what are polycyclic aromatic hydrocarbons and how do astronomers use them to study star forming regions?
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So, polycyclic aromatic hydrocarbons. Let’s break this down. A hydrocarbon is the most basic broad class of organic molecule – one whose molecules are composed of just carbon and hydrogen. Since hydrogen only has one electron to share, this means a hydrogen atom can only bind to one other atom, so the structure of hydrocarbons is formed of a carbon ‘skeleton’ with hydrogens generally hanging off most of those carbons.
Now, polycyclic means ‘many cycles’. This means the ‘skeleton’ of this molecule is composed of multiple cycles – typically six, sometimes five or seven and sometimes other numbers. We usually specify that each cycle must share a side with another cycle. So imagine two hexagons next to each other sharing a side, where each corner is a carbon atom (where we add hydrogens wherever necessary to make sure each carbon has four bonds). We can add even more, like a honeycomb.
Aromatic? Historically, compounds were called aromatic because they tend to give an ‘aromatic’ smell, the simplest example being benzene. But it means something more complicated. Now that’s the hardest one to explain, and usually requires at least one lecture in intro organic chemistry to motivate and define properly. But the ELI5 version is to take benzene as an example. One depiction of benzene is a setup with a ‘hexagon’ of carbons where each carbon atom is bonded to one hydrogen atom, to one neighbouring carbon once, and the other *twice* (with a double bond) – so we see something like …C=C-C=C-C=C-…[back to the start, in a ring], with each C also bonded to an H ‘hanging off it’. But something weird happens! For the double bonds, the second bonds are just as happy to hop to the next ‘step’ in the skeleton, and preserve the same structure… in fact the three ‘second’ electrons in those (three) double bonds are really not between two specific carbons at all, but swimming along the whole ring. Quantum mechanics allows us to make mathematical sense of ‘many simple pictures happening at once’ (when really our pictures are an approximation). Having all these apparent ‘options’ available lowers the energy of the molecule and makes it very stable, which makes a benzene ring a very useful building block, and something more likely to form than one might otherwise think.
So… a PAH (polycyclic aromatic hydrocarbon) is a molecule composed of multiple benzene hexagons (these special kinds of cycles) joined together like a piece of a honeycomb. And these can form quite naturally.
So naturally and stably in fact, that as specific as these organic molecules seem to be, they form a significant component not only of coal tar (which is composed of organic matter from ancient plants and such that has massively broken down but can’t so easily break these down)… but can even form in space!
Now the middle of your typical shining star is not a good place for a molecule to be – it’s so hot that bonds just break and instead we have atoms and even they see their electrons ripped away. But at the *relatively* ‘cooler’ surface of some cooler stars, and in the matter between stars and in nebulae (which are regions of dust dense enough that some of it might clump together and form stars), we often see organic molecules, and even PAHs.
Now when a molecule like a PAH is hit by a particle of light (and similar…) – a photon – that has just the right amount of energy, it will shake (we say that the photon activates a vibrational mode), and this will absorb the photon. They may also do other things that will emit or absorb photons of certain energies that are at very specific energies determined by the molecule. These energies correspond to ‘colours’ or more generally frequencies, and we see a bunch of these photons show up as ‘missing’ – instead of a nice smooth curve showing the amount of light we get for different frequencies (a ‘spectrum’), we see sudden drops at the frequencies that give those modes where absorbed – or spikes where they’re emitted. Different molecules have different modes, and this helps us figure out what molecules we see there! This is part of spectroscopy. For vibrational modes, these are usually in the infrared. For other sorts of actions like electrons ‘hopping’, they might be ultra-violet.
PAHs are cool because, although they form only a tiny fraction of the plasma and dust out there (most is hydrogen), not only do they have lots of these drops and spikes in their spectra (which give us info), but because of other important frequencies of photons that they absorb and emit, they also play a big role in heating up the plasma or ‘dust’ and how many other atoms get electrons ripped away. It’s a *bit* like how ozone and carbon dioxide have a role in the temperatures and radiation we see on earth. This has an impact on how quickly and how much stars will form, because when clumping and star formation is going on, the conditions for that all come from a balance of gravity squeezing the matter ‘in’ (towards becoming a star) fighting radiation pressure (from photons interacting in various ways) pushing ‘out’. This not only tells us whether a star will form, but how big and what kinds of stars.
Unfortunately, PAHs have lots of modes and especially if there are many PAHs it can be very hard to disentangle exactly which ones we are seeing, as at low resolutions a lot of our data will just end up looking like a mess. But it’s hoped that new machines for spectroscopy, with new data from new telescopes, will be able to figure that out much more easily! And from that, not only find out how common different PAHs are out there, and which… but test our theories and find more laws about all the different factors affecting how stars form.
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