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We analyze the behavior of the outer envelope in a massive star during andafter the collapse of its iron core into a protoneutron star (PNS) in terms ofthe equations of one-dimensional spherically symmetric ideal hydrodynamics. Theprofiles obtained in the studies of the evolution of massive stars up to thefinal stages of their existence, immediately before a supernova explosion(Boyes et al. 1999), are used as the initial data for the distribution ofthermodynamic quantities in the envelope.We use a complex equation of state formatter with allowances made for arbitrary electron degeneracy and relativity,the appearance of electron-positron pairs, the presence of radiation, and thepossibility of iron nuclei dissociating into free nucleons and helium nuclei.We performed calculations with the help of a numerical scheme based onGodunov's method. These calculations allowed us to ascertain whether theemersion of the outer envelope in a massive star is possible through thefollowing two mechanisms: first, the decrease in the gravitational mass of thecentral PNS through neutrino-signal emission and, second, the effect of hotnucleon bubbles, which are most likely formed in the PNS corona, on theenvelope emersion. We show that the second mechanism is highly efficient in therange of acceptable masses of the nucleon bubbles ($\leq 0.01M_\odot$)simulated in our hydrodynamic calculations in a rough, spherically symmetricapproximation.Comment: 14 pages, 11 figure

The possibility of emersion of the outer layers in a massive star simultaneously with iron-core collapse: A hydrodynamic model