High-velocity features of calcium and silicon in the spectra of Type Ia supernovae

Abstract

High-velocity features" (HVFs) are spectral features in Type Ia supernovae (SNe Ia) that have minima indicating significantly higher (by greater than about 6000 km s −1 ) velocities than typical "photospheric-velocity features" (PVFs). The PVFs are absorption features with minima indicating typical photospheric (i.e., bulk ejecta) velocities (usually ∼9000-15,000 km s −1 near B-band maximum brightness). In this work we undertake the most in-depth study of HVFs ever performed. The dataset used herein consists of 445 low-resolution optical and near-infrared (NIR) spectra (at epochs up to 5 d past maximum brightness) of 210 low-redshift SNe Ia that follow the "Phillips relation." A series of Gaussian functions is fit to the data in order to characterise possible HVFs of Ca II H&K, Si II λ6355, and the Ca II NIR triplet. The temporal evolution of the velocities and strengths of the PVFs and HVFs of these three spectral features is investigated, as are possible correlations with other SN Ia observables. We find that while HVFs of Ca II are regularly observed (except in underluminous SNe Ia, where they are never found), HVFs of Si II λ6355 are significantly rarer, and they tend to exist at the earliest epochs and mostly in objects with large photospheric velocities. It is also shown that stronger HVFs of Si II λ6355 are found in objects that lack C II absorption at early times and that have red ultraviolet/optical colours near maximum brightness. These results lead to a self-consistent connection between the presence and strength of HVFs of Si II λ6355 and many other mutually correlated SN Ia observables, including photospheric velocity.

Time series of high-resolution spectra of SN 2014J observed with the TIGRE telescope

“…Like in our study, they did not detect a clear high velocity component of the Si II feature. All these observations are consistent with the study of high velocity components in other type Ia supernova performed by Silverman et al (2015). Observing a high velocity component of Ca II is much more common than one of Si II.…”

Spectropolarimetry of the Type Ib Supernova iPTF 13bvn: revealing the complex explosion geometry of a stripped-envelope core-collapse supernova

“…While separate HV and LV components are observed for Ca II in the polarised flux spectrum (at +9 days), an analysis of the Ca II IR3 line profile in the flux spectrum, following the prescription of Silverman et al (2015), was not able to recover the two separate components. The asymmetric line profile observed in the flux spectrum, from +7 days onwards, was found to result from a superposition of the individual components of the triplet (at ∼ −8800 km s −1 ) becoming partially resolved.…”

Discovery and Follow-up Observations of the Young Type Ia Supernova 2016coj

“…For each of these four SNe, we consequently try to fit the early-time velocities (typically within 10 days after first light) with a power-law function, v C t 1 = ¢ a , where t¢ is the time after first light (t 0 ); the results are shown in the top panel of Figure 6 for each SN. This is very similar to the method that of Silverman et al (2015, Figure 12) adopted, but they used a natural exponential function to fit the velocities before +5 days after peak brightness and also obtained reasonable fitting results. In fact, Piro & Nakar (2013, Equation (13)) mathematically show that the photospheric velocity could decay as a power law at early times.…”

“…In addition to the usual photospheric-velocity feature (PVF) of Ca II H&K, SN2016coj exhibits a high-velocity feature (HVF; e.g., Mazzali et al 2005;Maguire et al 2012Maguire et al , 2014Folatelli et al 2013;Childress et al 2014;Silverman et al 2015) in nearly all of the early-time spectra. This HVF appears to be detached from the rest of the photosphere, with a velocity of ∼25,000 km s 1 -at discovery and slowing down to ∼20,000 km s 1 -at ∼8 days after the fitted first-light time.…”