Locating Type II-P Supernovae Using the Expanding Photosphere Method. I. Comparing Distances from Different Line Velocities

Abstract

W present the results of our work testing a version of the Expanding Photosphere Method (EPM) used by Hamuy et al. and Dessart & Hillier to calculate distances to Type II-P supernovae, accounting for the deviations of their luminosities from those of true blackbodies. This method was applied to a sample of supernovae with data sets covering different postexplosion time periods. Different spectral lines in visible wavelengths—H β , He i , Fe ii , Sc ii , Na i , and Ba ii —are used to measure the expansion velocity with the goal of determining the species that produces the most reliable distance determination when combined with the blackbody temperature of the effective photosphere. This research suggests that H β , Fe ii , and Ba ii lines are most likely to yield accurate distances when combined with blackbody temperature, and provides further evidence of EPM’s effectiveness as an indicator of distance, provided we have a minimum of three data sets covering a broad range of postexplosion phases of the supernova.

Keywords

• Type II supernovae
• Spectral line identification
• Effective temperature
• Galaxy distances
• Baade-Wesselink method
• Radial velocity