2024A&A...692A.204D

Much interest surrounds the nature of the compact remnant that formed in core collapse supernovae (SNe). One means to constrain its nature is to search for signatures of power injection from the remnant in the SN observables years after explosion. In this work, we conduct a large grid of 1D nonlocal thermodynamic equilibrium radiative transfer calculations of He-star explosions under the influence of magnetar-power injection from post-explosion age of about one to ten years. Our results for SN observables vary with He-star mass, SN age, injected power, or ejecta clumping. At high mass (model he12p00), the ejecta coolants are primarily O and Ne, with [O I] λλ6300.3, 6363.8, [O II] λλ7319.5, 7330.2, and [O III] λλ4958.9, 5006.8 dominating in the optical, and with strong [Ne II] 12.81 μm in the infrared - this line may carry more than half the total SN luminosity. For lower He-star masses (models he6p00 and he3p30), a greater diversity of coolants appear, in particular Fe, S, Ar, or Ni from the Si- and Fe-rich regions. All models tend to rise in ionization in time, with twice-ionized species (i.e., O III, Ne III, S III, or Fe III) dominating at ∼10 yr, although this ionization is significantly reduced if clumping is introduced. Our treatment of magnetar power in the form of high-energy electrons or X-ray irradiation yields similar results - no X-rays emerge from our ejecta even at ten years because of high-optical depth in the kilo-electronvolt range. An uncertainty of our work concerns the power deposition profile, which is not known from first principles, although this profile could be constrained from observations. Our magnetar-powered model he8p00 with moderate clumping yields a good match to the optical and near-infrared observations of Type Ib SN 2012au at both 289-335 d (power of 1 - 2 × 10⁴¹ erg s^–1) and 2269 d (power of 10⁴⁰ erg s^–1). Unless overly ionized (i.e., if the optical spectrum shows only strong [O III] λλ4958.9, 5006.8), we find that all massive magnetar-powered ejecta should be infrared luminous at 5-10 yr through strong [Ne II] 12.81 μm line emission.