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 the ultraviolet (UV) to the near-infrared (NIR). ASASSN-15hy shares many of the hallmark characteristics of 03fg-like SNe Ia, previously referred to as "super-Chandrasekhar" SNe Ia. It is bright in the UV and NIR, lacks a clear i-band secondary maximum, shows a strong and persistent C II feature, and has a low Si II λ6355 velocity. However, some of its properties are also extreme among the subgroup. ASASSN-15hy is underluminous (M_B,peak = $-{19.14}_{-0.16}^{+0.11}$ mag), red ( ${(B-V)}_{B\max }={0.18}_{-0.03}^{+0.01}$ mag), yet slowly declining (Δm₁₅(B) = 0.72 ± 0.04 mag). It has the most delayed onset of the i-band maximum of any 03fg-like SN. ASASSN-15hy lacks the prominent H-band break emission feature that is typically present during the first month past maximum in normal SNe Ia. Such events may be a potential problem for high-redshift SN Ia cosmology. ASASSN-15hy may be explained in the context of an explosion of a degenerate core inside a nondegenerate envelope. The explosion impacting the nondegenerate envelope with a large mass provides additional luminosity and low ejecta velocities. An initial deflagration burning phase is critical in reproducing the low ⁵⁶Ni mass and luminosity, while the large core mass is essential in providing the large diffusion timescales required to produce the broad light curves. The model consists of a rapidly rotating 1.47 M_⊙ degenerate core and a 0.8 M_⊙ nondegenerate envelope. This "deflagration core-degenerate" scenario may result from the merger between a white dwarf and the degenerate core of an asymptotic giant branch star.