Is oxygen an alpha element?

Question

Type II (core-collapse) supernovae occur shortly after star formation and enrich a galaxy with $\alpha$ elements such as O, C, NE, Mg, Ca and Si. On the other hand, Type Ia supernovae occur on a more delayed timescale and produce mostly Fe and heavier elements and negligible amounts of $\alpha$ elements. There is great interest in constraining the [$\alpha$/Fe] ratio of galaxies since that tells you something about the rate at which stars formed in the galaxy as a function of cosmic age.

I have usually seen studies rely on measuring abundances of Mg, Si and Ca relative to Fe, but not oxygen. For reference, the Wikipedia page for the alpha process says

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 stars. It is produced in Type II supernovas and its enhancement is well correlated with an enhancement of other alpha process elements.

Why would oxygen not be an $\alpha$ element in high-metallicity environments even though it is produced by Type II supernovae?

Do studies prefer [Mg/Fe], [Ca/Fe] and [Si/Fe] as a proxy for [$\alpha$/Fe] instead of [O/Fe] for an astrophysical reason, or just because the spectral features of Mg, Ca and Si are easier to measure than O?

Answer 1

Two things.

1. The abundance of oxygen is a difficult thing to measure in optical spectra - much harder than Mg, Ca, and Si. So these latter are usually used to represent "the alpha elements". There is a strong OI triplet at 777 nm and a much weaker OI forbidden line at 630 nm. But these often give contrary results because they are blended with other lines and suffer from uncertain NLTE effects (e.g. Ting et al. 2018). Mg, Ca and Si in contrast have many easily measured and well-understood absorption lines in stellar spectra.

2. Oxygen isn't only produced in massive stars and spewed out in supernovae (though that is probably the major cause of early galactic enrichment - e.g. Meyer et al. 2008). Oxygen is also formed in the He-burning zones of lower mass stars via the capture of a He nucleus by a $^{12}$C nucleus. Helium burning eventually happens in all stars that have left the main sequence in our galaxy. In particular, asymptotic giant branch stars of mass $2<M/M_\odot<8$ will never explode as supernovae, yet are much more numerous than high-mass stars and can distribute some of the products of their internal He-burning into the interstellar medium via deep convective mixing, pulsations and winds - the so-called third dredge-up (which brings up more carbon than oxygen actually). But this will happen later and in more metal-rich environments because the lifetimes of lower-mass stars are much longer. Thus even though oxygen is produced by alpha capture it may not behave like a typical "alpha element" in that it may show continuing enrichment beyond that produced by an initial burst of massive star formation in metal-poor populations. The oxygen abundance of the interstellar medium (and the stars born there-in) could continue to rise more slowly thanks to the contributions of numerous longer-lived AGB stars, not just the rarer supernovae. This complication is absent for Mg, Ca and Si because these are not produced in lower mass AGB stars.

Having said that, my understanding is that most authors treat oxygen as an alpha element and the contribution from AGB stars is not sufficient to stop [O/Fe] behaving very much like [Mg/Fe], [Si/Fe] and [Ca/Fe] in stellar populations (albeit with more noise, because it is difficult to measure). The plot below is from a well-cited paper by Bensby et al. (2014), and clearly shows that [O/Fe] behaves like the other alpha elements, although there is perhaps signs of a continuing decline for [Fe/H]>0 that is weaker in the other alpha elements.