Formation of an optically thick shocked shell in the very fast nova V1674 Herculis: the origin of superbrightness

Abstract: V1674 Her is the fastest ($t_2\sim 1$ day) classical nova in our Galaxy and its absolute $V$ peak of $M_{V,\rm max}\sim -10.2$ is one magnitude brighter than typical very fast novae. Such a nova is sometimes called a superbright nova. Using our fully self-consistent nova outburst model combined with the optically thick winds on a $1.35 ~M_\odot$ white dwarf (WD) with a mass accretion rate of $1\times 10^{-11} ~M_\odot$ yr$^{-1}$, we have clarified that a strong reverse shock arises $0.3$ days after the outburst, which is just after the maximum expansion of the WD photosphere. The shocked shell is optically thick and expanding with the velocity of $\sim 3500$ km~s$^{-1}$. Its $V$ brightness reaches maximum of $M_{V,\rm max}=-10.2$ when the shocked shell expands to $R_{\rm shell}\sim 300 ~R_\odot$ on day $\sim 0.7$. After that, the shocked shell turns to optically thin and becomes fainter than the brightness of free-free emission from the nova wind. In chronological order, the optical brightness of free-free emission reaches maximum of $M_V=-9$ on day 0.3. However, it is overtaken on day 0.5--0.7 by the $\sim$1 mag brighter luminosity of the optically thick shocked shell. The GeV gamma-ray flux reaches maximum on day 0.4 because the gamma-rays are emitted by the shock that arises on day 0.3. Our model consistently explains both the superbrightness and chronological order that the gamma-ray peak precedes substantially before the optical $V$ peak. We also present a similar light curve model for another superbright nova V1500 Cyg.