ASASSN-15hy: An Underluminous, Red 03fg-like Type Ia Supernova

We present photometric and spectroscopic observations of the 03fg-like Type Ia supernova (SN Ia) ASASSN-15hy from ultraviolet (UV) to near-infrared (NIR). shares many hallmark characteristics SNe Ia, previously referred as "super-Chandrasekhar" Ia. It is bright in UV NIR, lacks a clear i-band secondary maximum, shows strong persistent C ii feature, has low Si λ6355 velocity. However, some its properties are also extreme among subgroup. underluminous (MB,peak = mag), red ( yet slowly declining (Δm15(B) 0.72 ± 0.04 mag). most delayed onset maximum any SN. prominent H-band break emission feature that typically during first month past normal Such events may be potential problem for high-redshift SN cosmology. explained context an explosion degenerate core inside nondegenerate envelope. The impacting envelope with large mass provides additional luminosity ejecta velocities. An initial deflagration burning phase critical reproducing 56Ni luminosity, while essential providing diffusion timescales required produce broad light curves. model consists rapidly rotating 1.47 M⊙ 0.8 This "deflagration core-degenerate" scenario result merger between white dwarf asymptotic giant branch star.