Helium emission from Balmer-dominated shocks in Type Ia supernova remnants provides constraints to their progenitor systems

ABSTRACT Balmer-dominated shocks in Type Ia supernova remnants offer powerful probes into collisionless shock physics and hints towards supernova progenitor environments. Prior studies focused on the hydrogen Balmer lines, which manifest as a superposition of broad and narrow emission lines. Using integral-field spectroscopy with Multi Unit Spectroscopic Explorer (MUSE), we discovered broad and narrow helium emission lines from Balmer-dominated filaments of three Type Ia supernovae remnants in the Large Magellanic Cloud: SNR 0509-67.5, SNR 0519-69.0, and N103B. We detect broad and narrow He i 5876 Å, 7065 Å emission in SNR 0519 and N103B and He ii 8236 Å in SNR 0519. In SNR 0509, we detect narrow He i 5015 Å, 6678 Å, 7065 Å, and 7281 Å, with only 7065 Å exhibiting a broad component. The detection of narrow He ii challenges existing shock models, where such emission is not expected, and may indicate either incomplete ion–ion equilibration behind the shock or an origin in shock precursors. For SNR 0509 and N103B, the neutral He/H line ratios indicate enhanced helium abundances, whereas SNR 0519 is consistent with the primordial He/H value. We therefore propose helium emission in Balmer-dominated shocks as a new diagnostic of shock physics and Type Ia supernova circumstellar environments. Although our modelling is primarily a proof of concept, it demonstrates the possibility to infer the total He-to-H abundance ratio, with dominant uncertainties arising from the assumed initial ionization fractions. Despite the uncertainties, we demonstrate that narrow helium lines can serve as effective probes of circumstellar conditions and progenitor evolution when analysed alongside reliable constraints on the pre-shock neutral H/He abundance ratio.