An environmental analysis of the fast transient AT2018cow and implications for its progenitor and late-time brightness

Abstract

The nature of the newly discovered fast blue optical transients (FBOTs) is still puzzling astronomers. In this paper we carry out a comprehensive analysis of the molecular gas, ionized gas and stellar populations in the environment of the nearby FBOT AT2018cow based on ALMA, VLT/MUSE and HST/WFC3 observations. A prominent molecular concentration of 6 (± 1) × 10 6 𝑀 is found in the vicinity of AT2018cow, which has given rise to two active star-forming complexes with ages of 4 ± 1 Myr and 2.5 Myr, respectively. Each star-forming complex has a stellar mass of 3 × 10 5 𝑀 and has photoionized a giant H region with H𝛼 luminosity even comparable to that of the 30 Dor mini-starburst region. AT2018cow is spatially coincident with one of the star-forming complexes; however, it is most likely to reside in its foreground since it has a much smaller extinction than the complex. Its progenitor could have been formed at an earlier epoch in this area; if it were from a major star-forming event, the non-detection of the associated stellar population constrains the progenitor's age to be 10 Myr and initial mass to be 20 𝑀 . We further find the late-time brightness of AT2018cow is unlikely to be a stellar object. Its brightness has slightly declined from 2 yr to 4 yr after explosion and is most likely to originate from AT2018cow itself due to some powering mechanism still working at such late times.

A flash of polarized optical light points to an aspherical ‘cow’

“…A diverse range of scenarios have been proposed to explain the peculiar behaviour of AT 2018cow and similar FBOTs: the failed explosion of a supergiant star that forms a compact object, which accretes material from the progenitor (Margutti et al 2019;Quataert et al 2019); an electron-capture SN resulting from a merger of white dwarfs (Lyutikov & Toonen 2019); a tidal disruption event (Kuin et al 2019;Perley et al 2019), although associated with an intermediate or stellar mass blackhole, due to both the location of AT 2018cow in its host galaxy (see Sun et al 2022) and the fast rise-time (Kuin et al 2019;Ho et al 2019); or a jet originating from a merger of a neutron star and a massive star evolving in a common envelope (Soker 2022).…”