Multi-zone models of Type I X-ray bursts are presented that use an adaptive nuclear reaction network of unprecedented size , up to 1300 isotopes , for energy generation and include the most recent measurements and estimates of critical nuclear physics .
Convection and radiation transport are included in calculations that carefully follow the changing composition in the accreted layer , both during the bursts themselves and in their ashes .
Sequences of bursts , up to 15 in one case , are followed for two choices of accretion rate and metallicity , up to the point where quasi-steady state is achieved .
For { \dot { M } } = 1.75 { { \times } { 10 ^ { -9 } } } { { \mathrm { M } _ { \odot } } { \mathrm { yr } } ^ { -1 } } ( and { \dot { M } } = 3.5 { { \times } { 10 ^ { -10 } } } { { \mathrm { M } _ { \odot } } { \mathrm { yr } } ^ { -1 } } , for low metallicity ) , combined hydrogen-helium flashes occur .
These bursts have light curves with slow rise times ( seconds ) and long tails .
The rise times , shapes , and tails of these light curves are sensitive to the efficiency of nuclear burning at various waiting points along the rp -process path and these sensitivities are explored .
Each displays “ compositional inertia ” in that its properties are sensitive to the fact that accretion occurs onto the ashes of previous bursts which contain left-over hydrogen , helium and CNO nuclei .
This acts to reduce the sensitivity of burst properties to metallicity .
Only the first anomalous burst in one model produces nuclei as heavy as A = 100 .
For the present choice of nuclear physics and accretion rates , other bursts and models make chiefly nuclei with A \approx 64 .
The amount of carbon remaining after hydrogen-helium bursts is typically \lesssim 1 % by mass , and decreases further as the ashes are periodically heated by subsequent bursts .
For { \dot { M } } = 3.5 { { \times } { 10 ^ { -10 } } } { { \mathrm { M } _ { \odot } } { \mathrm { yr } } ^ { -1 } } and solar metallicity , bursts are ignited in a hydrogen-free helium layer .
At the base of this layer , up to 90 % of the helium has already burned to carbon prior to the unstable ignition of the helium shell .
These helium-ignited bursts have a ) briefer , brighter light curves with shorter tails ; b ) very rapid rise times ( < 0.1 s ) ; and c ) ashes lighter than the iron group .