ON THE POSSIBILITY OF THE NONEXPLOSIVE CORE CONTRACTION OF MASSIVE STARS - NEW EVOLUTIONARY PATHS FROM ROTATING WHITE-DWARFS TO ROTATING NEUTRON-STARS

We have found that a new and nonexplosive evolutionary path from stars in the white dwarf region to stars in the neutron star region may be possible. Such a process can be realized if we incorporate both a larg e amount of rotation and the temperature effect on the equation of sta te. The large value of angular momentum is required to make stars secu larly unstable because of gravitational radiation emission. For high t emperature matter, the contribution of the temperature and the electro n fraction to the pressure becomes large enough to make rotating stars dynamically stable against axisymmetric perturbations. Thus; equilibr ium states may exist for rotating compact stars that are dynamically s table against axisymmetric collapse but secularly unstable. The evolut ion of dynamically stable and secularly unstable rotating stars can pr oceed as follows. The secular instability is caused by the emission of gravitational waves that carry away the angular momentum of the star. Stars with less angular momentum will contract to higher density stat es. This process occurs rather slowly, i.e., not on a dynamical timesc ale but on a secular timescale of gravitational radiation emission. Co nsequently, compact configurations such as white dwarfs in this catego ry may undergo nonexplosive and slow contraction. This contraction lea ds some configurations to neutron stars and others to black holes, dep ending on the mass and the angular momentum. If the final outcomes are neutron stars, they are both dynamically and secularly stable because some of the angular momentum is lost. Therefore, we have succeeded in showing that ''fizzlers'' can exist, although Newtonian gravity is us ed. This evolution is likely to occur within the central region of mas sive stars. Since the central region of a massive star is hot, high-te mperature effects become important. Concerning high angular-momentum, massive stars in the main sequence stage usually rotate rather rapidly . It implies that the angular momentum of the core can also be large e nough to lead to secular instability. Thus, cores of massive stars may contract on a long timescale without being accompanied by supernova e xplosions.