High-velocity feature as the indicator of the stellar population of Type Ia supernovae

Abstract: Although type Ia supernovae (SNe Ia) are very useful in many astrophysical fields, their exact nature is still unclear, e.g. the progenitor and explosion models. The high-velocity features (HVFs) in optical spectra of SNe Ia could provide some meaningful information to constrain the nature of SNe Ia. Here, I show strong evidence that the SNe Ia with strong CaII infrared triple (CaII IR3) HVF around maximum brightness associate with relatively younger population than those with weak CaII IR3 HVF, e.g. the SNe Ia with strong CaII IR3 HVF tend to occur in late-type galaxy, or early-type galaxy with significant star formation. In addition, using pixel statistics I find that the SNe Ia with strong maximum-light CaII IR3 HVF show a higher degree of association with star formation index, e.g. ${\rm H}\alpha$ or near-UV emission, than those with weak CaII IR3 HVF. Moreover, I find that the strength of the CaII IR3 HVF is linearly dependent on the difference of the absorption-weighted velocities between CaII IR3 and SiII 635.5 nm absorption lines, which then is a good index to diagnose whether or not there is a high-velocity component in the CaII IR3 absorption feature in the spectra of SNe Ia. I finally discussed the origin of the HVFs and the constraints from our discoveries on the progenitor model of SNe Ia.