SN 2020jgb: A Peculiar Type Ia Supernova Triggered by a Helium-Shell Detonation in a Star-Forming Galaxy

Abstract

The detonation of a thin ($\lesssim$$0.03{\mathrm{M}}_{\odot }$) helium shell (He-shell) atop a $\sim$$1{\mathrm{M}}_{\odot }$ white dwarf (WD) is a promising mechanism to explain normal Type Ia supernovae (SNe Ia), while thicker He-shells and less massive WDs may explain some recently observed peculiar SNe Ia. We present observations of SN 2020jgb, a peculiar SN Ia discovered by the Zwicky Transient Facility (ZTF). Near maximum light, SN 2020jgb is slightly subluminous (ZTF $g$-band absolute magnitude $M_g$ between $-18.2$ and $-18.7$ mag depending on the amount of host galaxy extinction) and shows an unusually red color ($g_{\mathrm{ZTF}}-r_{\mathrm{ZTF}}$ between 0.4 and 0.2 mag) due to strong line-blanketing blueward of $\sim$5000 $\tilde{A}$. These properties resemble those of SN 2018byg, a peculiar SN Ia consistent with a thick He-shell double detonation (DDet) SN. Using detailed radiative transfer models, we show that the optical spectroscopic and photometric evolution of SN 2020jgb are broadly consistent with a $\sim$$0.95{\mathrm{M}}_{\odot }$ (C/O core + He-shell; up to $\sim$$1.00{\mathrm{M}}_{\odot }$ depending on the total host extinction) progenitor ignited by a thick ($\sim$$0.13{\mathrm{M}}_{\odot }$) He-shell. We detect a prominent absorption feature at $\sim$1 $\hat{I}¼\mathrm{m}$ in the near-infrared (NIR) spectrum of SN 2020jgb, which could originate from unburnt helium in the outermost ejecta. While the sample size is limited, similar 1 $\hat{I}¼\mathrm{m}$ features have been detected in all the thick He-shell DDet candidates with NIR spectra obtained to date. SN 2020jgb is also the first subluminous, thick He-shell DDet SN discovered in a star-forming galaxy, indisputably showing that He-shell DDet objects occur in both star-forming and passive galaxies, consistent with the normal SN Ia population.

23 pages, 10 figures. Updated to accepted version (ApJ)