The Progenitors of Calcium-Strong Transients
  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/Zsol < 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.
