The long-term influence of a magnetar power in stripped-envelope supernovae. Radiative-transfer modeling of He-star explosions from 1 to 10 years

Much interest surrounds the nature of the compact remnant 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, the ejecta coolants are primarily O and Ne, with [OI]6330A, [OII]7325A, and [OIII]5000A dominating in the optical, and with strong [NeII]12.81micron in the infrared -- this line may carry more than half the total SN luminosity. For lower He-star masses, 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., OIII, NeIII, SIII, or FeIII) dominating at ~10yr, 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 keV 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 SN2012au at both ~300d (power of 1-2x10^41erg/s) and 2269d (power of 10^40erg/s). Unless overly ionized, we find that all massive magnetar-powered ejecta should be infrared luminous at 5-10yr through strong [NeII]12.81micron line emission.