The UBV Color Evolution of Classical Novae. III. Time-stretched Color-Magnitude Diagram of Novae in Outburst

We propose a modified color-magnitude diagram for novae in outburst, i.e., (B - V) ₀ versus (M _V - 2.5 log f _s ), where f _s is the time-scaling factor of a (target) nova against a comparison (template) nova, (B - V) ₀ is the intrinsic B - V color, and M _V is the absolute V magnitude. We dub it the time-stretched color-magnitude diagram. We carefully reanalyzed 20 novae based on the time-stretching method and revised their extinctions E(B - V), distance moduli in the V-band (m - M) _V , distances d, and time-scaling factors f _s against the template nova LV Vul. We have found that these 20 nova outburst tracks broadly follow one of the two template tracks, the LV Vul/V1668 Cyg or V1500 Cyg/V1974 Cyg group, in the time-stretched color-magnitude diagram. In addition, we estimate the white dwarf masses and (m - M) _V of the novae by directly fitting the absolute V model light curves (M _V ) with observational apparent V magnitudes (m _V ). A good agreement of the two estimates of (m - M) _V confirms the consistency of the time-stretched color-magnitude diagram. Our distance estimates are in good agreement with the results of Gaia Data Release 2.