2020ApJ...902...91H

We propose a theory for the maximum magnitude versus rate of decline (MMRD) relation of novae using free-free emission model light curves built on the optically thick wind theory. We calculated (t₃,M_V,{max}) for various sets of ({dot}M_acc,M_WD), where M_V,{max} is the peak absolute V magnitude, t₃ is the 3 mag decay time from the peak, and {dot}M_acc is the mass accretion rate onto the white dwarf (WD) of mass M_WD. The model light curves are uniquely characterized by x≡M_env/M_sc, where M_env is the hydrogen-rich envelope mass and M_sc is the scaling mass at which the wind has a certain wind mass-loss rate. For a given ignition mass M_ig, we can specify the first point x₀=M_ig/M_sc on the model light curve and calculate the corresponding peak brightness and t₃ time from this first point. Our (t₃,M_V,{max}) points cover well the distribution of existing novae. The lower the mass accretion rate, the brighter the peak. The maximum brightness is limited to M_V,{max}≳-10.4 by the lowest mass accretion rate of {dot}M_acc≳1×10^–11M_☉ yr^–1. A significant part of the observational MMRD trend corresponds to the {dot}M_acc∼5×10^–9M_☉ yr^–1 line with different WD masses. A scatter from the trend line indicates a variation in their mass accretion rates. Thus, the global trend of an MMRD relation does exist, but its scatter is too large for it to be a precision distance indicator of individual novae.