Type II supernovae as distance indicators

Abstract

I report photometry and spectroscopy for 16 Type II supernovae (SNe) observed during the Calan/Tololo, SOIRS, and CTIO SN programs, a valuable resource for astrophysical studies. I perform a detailed assessment of the performance of the "expanding photosphere method" (EPM) in the determination of extragalactic distances. EPM proves very sensitive to the many steps involved in the analysis which can make it an art instead of an objective measurement tool. To minimize biases I implement objective procedures to compute synthetic magnitudes, measure true photospheric velocities, interpolate velocities, estimate dust extinction and realistic errors. While EPM performs well during the initial phases of SN evolution, I find distance residuals as large as 50% as the photosphere approaches the H recombination temperature. Despite the effort to lend credence to EPM, it proves necessary to exercise great care to avoid biasing the results. The main sources of uncertainties are observational errors (8%), dilution factors (11%), velocity interpolations (12%), and dust extinction (14%). The EPM Hubble diagram suggests the true error in an individual EPM distance is 20%. I find values of 63 ± 8 and 67 ± 7 km s⁻¹ Mpc⁻¹ for the Hubble constant, depending on the redshift sample chosen for the analysis. This result is independent of the extragalactic distance scale which yields 65 ± 5 from Cepheid/SNe la distances. From four objects the comparison of EPM and Tully-Fisher yields D(EPM)/D(TF) = 0.82 ± 0.12. I derive bolometric corrections for plateau SNe (SNe II-P) that permit me to obtain reliable bolometric luminosities from BVI photometry. Despite the great diversity displayed by SNe II-P, the duration of the plateau is approximately the same and the luminosities and expansion velocities measured in the middle of the plateau prove highly correlated. From the luminosity of the exponential tail I obtain ⁵⁶Co masses ranging between 0.02 and 0.28 M(⊙), and some evidence that SNe with brighter plateaus produce more Ni (and its daughter Co). The correlation between expansion velocity and luminosity permits me the use of SNe II-P as standard candles with a magnitude dispersion between 0.39-0.20 mag. Using SN 1987A to calibrate the Hubble diagram I get H₀ = 55 ± 12 and H₀ = 56 ± 9 from the V and I filters, respectively.