Radiation Transport for Explosive Outflows: Opacity Regrouping

Abstract

Implicit Monte Carlo (IMC) and Discrete Diffusion Monte Carlo (DDMC) are methods used to stochastically solve the radiative transport and diffusion equations, respectively. These methods combine into a hybrid transport-diffusion method we refer to as IMC-DDMC. We explore a multigroup IMC-DDMC scheme that, in DDMC, combines frequency groups with sufficient optical thickness. We term this procedure "opacity regrouping". Opacity regrouping has previously been applied to IMC-DDMC calculations for problems in which the dependence of the opacity on frequency is monotonic. We generalize opacity regrouping to non-contiguous groups and implement this in SuperNu, a code designed to do radiation transport in high-velocity outflows with non-monotonic opacities. We find that regrouping of non-contiguous opacity groups generally improves the speed of IMC-DDMC radiation transport. We present an asymptotic analysis that informs the nature of the Doppler shift in DDMC groups and summarize the derivation of the Gentile-Fleck factor for modified IMC-DDMC. We test SuperNu using numerical experiments including a quasi-manufactured analytic solution, a simple ten-group problem, and the W7 problem for Type Ia supernovae. We find that the opacity regrouping is necessary to make our IMC-DDMC implementation feasible for the W7 problem and possibly Type Ia supernova simulations in general. We compare the bolometric light curves and spectra produced by the SuperNu and PHOENIX radiation transport codes for the W7 problem. The overall shape of the bolometric light curves are in good agreement, as are the spectra and their evolution with time. However, for the numerical specifications we considered, we find that the peak luminosity of the light curve calculated using SuperNu is ∼10% less than that calculated using PHOENIX.

Faint rapid red transients from neutron star–CO white dwarf mergers

“…Transients from NS-WD mergers may, therefore, represent a completely different class of SNe that might be observed mostly in close-by galaxies using large telescopes or possibly with next-generation surveys such as LSST, as discussed above. As we show in Toonen et al (2018) and discuss above, the rates of NS-WD mergers could be sufficiently high as to be observed by LSST. The delay-time distribution for such mergers peaks at early times of hundreds of Myr up to one Gyr, and they are therefore expected to occur preferentially in late-type host galaxies.…”

“…Following the nuclear post-processing, we mapped the physical properties from this step as inputs for the openly available radiation transfer code SuperNu (Wollaeger et al 2013;Wollaeger & van Rossum 2014) in order to calculate the light-curves (LC) and spectra expected from the mergers. SuperNu uses Implicit Monte Carlo (IMC) and Discrete Diffusion Monte Carlo (DDMC) methods to stochastically solve the special-relativistic radiative transport and diffusion equations to first order in v/c in three dimensions.…”

Infrared Emission from Kilonovae: The Case of the Nearby Short Hard Burst GRB 160821B

“…We calculate radioactive heating energy partitioning between different decay species and their thermalization, using empirical formulae derived in Barnes et al (2016). Radiative transfer simulations are performed using the open source code SuperNu 3 (Wollaeger & van Rossum 2014), which implements a 3D semi-implicit multigroup Monte Carlo solver.…”

Light Curves and Spectra from a Unimodal Core-collapse Supernova

“…SuperNu has features specialized for homologous outflows and structured opacity (Wollaeger et al 2013;Wollaeger & Van Rossum 2014). DDMC is one of several techniques that use diffusion to accelerate Monte Carlo transport (see, for instance, Fleck & Canfield (1984); Gentile (2001); Densmore et al (2007Densmore et al ( , 2008Densmore et al ( , 2012; Abdikamalov et al (2012); Cleveland & Gentile (2014).…”

“…For the group resolution test, the heights of the peaks in the spectra vary with group resolution but do not shift in wavelength. For SuperNu, group resolutions on the order of 500 to 1000 yield reasonable results for the SN regime of domain properties (Wollaeger & Van Rossum 2014). Similarly, the multigroup transport code EDDINGTON has applied 500 to 5000 groups for various SN problems (Eastman & Pinto 1993;Eastman et al 1994;Pinto & Eastman 2000).…”