SN2019dge: a Helium-rich Ultra-Stripped Envelope Supernova

TL;DR

SN2019dge is a helium-rich ultra-stripped supernova with a low ejecta mass and kinetic energy, showing early shock cooling and circumstellar interaction, originating from a close binary system with a helium star.

Contribution

This paper reports the discovery and detailed analysis of SN2019dge, a helium-rich ultra-stripped supernova, highlighting its unique properties and progenitor system, and estimating its local birth rate.

Findings

Low ejecta mass (~0.3 M_sun) and kinetic energy (~1.2×10^{50} erg).

Evidence of shock cooling from an extended helium-rich envelope.

Estimated birth rate of 1400–8200 Gpc^{-3} yr^{-1}.

Abstract

We present observations of ZTF18abfcmjw (SN2019dge), a helium-rich supernova with a fast-evolving light curve indicating an extremely low ejecta mass ($\approx 0.3\,M_\odot$) and low kinetic energy ($\approx 1.2\times 10^{50}\,{\rm erg}$). Early-time (<4 d after explosion) photometry reveal evidence of shock cooling from an extended helium-rich envelope of $\sim0.1\,M_\odot$ located at $\sim 3\times 10^{12}\,{\rm cm}$ from the progenitor. Early-time He II line emission and subsequent spectra show signatures of interaction with helium-rich circumstellar material, which extends from $\gtrsim 5\times 10^{13}\,{\rm cm}$ to $\gtrsim 2\times 10^{16}\,{\rm cm}$. We interpret SN2019dge as a helium-rich supernova from an ultra-stripped progenitor, which originates from a close binary system consisting of a mass-losing helium star and a low-mass main sequence star or a compact object (i.e., a white dwarf, a neutron star, or a black hole). We infer that the local volumetric birth rate of 19dge-like ultra-stripped SNe is in the range of 1400--8200$\,{\rm Gpc^{-3}\, yr^{-1}}$ (i.e., 2--12% of core-collapse supernova rate). This can be compared to the observed coalescence rate of compact neutron star binaries that are not formed by dynamical capture.