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Photospheric Magnitude Diagrams for Type II Supernovae: A Promising Tool to Compute Distances

We develop an empirical color-based standardization for Type II supernovae (SNe II), equivalent to the classical surface brightness method given in Wesselink (1969). We calibrate it with SNe II with host galaxy distance measured with Cepheids, and well-constrained shock breakout epoch and extinction due to the host galaxy. We estimate the reddening with an analysis of the B-V versus V-I color-color curves, similar to that of Natali et al. (1994). With four SNe II meeting the above requirements, we build a photospheric magnitude versus color diagram (similar to an HR diagram) with a dispersion of 0.29 mag. We also show that when using time since shock breakout instead of color as independent variable, the same standardization gives a dispersion of 0.09 mag. Moreover, we show that the above time-based standardization corresponds to the generalization of the standardized candle method of Hamuy & Pinto (2002) for various epochs throughout the photospheric phase. To test the new tool, we construct Hubble diagrams to different subsamples of 50 low-redshift (cz<10^4 km s^-1) SNe II. For 13 SNe within the Hubble flow (cz_CMB>3000 km s^-1) and with well-constrained shock breakout epoch we obtain values of 68-69 km s^-1 Mpc^-1 for the Hubble constant, and an mean intrinsic scatter of 0.12 mag or 6% in relative distances.