Spectral signatures of H-rich material stripped from a non-degenerate companion by a Type Ia supernova

¹ Institut d’Astrophysique de Paris, CNRS-Sorbonne Université, 98bis boulevard Arago, 75014 Paris, France
e-mail: dessart@iap.fr
² Department of Astronomy, San Diego State University, San Diego, CA 92182-1221, USA
³ Núcleo de Astronomía de la Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército 441 Santiago, Chile
⁴ Millennium Institute of Astrophysics, Santiago, Chile

Received: 29 February 2020
Accepted: 8 April 2020

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 (M_st), 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 M_st up to at least 300 days, while Hα should be observed after ~100 d for all M_st ≥ 0.01 M_⊙. Observational non-detections capable of limiting the Hα equivalent width to <1 Å set a formal upper limit of M_st < 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 M_st 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.