The type II-plateau supernova 2017eaw in NGC 6946 and its red supergiant progenitor

We present extensive optical photometric and spectroscopic observations, from 4 to 482 days after explosion, of the Type II-plateau (II-P) supernova (SN) 2017eaw in NGC 6946. SN 2017eaw is a normal SN II-P intermediate in properties between, for example, SN 1999em and SN 2012aw and the more luminous SN 2004et, also in NGC 6946. We have determined that the extinction to SN 2017eaw is primarily due to the Galactic foreground and that the SN site metallicity is likely subsolar. We have also independently confirmed a tip-of-the-red-giant-branch (TRGB) distance to NGC 6946 of 7.73 ± 0.78 Mpc. The distances to the SN that we have also estimated via both the standardized candle method and expanding photosphere method corroborate the TRGB distance. We confirm the SN progenitor identity in pre-explosion archival Hubble Space Telescope (HST) and Spitzer Space Telescope images, via imaging of the SN through our HST Target of Opportunity program. Detailed modeling of the progenitor's spectral energy distribution indicates that the star was a dusty, luminous red supergiant consistent with an initial mass of ~15 M ⊙

A benchmark comparison of subduction models

Numerically modelling the dynamics of a self-consistently subducting lithosphere is a challenging task because of the decoupling problems of the slab from the free surface. We address this problem with a benchmark comparison between various numerical codes (Eulerian and Lagrangian, Finite Element and Finite Difference, with and without markers) as well as a laboratory experiment. The benchmark test consists of a prescribed setup of viscous flow, driven by compositional buoyancy, and with a low viscosity, zero-density top layer to approximate a free surface. Alternatively, a fully free surface is assumed. Our results indicate that the convergence of the subduction behaviour with increasing resolution strongly depends on the averaging scheme for viscosity near moving rheological boundaries. Harmonic means result in fastest subduction, arithmetic means produces slow subduction and geometric mean results in intermediate behaviour. Complete convergence of results appears mostly beyond presently feasible grid resolution. Analysing the behaviour reveals that this problem is caused by the entrainment of the weak zero-density material into a lubrication layer on top of the subducting slab whose thickness turns out to be smaller than even the finest grid resolution. Agreement between the free surface runs and the weak top layer models is satisfactory only if both approaches use high resolution. Comparison of numerical models with a free surface laboratory experiment shows that (1) Lagrangian-based free-surface numerical models can closely reproduce the laboratory experiments provided that sufficient numerical resolution is employed and (2) Eulerian-based codes with a weak surface layer reproduce the experiment if harmonic averaging of viscosity is used. The harmonic mean is also preferred if circular high viscosity bodies with or without a lubrication layer are considered. We conclude that modelling the free surface by a weak zero-density layer gives good results but care has to be taken in 1) handling the associated entrainment and formation of a lubrication layer and 2) choosing the appropriate averaging scheme for viscosity at rheological boundaries