Measuring Expansion Velocities in Type II-P Supernovae

TL;DR

This paper compares different methods for estimating photospheric velocities in Type II-P supernovae, demonstrating that SYNOW provides accurate, less computationally intensive velocities and refining velocity relations for better distance measurements.

Contribution

The study introduces a simplified SYNOW-based method for velocity estimation and refines velocity relations to improve supernova distance measurements.

Findings

SYNOW velocities are comparable to NLTE models but less computationally intensive.

FeII velocities correlate more tightly with model velocities than Hbeta velocities.

Refined velocity relations enhance the accuracy of supernova distance estimates.

Abstract

We estimate photospheric velocities of Type II-P supernovae using model spectra created with SYNOW, and compare the results with those obtained by more conventional techniques, such as cross-correlation, or measuring the absorption minimum of P Cygni features. Based on a sample of 81 observed spectra of 5 SNe, we show that SYNOW provides velocities that are similar to ones obtained by more sophisticated NLTE modeling codes, but they can be derived in a less computation-intensive way. The estimated photospheric velocities (v_model) are compared to ones measured from Doppler-shifts of the absorption minima of the Hbeta and the FeII \lambda5169 features. Our results confirm that the FeII velocities (v_Fe) have tighter and more homogeneous correlation with the estimated photospheric velocities than the ones measured from Hbeta, but both suffer from phase-dependent systematic deviations from those. The same is true for comparison with the cross-correlation velocities. We verify and improve the relations between v_Fe, v_Hbeta and v_model in order to provide useful formulae for interpolating/extrapolating the velocity curves of Type II-P SNe to phases not covered by observations. We also discuss the implications of our results for the distance measurements of Type II-P SNe, and show that the application of the model velocities is preferred in the Expanding Photosphere Method.