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Abstract Was Betelgeuse once in a binary star system? What causes it to vary over a vast range of timescales? Why did it dim dramatically in 2020? When and how will it explode? J. Craig Wheeler and Manos Chatzopoulos present a host of challenges to both observers and theorists. 

Abstract In this paper, we introduceSuperLite, an open-source Monte Carlo radiation transport code designed to produce synthetic spectra for astrophysical transient phenomena affected by circumstellar interaction.SuperLiteutilizes Monte Carlo methods for semi-implicit, semirelativistic radiation transport in high-velocity shocked outflows, employing multigroup structured opacity calculations. The code enables rapid post-processing of hydrodynamic profiles to generate high-quality spectra that can be compared with observations of transient events, including superluminous supernovae, pulsational pair-instability supernovae, and other peculiar transients. We present the methods employed inSuperLiteand compare the code’s performance to that of other radiative transport codes, such asSuperNuand CMFGEN. We show thatSuperLitehas successfully passed standard Monte Carlo radiation transport tests and can reproduce spectra of typical supernovae of Type Ia, Type IIP, and Type IIn. 

ABSTRACT The R Coronae Borealis (RCB) stars are extremely hydrogen-deficient carbon stars that produce large amounts of dust, causing sudden deep declines in brightness. They are believed to be formed primarily through white dwarf mergers. In this paper, we use mesa to investigate how post-merger objects with a range of initial He-burning shell temperatures from 2.1 to 5.4 × 108 K with solar and subsolar metallicities evolve into RCB stars. The most successful model of these has subsolar metallicity and an initial temperature near 3 × 108 K. We find a strong dependence on initial He-burning shell temperature for surface abundances of elements involved in the CNO cycle, as well as differences in effective temperature and radius of RCBs. Elements involved in nucleosynthesis present around 1 dex diminished surface abundances in the 10 per cent solar metallicity models, with the exception of carbon and lithium that are discussed in detail. Models with subsolar metallicities also exhibit longer lifetimes than their solar counterparts. Additionally, we find that convective mixing of the burned material occurs only in the first few years of post-merger evolution, after which the surface abundances are constant during and after the RCB phase, providing evidence for why these stars show a strong enhancement of partial He-burning products.