2.2 Formation rate
Update
The AIC occurrence rate is difficult to determine for a number of reasons. These include incomplete
understanding of binary star evolution and determining how much matter is truly accreted onto the white
dwarf versus the amount that is ejected through novae [48, 137, 234
]. Another uncertainty is whether the
collapse of an accreting Chandrasekhar mass white dwarf results in a supernovae explosion or a complete
AIC (with accompanying neutron star formation). Figure 1 shows one estimate of the region in the space of
initial white dwarf mass and accretion rate that produces AICs. New results continue to alter the dividing
lines between these fates [234].
The AIC rate can be indirectly inferred from the observed amount of rare, neutron rich
isotopes present in the Galaxy. These isotopes (formed via electron capture) are present in the
portion (
) of the outer envelope ejected by the star during an AIC. The exact yield is
sensitive to the neutron fraction in this ejecta, which depends both on the neutrino transport
and the electron capture rates, but if all of these isotopes present in the Galaxy are assumed
to have originated in AICs, an upper limit of
can be set for the Galactic AIC
rate [82
].
Binary population synthesis analysis can be used to determine which accreting white dwarfs will
undergo AIC. The results of Yungelson and Livio [268] predict that the galactic AIC rate is between
and
. Thus, a reasonable occurrence rate can be found for an observation
distance of
.