Probing the Progenitors of Type Ia Supernovae through Near-Infrared Spectroscopy: A Case Study of SN 2018aoz

Type Ia supernovae (SNe Ia) are key cosmological distance indicators, yet their progenitor systems remain uncertain. In particular, it is still debated whether SNe Ia arise from a white dwarf accreting from a non-degenerate companion or from a double-degenerate merger. SN 2018aoz, a nearby normal SN Ia exhibiting early excess light, provides an excellent opportunity to investigate these progenitors. We present four near-infrared (NIR) spectra obtained with the FIRE spectrograph on the 6.5-m Magellan telescope as part of the Carnegie Supernova Project-II. NIR spectra provide unique diagnostic information not accessible in the optical, as supernova ejecta become optically thin at NIR wavelengths much earlier than at optical. Focusing on the spectrum at ~+53 days after maximum light, we searched for hydrogen Paβ (1.282 μm) emission, which would indicate material stripped from a non-degenerate companion by the collision with supernova ejecta. Using a novel technique to construct a hydrogen-free template spectrum and analyze residuals, we identify a tentative feature near Paβ with a slight redshift of ≈+700 km s⁻¹. This feature is consistent with models of a ~0.1M⊙ hydrogen envelope from a red-giant companion located behind the supernova. This first tentative detection of hydrogen Paβ in a normal SN Ia at post-maximum phase provides a new probe of progenitors and binary interaction geometry and offers crucial constraints on ejecta–companion interactions.