2012MNRAS.419.2783T

We estimate photospheric velocities of Type II-P supernovae (SNe) using model spectra created with SYNOW, and compare the results with those obtained either by more conventional techniques, such as cross-correlation, or by measuring the absorption minimum of P Cygni features. Based on a sample of 81 observed spectra of five SNe, we show that SYNOW provides velocities that are similar to those obtained by more sophisticated non-local thermodynamic equilibrium modelling codes, but they can be derived in a less computation-intensive way. The estimated photospheric velocities (V_model) are compared to those measured from Doppler shifts of the absorption minima of the Hβ and the Fe IIλ5169 features.

Our results confirm that Fe II velocities (v_Fe) have a tighter and more homogeneous correlation with the estimated photospheric velocities than those measured from Hβ, but both suffer from phase-dependent systematic deviations. The same is true for a comparison with the cross-correlation velocities. We verify and improve the relations between v_Fe, v_Hβ and v_modelin 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 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.