High-Velocity Features in Type Ia Supernovae—A puzzle still waiting to be solved after 26 years

High-velocity (25-30000 km/s) lines (HVFs) of Ca II were first discovered in the spectrum of SN1994D in 1999, and since then, they have been proven to be ubiquitous. Despite this, their exact origin remains unclear to this day. They appear to form in a higher-velocity layer above the photosphere (10-15000 km/s) and show varying strengths and velocity evolutions from object to object.

We used early-phase optical and near-infrared spectra of 56 Type Ia supernovae, observed over the span of 15 years by the Hobby–Eberly Telescope (McDonald Observatory, Texas) to determine the velocity, strength, and evolution of these features.

After using SYNOW to model the high-velocity features of these spectra, we compare them to Gaussian fits of the lines. We look at how velocity determination from the Gaussian fitting of spectral lines compares to velocities gained from the modelling of spectra. We confirm a connection between the light-curve width and the strength (and velocity) of the HVFs, with slow decliners showing stronger (and faster) high-velocity lines. We also demonstrate how high-velocity (HV) and normal-velocity (NV) Wang subtypes have differently behaving high-velocity features, which leads to interesting possibilities in the discussion of the progenitor systems of Type Ia supernovae.