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

The detonation of a thin ($\\lesssim$$0.03\\,\\mathrm{M_\\odot}$) helium shell\n(He-shell) atop a $\\sim$$1\\,\\mathrm{M_\\odot}$ white dwarf (WD) is a promising\nmechanism to explain normal Type Ia supernovae (SNe Ia), while thicker\nHe-shells and less massive WDs may explain some recently observed peculiar SNe\nIa. We present observations of SN 2020jgb, a peculiar SN Ia discovered by the\nZwicky Transient Facility (ZTF). Near maximum light, SN 2020jgb is slightly\nsubluminous (ZTF $g$-band absolute magnitude $M_g$ between $-18.2$ and $-18.7$\nmag depending on the amount of host galaxy extinction) and shows an unusually\nred color ($g_\\mathrm{ZTF}-r_\\mathrm{ZTF}$ between 0.4 and 0.2 mag) due to\nstrong line-blanketing blueward of $\\sim$5000 $\\r{A}$. These properties\nresemble those of SN 2018byg, a peculiar SN Ia consistent with a thick He-shell\ndouble detonation (DDet) SN. Using detailed radiative transfer models, we show\nthat the optical spectroscopic and photometric evolution of SN 2020jgb are\nbroadly consistent with a $\\sim$$0.95\\,\\mathrm{M_\\odot}$ (C/O core + He-shell;\nup to $\\sim$$1.00\\,\\mathrm{M_\\odot}$ depending on the total host extinction)\nprogenitor ignited by a thick ($\\sim$$0.13\\,\\mathrm{M_\\odot}$) He-shell. We\ndetect a prominent absorption feature at $\\sim$1 $\\mu\\mathrm{m}$ in the\nnear-infrared (NIR) spectrum of SN 2020jgb, which could originate from unburnt\nhelium in the outermost ejecta. While the sample size is limited, similar 1\n$\\mu\\mathrm{m}$ features have been detected in all the thick He-shell DDet\ncandidates with NIR spectra obtained to date. SN 2020jgb is also the first\nsubluminous, thick He-shell DDet SN discovered in a star-forming galaxy,\nindisputably showing that He-shell DDet objects occur in both star-forming and\npassive galaxies, consistent with the normal SN Ia population.\n