A study of the low-luminosity Type II-Plateau supernova 2008bk

Abstract

Supernova (SN) 2008bk is a well-observed low-luminosity Type II event visually associated with a low-mass red-supergiant progenitor. To model SN 2008bk, we evolve a 12M⊙ star from the main sequence until core collapse, when it has a total mass of 9.88 M⊙, a He-core mass of 3.22 M⊙ and a radius of 502 R⊙. We then artificially trigger an explosion that produces 8.29M⊙ of ejecta with a total energy of 2.5 × 1050 erg and ~0.009M⊙ of 56Ni.We model the subsequent evolution of the ejecta with non-local thermodynamic equilibrium time-dependent radiative transfer. Although somewhat too luminous and energetic, this model reproduces satisfactorily the multiband light curves and multi-epoch spectra of SN 2008bk, confirming the suitability of a low-mass massive star progenitor. As in other low-luminosity SNe II, the structured Ha profile at the end of the plateau phase is probably caused by Ba II 6496.9 Å rather than asphericity.We discuss the sensitivity of our results to changes in progenitor radius and mass, as well as chemical mixing. A 15 per cent increase in progenitor radius causes a 15 per cent increase in luminosity and a 0.2 mag V-band brightening of the plateau but leaves its length unaffected. An increase in ejecta mass by 10 per cent lengthens the plateau by~10 d. Chemical mixing introduces slight changes to the bolometric light curve, limited to the end of the plateau, but has a large impact on colours and spectra at nebular times.