Analysis of blueshifted emission peaks in Type II supernovae

In classical P-Cygni profiles, theory predicts emission to peak at zero rest velocity. However, supernova spectra exhibit emission that is generally blueshifted. While this characteristic has been reported in many SNe, it is rarely discussed in any detail. Here, we present an analysis of H alpha emission peaks using a data set of 95 Type II supernovae, quantifying their strength and time evolution. Using a post-explosion time of 30 d, we observe a systematic blueshift of H alpha emission, with a mean value of -2000 km s(-1). This offset is greatest at early times but vanishes as supernovae become nebular. Simulations of Dessart et al. match the observed behaviour, reproducing both its strength and evolution in time. Such blueshifts are a fundamental feature of supernova spectra as they are intimately tied to the density distribution of ejecta, which falls more rapidly than in stellar winds. This steeper density structure causes line emission/absorption to be much more confined; it also exacerbates the occultation of the receding part of the ejecta, biasing line emission to the blue for a distant observer. We conclude that blueshifted emission-peak offsets of several thousand km s(-1) are a generic property of observations, confirmed by models, of photospheric-phase Type II supernovae.