Abstract
The single-degenerate scenario for Type Ia supernovae should yield metal-rich ejecta that enclose some stripped material from the non-degenerate H-rich companion star. We present a large grid of non-local thermodynamic equilibrium steady-state radiative transfer calculations for such hybrid ejecta and provide analytical fits for the Hα luminosity and equivalent width. Our set of models covers a range of masses for 56Ni and the ejecta, for the stripped material (Mst), and post-explosion epochs from 100 to 300 d. The brightness contrast between stripped material and metal-rich ejecta challenges the detection of H I and He I lines prior to ~100 d. Intrinsic and extrinsic optical depth effects also influence the radiation emanating from the stripped material. This inner denser region is marginally thick in the continuum and optically thick in all Balmer lines. The overlying metal-rich ejecta blanket the inner regions, completely below about 5000 Å, and more sparsely at longer wavelengths. As a consequence, Hβ should not be observed for all values of Mst up to at least 300 days, while Hα should be observed after ~100 d for all Mst ≥ 0.01 M⊙. Observational non-detections capable of limiting the Hα equivalent width to <1 Å set a formal upper limit of Mst < 0.001M⊙. This contrasts with the case of circumstellar-material (CSM) interaction, not subject to external blanketing, which should produce Hα and Hβ lines with a strength dependent primarily on CSM density. We confirm previous analyses that suggest low values of order 0.001 M⊙ for Mst to explain the observations of the two Type Ia supernovae with nebular-phase Hα detection, in conflict with the much greater stripped mass predicted by hydrodynamical simulations for the single-degenerate scenario. A more likely solution is the double-degenerate scenario, together with CSM interaction, or enclosed material from a tertiary star in a triple system or from a giant planet.
Highlights
Unambiguous signatures of the single-degenerate scenario for Type Ia supernovae (SNe Ia) are much desired but hard to secure. One such signature is the identification of material stripped from the companion star by the SN Ia ejecta
Numerical approach We investigated the radiative signatures of stripped material in the innermost layers of SN Ia ejecta using a parametrized approach
Since we focus on the optical depth of the SN Ia ejecta overlying the putative stripped material located below 1000 km s−1, these simulations are suitable for estimating the optical depth of the metal-rich ejecta in the time range 30–100 d
Summary
Unambiguous signatures of the single-degenerate scenario for Type Ia supernovae (SNe Ia) are much desired but hard to secure. This setup is technically unphysical but operationally viable Inferred masses of stripped material for ASASSN-18tb and SN2018 cqj suggest very low values of about 0.001 M (Kollmeier et al 2019; Prieto et al 2020) Such low values are incompatible with the 0.1−0.5 M stripped material masses expected from the standard single-degenerate scenario for SNe Ia, and this holds whether one uses the model results of Mattila et al (2005) or those of Botyánszki et al (2018)
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