. Consequently, the time-dependent photospheric radius is strongly affected by the assumptions and approximations made in a light curve model. At maximum light the photospheric radius can be wrong by up to 70% with the error; increasing at later epochs. This fact can have important implications for Type Ia supernovae both for their use as distance indicators via the Baade-Wesselink method, and for their spectral analysis. Our results further show that although the bolometric light curve only depends on the total energy deposition due to the decay of ^56^Co at late epochs, its early rise, its maximum luminosity L_bol_, the time of maximum light t_bol_(max), and its early decline sensitively depend on the physical assumptions. In particular, using a time-independent opacity may cause an error in L_bol_(max) of about 50% and in t_bol_(max) of about 3.5 days, respectively. In order to obtain accurate monochromatic light curves line scattering effects must be taken into account, because otherwise most of the flux will erroneously be emitted in the infrared.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- February 1993
- Bibcode:
- 1993A&A...268..570H
- Keywords:
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- Astronomical Models;
- Light Curve;
- Supernovae;
- Bolometers;
- Computational Astrophysics;
- Local Thermodynamic Equilibrium;
- Opacity;
- Photoionization;
- Radiative Transfer;
- Astrophysics