The Progenitors of Calcium-strong Transients

Abstract

A new class of faint, spectroscopically peculiar transients has emerged in the last decade. We term these events “calcium-strong transients” (CaSTs) because of their atypically high calcium-to-oxygen nebular line ratios. Previous studies have struggled to deduce the identity of their progenitors, due to a combination of their extremely extended radial distributions with respect to their host galaxies and their relatively high rate of occurrence. In this work, we find that the CaST radial distribution is consistent with the radial distribution of two populations of stars: old (ages >5 Gyr), low-metallicity ( Z / Z ⊙  < 0.3) stars, and globular clusters. While no obvious progenitor scenario arises from considering old, metal-poor stars, the alternative production site of globular clusters leads us to narrow down the list of possible candidates to three binary scenarios: mergers of helium and oxygen/neon white dwarfs; tidal disruptions of helium white dwarfs by neutron stars; and stable accretion from low-mass helium-burning stars onto white dwarfs. While rare in the field, these binary systems can be formed dynamically at much higher rates in globular clusters. Subsequent binary hardening both increases their interaction rate and ejects them from their parent globular clusters prior to mass transfer contact. Their production in, and ejection from, globular clusters may explain their radial distribution and the absence of globular clusters at their explosion site. This model predicts a currently undiscovered high rate of CaSTs in nuclear star clusters. Alternatively, an undetermined progenitor scenario involving old, low-metallicity stars may instead hold the key to understanding CaSTs.