The evolution of helium stars in the mass range 2.0 to 4.0 solar masses

The evolution of single helium stars is calculated from the helium zero age main sequence up to neon ignition in the core for stellar masses of 2.2 M_sun;, 2.5 M_sun;, 2.9 M_sun;, 3.2 M_sun;, 3.5 M_sun;, and 4.0 M_sun;. For a 2.0 M_sun; helium star the evolution is calculated from the helium zero age main sequence, through ignition of carbon in the core (off-centre), up to the time when the convective carbon-burning shell reaches the centre. The aim of this investigation is: to determine the lower mass limit for neutron star formation by helium star evolution. This mass limit is found here to be most likely lower than the mass limit above which neon ignites in the core of a helium star (2.2 M_sun;). The evolutionary tracks in the p_c, T_c plane and in the HR diagram are intercompared and are also compared with tracks calculated by other investigators.