# Why is the carbon/oxygen ratio at low metallicities important to study?

Oxygen and carbon are the most abundant elements in the Universe (formed via stellar nucleosynthesis), following only hydrogen and helium (formed via the Big Bang). There seems to be a lot of work to constrain the carbon-to-oxygen ratio [C/O] for low-metallicity stars, but I'm struggling to understand the motivation/reasoning. I think the idea is that very low metallicity stars that exist today should have enhanced carbon and/or oxygen abundances, otherwise they would've formed as Population III stars and died as supernovae long ago. Is that right? Then does this automatically imply that low metallicities should coincide with low [C/O] since oxygen is more abundant than carbon? Wouldn't this depend on the gas out of which a star formed and how many high-mass vs. low-mass stars chemically enriched that pre-existing gas?

Relatedly, what is the motivation for studying [C/O] vs. metallicity (e.g., as traced by oxygen abundance [O/H]) for planets and galaxies, and are the correlations expected to be similar (i.e., low overall metallicities coincide with low [C/O] ratios)?

• Your follow-up questions about planets and galaxies are separate questions in my mind. However just briefly: In planetary atmospheres we expect exotic chemistry for [C/O]>1. However interesting those ideas are, it seems that the most planets will host a [C/O]<1, just like the Solar system planets. Oct 26, 2021 at 20:33

I think I can in part answer your questions. The [CII] ($$\lambda=158\,\mu m$$) and [OIII] ($$\lambda=88\,\mu m$$) are the most brightest IR emission lines in the local Universe Stacey et al. (1991). The former has been observed to be the dominant coolant of the interstellar medium (ISM) DeLooze et al. (2014) metal-poor local Universe dwarfs galaxies. Cooling of the gas is necessary in order for a cloud of gas to lose energy and collapse under the influence of gravity, so to form stars and galaxies.

The latter has a high ionisation potential energy (IP), therefore the emission of its lines is possible only in presence of a strong ionising radiation, meaning it can be correlated with ionised gas.

Therefore, the ratio [OIII]/[CII] is useful to investigate the ionisation state of star-forming galaxies. For instance, a larger value indicates the presence of hot ionised gas, so a region of newly formed stars.

The ALMA telescope has recently detected very bright emissions of these, among others, important lines for galaxies at redshift z>6 Harikane et al. (2020). At that period of the Universe (i.e. at that redshift), these galaxies are extremely metal-poor, making this a good opportunity to study the formation of such stars (POPII stars).

I suggest you read the paper I cited, especially the recent one of Harikane et al. (2020). It gives a good explanation of what we can achieve with the study of these emission lines.

Cheers!