# What does “O I” mean in the context of detected molecules?

I've recently been analysing the Exoplanet Database, and have been looking at the 'molecules' field (molecules detected on the planet or in its atmosphere). They all make sense apart from "O I". It's not iodine oxide (which would be I2O), so I'm at a loss as to what it could be, especially since I'm very new to this field.

Any help is very much appreciated!

• Hint: Search for "ionization states of atomic oxygen;" a single atom molecule. – uhoh Mar 1 '18 at 10:17
• Please edit and gives us an example (text or link) of where we can find that abbreviation – user1569 Mar 1 '18 at 10:34

## 1 Answer

This is the standard astronomer notation for neutral oxygen. Where chemists would write $\mathrm{O}$ (or $\mathrm{O}^0$), $\mathrm{O}^{+}$, $\mathrm{O}^{++}$, and $\mathrm{O}^{+++}$ (or $\mathrm{O}^{3+}$), for neutral, once-, twice-, and thrice-ionized oxygen, respectively, astronomers almost exclusively write $\mathrm{O}\,\mathrm{I}$, $\mathrm{O}\,\mathrm{II}$, $\mathrm{O}\,\mathrm{III}$, $\mathrm{O}\,\mathrm{IV}$, etc.

The same is of course the case for other elements. For instance, you may have an $\mathrm{H}\,\mathrm{II}$ region (i.e. ionized hydrogen) surrounding hot stars, in a larger $\mathrm{H}\,\mathrm{I}$ (i.e. neutral) envelope.

In other words, we start at 1, where chemists start at 0.

• Follow-on question: why does this oxygen not bond covalently with other oxygen atoms to form O2 in the atmospheres of planets containing O I? – JThistle Mar 1 '18 at 10:23
• @JThistle Atmospheres are far from my comfort zone, but in general to form molecules, you must have high densities and low temperatures. The former should be satisfied in a atmosphere, I suppose, but perhaps it's too close to its star so the temperature is too high, or it is exposed to a strong UV field from the star, so $\mathrm{O}_2$ is dissociated? – pela Mar 1 '18 at 10:40
• @pela: high densities are the right keyword. If you just mix a bunch of oxygen atoms together and leave them at low densities, they will stay in atomic form instead of forming molecules. Thus, atomic species probe low-density or low-pressure environments in atmospheres. – AtmosphericPrisonEscape Mar 1 '18 at 11:58
• @JThistle In general we prefer people to ask new questions as proper new full questions, not comments. You can link to the original question in your new one. – StephenG Mar 1 '18 at 12:03