Possible explanations for the formation of ear-like structures in young Type Ia supernova remnants

Several young type Ia supernova remnants (SNRs) exhibit apparent axisymmetrical deviations from spherical symmetry, manifested as two opposite ear-like protrusions in their projected morphologies. The origin of this specific feature remains debated, with several physical mechanisms proposed as possible explanations. In this work, we propose two scenarios to explain the formation of ear-like structures: (i) the feedback of cosmic-ray (CR) acceleration via an effective adiabatic index, γeff, with spatial variations, and (ii) the large-scale pre-existing density gradient in the local interstellar medium (ISM). Our cylindrical hydrodynamic simulation results reveal that both scenarios can produce prominent protrusions well beyond the main shell, resembling the peculiar features observed in several young type Ia SNRs. Additionally, based on the detailed analysis of the simulation data, we attempt to elucidate the ear-formation process and suggest that the relative positions of contact discontinuity can serve as an observational diagnosis in pursuing the origin of ear-like structures. We also discuss the following points: (i) the theoretical prediction of type Ia SNRs with “ears” and the visibility of an ear-like structure in observations; (ii) the simulation-based inference concerning the emission properties of the ear-like regions and the implications of a qualitative comparison of our results with X-ray observations; and (iii) the potential combination of different scenarios. Despite various existing models, we tend to regard our proposed scenarios as potential alternatives, whereby the ear-like structures originate in pure ejecta-ISM interaction, distinct from the ejecta-circumstellar medium (CSM) or jet-CSM models.