Alpha process

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The Alpha process (or alpha reactions) is one of two classes of nuclear fusion reactions by which stars convert helium into heavier elements, the other being the triple-alpha process. While the triple-alpha process only requires helium, once some carbon is present, other reactions that consume helium are possible:

126C	+	42He	→	168O	+	γ
168O	+	42He	→	2010Ne	+	γ
2010Ne	+	42He	→	2412Mg	+	γ

All these reactions have a very low rate and therefore do not contribute significantly to the energy production in stars; with elements heavier than neon (atomic number > 10) they occur even less easily due to the increasing Coulomb barrier.

Alpha process elements (or alpha elements) are so-called since their most abundant isotopes are integer multiples of the mass of the helium nucleus (the alpha particle). Alpha elements are Z ≤ 22: (C, N), O, Ne, Mg, Si, S, Ar, Ca, Ti. They are synthesized by alpha-capture in the silicon fusing precursor state to Type II supernovae. Silicon and calcium are purely alpha process elements. Magnesium can be burned by proton capture reactions. As for oxygen, some authors consider it an alpha element, while others do not. Oxygen is surely an alpha element in low metallicity population II stars. It is produced in Type II supernovae and its enhancement is well correlated with an enhancement of other alpha process elements. Sometimes C and N are considered alpha process elements, since they are synthesized in nuclear alpha-capture reactions.

The abundance of alpha elements in stars is usually expressed in a logarithmic manner:

,

Here N_α and N_Fe are the number of alpha element atoms and Fe atoms per unit volume. Theoretical galactic evolution models predict that early in the universe there were more alpha elements relative to Fe. Type II supernovae mainly synthesize oxygen and the alpha-elements (Ne, Mg, Si, S, Ar, Ca and Ti) while Type Ia supernovae produce elements of the iron peak (V, Cr, Mn, Fe, Co and Ni).

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The Age, Metallicity and Alpha-Element Abundance of Galactic Globular Clusters from Single Stellar Population Models

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