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 <ns0:sc>II</ns0:sc>/[Ca <ns0:sc>II</ns0:sc>] and He <ns0:sc>I</ns0:sc> lines observed in Ca-rich transients. Our study is based on a 1D representation of a hydrodynamic simulation of a 0.6 M⊙ CO+0.4 M⊙ He 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 <ns0:sc>I</ns0:sc> and [Ca <ns0:sc>II</ns0:sc>]. Consistent with Ca-rich transients, our simulation predicts a nebular spectrum dominated by emission from [Ca <ns0:sc>II</ns0:sc>] 7291, 7324 Å and the Ca <ns0:sc>II</ns0:sc> near-infrared (NIR) triplet. The photospheric-phase synthetic spectrum also exhibits a strong Ca <ns0:sc>II</ns0:sc> NIR triplet, prominent optical absorption due to He <ns0:sc>I</ns0:sc> 5876 Å and He <ns0:sc>I</ns0:sc> 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 <ns0:sc>I</ns0:sc> features, in particular both He <ns0:sc>I</ns0:sc> 6678 and 7065 Å, and shows a significant contribution from Ti <ns0:sc>II</ns0:sc>, which results in a spectral energy distribution that is substantially redder than most Ca-rich transients at peak. Additionally, the Ca <ns0:sc>II</ns0:sc> 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.