Naked pulsating core of a star discovered for the first time
Scientists at the University of Erlangen-Nuremberg (Germany), the University of Innsbruck (Austria), and the University of Geneva (Switzerland) have discovered for the first time the naked pulsating core of a massive star. It turned out to be an object in the Gamma Dove (γ Columbae) binary system, which was previously thought to be a typical white-blue star. About the discovery reported in an article published in the journal Nature Astronomy.
γ Columbae is located about 870 light-years from the Sun in the constellation of Dove and is a binary system, its main component is a pulsating star of spectral type B, whose mass is six times the mass of the Sun. Scientists have shown that although the temperature and gravity on the surface of this star are consistent with what one would expect from a typical white-blue subgiant, the helium, carbon, and nitrogen content of the outer envelope is dramatically different from normal stars and fits the pattern of the CNO cycle – thermonuclear reactions in the stellar core.
It is thought that stars can expose their inner cores only in very rare cases, when they evolve into unusual types of stars – Wolf-Raye stars, which represent a late stage in the life of massive stars, or hot helium subdwarfs. Wolf-Raye stars produce strong winds that can blow away their own outer envelope, while hot dwarfs are the result of the transfer of the outer envelope from a low-mass star to a neighboring star. In theory, main-sequence stars that burn hydrogen, turning it into helium, can also lose their envelope if the conditions are right for this, but so far this has not been observed.
Scientists have rejected alternative explanations for the unusual elemental composition of γ Columbae. According to one of them, helium, nitrogen and carbon, which catalyze thermonuclear reactions in the core, can move to the surface of the star by hydrodynamic mixing, but the ratios between N/C and N/O in Gamma Dove are too extreme for this process. A similar process can be observed in young stars with strong internal mixing, but this requires them to rotate very rapidly and to be much more massive.
The observed mass fractions of those elements affected by the CNO cycle correspond well to a 4-mass solar mass core in stars of 12 solar masses that are running out of hydrogen fuel. According to the theory of stellar evolution, at this stage the star is expanding. If it is in a binary system, the second star may sink into its outer shell and begin to approach its spiral neighbor, eventually leading to the removal of the shell from the system, which leaves behind a stripped core.
The model predicts that the star will live only 10,000 years in this state, making the likelihood of detecting other massive stars in the naked core stage very low. It is assumed that the object will end its life like Wolf-Raye stars, like Type Ib supernovae, which occur by the gravitational collapse of a core stripped of its hydrogen envelope.
