NLTE spectral modelling for a carbon-oxygen and helium white dwarf merger as a Ca-rich transient candidate

We carried out NLTE (non local thermodynamic equilibrium) radiative transfer simulations to determine whether an explosion during the merger of a carbon-oxygen (CO) white dwarf (WD) with a helium (He) WD can reproduce the characteristic Ca ii Ca ii and He i lines observed in Ca-rich transients. Our study is based on a 1D representation of a hydrodynamic simulation of a $0.6,msun,CO;+;0.4,$ WD merger. We calculated both the photospheric and nebular-phase spectra, including treatment for non-thermal electrons, as is required for accurate modelling of He i and Ca ii . Consistent with Ca-rich transients, our simulation predicts a nebular spectrum dominated by emission from Ca ii 7291, 7324,Å, and the Ca ii near-infrared (NIR) triplet. The photospheric-phase synthetic spectrum also exhibits a strong Ca ii NIR triplet, prominent optical absorption due to He i 5876,Å and He i 10830,Å in the NIR, which is commonly observed for Ca-rich transients. Overall, our results therefore suggest that CO+He WD mergers are a promising channel for Ca-rich transients. However, the current simulation overpredicts some He i features, in particular both He i 6678 and 7065,Å, and shows a significant contribution from Ti ii which results in a spectral energy distribution that is substantially redder than most Ca-rich transients at peak. Additionally, the Ca ii nebular emission features are too broad. Future work should investigate if these discrepancies can be resolved by considering full 3D models and exploring a range of CO+He WD binary configurations.