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.

SN 2019ehk: A Double-peaked Ca-rich Transient with Luminous X-Ray Emission and Shock-ionized Spectral Features

“…Perets (2014) finds the location of CaSTs to be mostly consistent with older stellar populations, with many of these objects having large separations from early-type host galaxies known to have large stellar halos. Shen et al (2019) also find that the radial distribution of CaSTs is consistent with old (>5 Gyr), metal-poor stellar populations. However, a non-negligible fraction of CaSTs were found in disk-shaped galaxies (Perets et al 2010;Perets 2014;De et al 2020).…”

“…A variety of progenitor scenarios have been proposed to explain the observed properties of CaSTs and their environments. Shen et al (2019) outline the three scenarios that are most consistent with current observations. First, ultra-stripped-envelope SNe could reproduce the low ejecta and Ni 56 masses and rapidly evolving light curves, but cannot reconcile the lack of star formation at most CaST explosion sites.…”

“…Early-time spectra of "gold sample" CaSTs (Shen et al 2019) resemble that of core-collapse type Ib SNe (SNe Ib) with detectable photospheric He I and no observed Hα emission. However, the large fraction of objects found in old stellar environments on the outskirts of early-type galaxies disfavors a massive star origin for most CaSTs (Perets et al 2011;Kasliwal et al 2012).…”

“…A potential explanation for the prominence of Ca II emission relative to other species is that the distribution of 56 Ni throughout the SN ejecta overexcites calcium ions (Polin et al 2019a). Because of this, we choose to adopt the label suggested by Shen et al (2019) and refer to these objects as "Calcium-Strong Transients" (CaSTs) from this point forward.…”

Type Ia supernova sub-classes and progenitor origin

“…Often far from potential galaxy host, see e.g. Shen et al 2019-likes: lower luminosity; faster than normal (e.g. narrow) light curves.…”