5 Thermonuclear X-ray Bursts
215
II If y ign is too shallow for runaway helium burning, the hydrogen flash does not
ignite helium. Instead, helium continues to pile up until it reaches its ignition
conditions at much larger y. This regime, therefore, exhibits brief hydrogen
flashes and long helium bursts. (No observations matching case I or case II
bursting have been identified.)
III At T 7 × 10 8 K, βCNO cycle burning of hydrogen is stable. It heats the
envelope and converts hydrogen into helium. At ˙
M 0.08 ˙
M Edd , the buildup
of the fuel layer is sufficiently slow for all hydrogen to burn stably to helium.
As most of the heating in the envelope comes from the hydrogen burning, T
hardly increases with depth once hydrogen is exhausted (see the lower series
of arrows in Fig. 5.2). Once the helium ignition curve is reached, a pure helium
burst ignites.
IV In a narrow range around 0.1 ˙
M Edd , heating by hydrogen burning creates the
conditions for helium to burn stably before reaching the helium ignition curve
[95]. This regime of stable hydrogen and helium burning produces pure carbon
ashes, which may in turn ignite upon reaching the carbon ignition curve and
power superbursts (see Sect. 5.7.2).
V At higher ˙
M, there is insufficient time for βCNO burning to deplete hydrogen.
Upon reaching helium ignition, hydrogen is still present in the fuel layer, and
it burns along with helium in the burst. In this case burst ignition is more
complicated because of the interplay between hydrogen and helium burning. As
3α burning of helium creates 12 C, the increase in the CNO abundance promotes
βCNO cycle burning of hydrogen. In turn, hydrogen burning produces helium,
which powers more 3α burning. This mode can be considered a combined
runaway of hydrogen and helium burning. Furthermore, once T 5 × 10 8 K,
break-out reactions from the βCNO cycle such as 15 O (α, γ ) 19 Ne boost nuc ,
and have an important effect on the ignition conditions [35, 44, 103]. See [46]
for a detailed description of the onset of a mixed hydrogen/helium burst.
VI Near ˙
M Edd helium burning is marginally stable due to competition between the
burning and cooling processes. It results in an oscillatory burning mode (see
Sect. 5.6).
VII For ˙
M ˙
M Edd the envelope is sufficiently hot and fuel is accumulated fast
enough for steady-state (stable) helium burning. Hydrogen also burns stably.
If the accreted fuel does not contain hydrogen, helium flashes are expected to
occur at all mass accretion rates below the transition to stable helium burning
(regime VII). Carbon burning is further discussed in Sect. 5.7.2.
5.1.1.5 Base Heating, Rotational Mixing, and Gravitational Separation
The above theoretical picture successfully describes many of the observed burning
regimes as a function of mass accretion rate. The range of ˙
M where each regime
occurs observationally are, however, different from the predicted values. For
example, the transition from bursts to stable burning (around regime VI) is observed
215
II If y ign is too shallow for runaway helium burning, the hydrogen flash does not
ignite helium. Instead, helium continues to pile up until it reaches its ignition
conditions at much larger y. This regime, therefore, exhibits brief hydrogen
flashes and long helium bursts. (No observations matching case I or case II
bursting have been identified.)
III At T 7 × 10 8 K, βCNO cycle burning of hydrogen is stable. It heats the
envelope and converts hydrogen into helium. At ˙
M 0.08 ˙
M Edd , the buildup
of the fuel layer is sufficiently slow for all hydrogen to burn stably to helium.
As most of the heating in the envelope comes from the hydrogen burning, T
hardly increases with depth once hydrogen is exhausted (see the lower series
of arrows in Fig. 5.2). Once the helium ignition curve is reached, a pure helium
burst ignites.
IV In a narrow range around 0.1 ˙
M Edd , heating by hydrogen burning creates the
conditions for helium to burn stably before reaching the helium ignition curve
[95]. This regime of stable hydrogen and helium burning produces pure carbon
ashes, which may in turn ignite upon reaching the carbon ignition curve and
power superbursts (see Sect. 5.7.2).
V At higher ˙
M, there is insufficient time for βCNO burning to deplete hydrogen.
Upon reaching helium ignition, hydrogen is still present in the fuel layer, and
it burns along with helium in the burst. In this case burst ignition is more
complicated because of the interplay between hydrogen and helium burning. As
3α burning of helium creates 12 C, the increase in the CNO abundance promotes
βCNO cycle burning of hydrogen. In turn, hydrogen burning produces helium,
which powers more 3α burning. This mode can be considered a combined
runaway of hydrogen and helium burning. Furthermore, once T 5 × 10 8 K,
break-out reactions from the βCNO cycle such as 15 O (α, γ ) 19 Ne boost nuc ,
and have an important effect on the ignition conditions [35, 44, 103]. See [46]
for a detailed description of the onset of a mixed hydrogen/helium burst.
VI Near ˙
M Edd helium burning is marginally stable due to competition between the
burning and cooling processes. It results in an oscillatory burning mode (see
Sect. 5.6).
VII For ˙
M ˙
M Edd the envelope is sufficiently hot and fuel is accumulated fast
enough for steady-state (stable) helium burning. Hydrogen also burns stably.
If the accreted fuel does not contain hydrogen, helium flashes are expected to
occur at all mass accretion rates below the transition to stable helium burning
(regime VII). Carbon burning is further discussed in Sect. 5.7.2.
5.1.1.5 Base Heating, Rotational Mixing, and Gravitational Separation
The above theoretical picture successfully describes many of the observed burning
regimes as a function of mass accretion rate. The range of ˙
M where each regime
occurs observationally are, however, different from the predicted values. For
example, the transition from bursts to stable burning (around regime VI) is observed