Hydrogen shell flash

Helium flash A rapid burst of nuclear reactions in the hydrogen-shell burning phase of stellar evolution. 

However, there are two sets of observations which indicate that AGB stars do indeed reach the AGB limit Mbol = -7.1. Firstly, the results of WBF and Reid, Glass and Catchpole (1987) for the LPVs show that such stars do reach the AGB limit. Secondly, a luminosity function for red stars in the bar of the LMC obtained by Hughes and Wood (1987) shows that AGB stars extend to Mbol = -7.1, although there is a steep fall-off with luminosity above Mboi = -6. One explanation for the rapid fall-off in the number of AGB stars above Mboi = -6 is provided by Wood and Faulkner (1986) who show that envelope ejection will occur at this point in all but the most massive AGB stars due to the luminosity of the star exceeding the Eddington limit at the base of the hydrogen-rich envelope during the surface luminosity peak of a helium shell flash. 

In aqueous solutions, it has been shown that the solvated electron e" circulates in the solvation shell until it is captured by I2 to given an I, radical ion which is finally stabilized to I . The solvated electrons have a characteristic absorption band near 700 nm which has been detected in flash pbotolytic studies of aqueous KI. The orbital of the excited electron may be considered to be spherically symmetric like that of a hydrogen atom, with its centre coinciding with the centre of the cavity containing the ion. 

Following the core helium flash, the star quickly ( 106 years) moves to the Horizontal Branch (HB), where it burns 4He in a convective core, and hydrogen in a shell (that provides most of the luminosity). This corresponds to points 10-13 in Figure 3. The coulomb repulsion is larger for He than for H,... 

This conversion is conducted at moderate temperature and pressure (100°C, 25.106 Pa absolute), and possibly in the presence of a hydrocarbon diluent, for better control of the temperature rise in the catalyst beds, due to the high exothermidty of the reaction, which is itself related to the high diolefinic content of the initial C3 cut As a rule, the feed is introduced in a downflow stream into the reactor, which contains several beds of a noble metal catalyst on alumina. Quench by recycling and diluent injection is carried out between the beds. The diluent is recovered, by distillation in a depentanizer, after flash to eliminate the inert compounds introduced with hydrogen gas at the same time as the feedstock. The leading licensors inclnde IFP and Shell, etc. 

During the next 100000 years the situation is as follows in the non degenerate core, helium burns to carbon, the luminosity of the star is now 1/100 as it was at the time of the helium flash. The star shrinks, its surface temperature increases. In the H-R diagram (Fig. 9.10) the stars move to the left of the red giant branch. In this horizontal branch, stars burn helium in the core to carbon and hydrogen to He in a shell. Differences in their chemical composition affect where these stars fall on the horizontal branch. 

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