The Origins of Calcium-rich Supernovae From Disruptions of CO White Dwarfs by Hybrid He–CO White Dwarfs

Abstract

Calcium-rich explosions are very faint (M B ∼ −15.5), type I supernovae (SNe) showing strong Ca lines, mostly observed in old stellar environments. Several models for such SNe have been explored and debated, but none were able to reproduce consistently the observed properties of Ca-rich SNe, nor their rates and host-galaxy distributions. Here we show that the disruptions of low-mass carbon–oxygen (CO) white dwarfs (WDs) by hybrid helium–CO (HeCO) WDs during their merger could explain the origin and properties of such SNe. We make use of detailed multidimensional hydrodynamical-thermonuclear (FLASH) simulations to characterize such explosions. We find that the accretion of CO material onto an HeCO-WD heats its He shell and eventually leads to its “weak” detonation and ejection and the production of a sub-energetic ∼1049 erg Ca-rich SN, while leaving the CO core of the HeCO-WD intact as a hot remnant WD, possibly giving rise to X-ray emission as it cools down. We model the detailed light curves and spectra of such explosions and find excellent agreement with the observations of Ia/c Ca-rich, and potentially Ib Ca-rich, SNe. We thereby provide a viable, consistent model for the origins of Ca-rich SNe. These findings can shed new light on the role of Ca-rich SNe in the chemical evolution of galaxies and the intracluster medium, and their contribution to the observed 511 Kev signal in the galaxy originating from positrons produced from 44Ti decay. Finally, the origins of such SNe points to the key role of HeCO-WDs as SN progenitors and their potential role as progenitors of other thermonuclear SNe including normal Ia.