Stars as Laboratories for Fundamental Physics - MPP Theory Group

398 Chapter 11
not much lower than the initial iron-core value, i.e. not much of the
lepton number is lost during infall. The conditions of chemical and
thermal equilibrium dictate that neutrinos take up only a relatively
small part (about 1 ) of the lepton number (Appendix D.2). A typical
4
Y L profile after collapse is shown in Fig. 11.2
The collapse is intercepted when the inner core reaches nuclear density
(ρ 0 ≈ 3×10 14 g cm −3 ), a point where the equation of state stiffens.
Because the inner core collapse is subsonic, the information about the
central condition spreads throughout, i.e. the collapse of the entire homologous
core slows down. However, this information cannot propagate
Fig. 11.2. Snapshots of the profiles of temperature T and lepton number
fraction Y L in the collapsed core of a massive star. The indicated times are
in seconds after collapse. The shaded arrows in the upper panel indicate
the motion of the temperature maximum. For a soft equation of state the
maximum temperature can be up to about 70 MeV. (Adapted from Burrows
and Lattimer 1986.)