Population II – a research primer

I mentioned something about my day job yesterday. I’ve spent much of today peering at plots and graphs and tables, trying to come up with something sensible to say about the science aims. Not much luck, unfortunately! Oh well, c’est la vie. Writer’s block is a funny thing.

Sometimes, when your brain locks itself down, it’s best to take a step or two back, remind yourself what it’s all about.

My PhD is on brown dwarfs; these are what happens when star formation fails. Why it sometimes fails, we don’t really know. There are various theories, and they’re all flawed. Better yet, none of them fit the data well. Some models can explain some bits, others other bits. (One of my pet theories is there are actually several pathways to brown dwarf formation – we’re not seeing one process but two or three, hence the confused data.)

Anyway, in particular I look at ‘metal poor’ T-dwarfs. The ‘T’ bit means brown dwarfs with temperatures of 1400 K and lower. (If it gets to the T spectral type then it’s definitely not an actual hydrogen-burning star. The situation gets muddled for L-dwarfs, which are hotter brown dwarfs; some of them actually are proper stars, albeit pretty much the coolest and faintest there are.)

The ‘metal-poor’ refers to chemical composition. In astronomy a ‘metal’ is any element heavier than helium. This sounds an odd definition but it actually makes sense. Hydrogen came direct from the Big Bang. So does most of the helium. With the exception of some lithium, though, everything else was made in stars, as by-products of nuclear burning. All the carbon, oxygen and uranium and everything else was either blown out in supernova explosions or it was exhaled on the stellar winds of dying red giants. Although it’s a cliched line, we are indeed ‘made of star dust’. (I know, I shuddered too while I typed that.)

Now, there is an implication hidden in all this. The very first generation of stars, the so-called ‘Population III’, would have contained next to no heavy elements. They would have been pure H/He. But, after these first stars died, they would have enriched the surrounding gas clouds with the first heavy elements. The next stars, the so-called ‘Population II’, started off with small but non-zero metallicities. Then, as some of these stars age and die, they dump more metals back into the Galaxy, leading to the formation of the modern stars, like our Sun – the so-called Population I.

Now, there are no observable Pop III stars (at least, that we know of). It’s thought that they’ve all evolved and died, although some of their light may still be visible in the cosmological background, at extreme red-shifts. Population II, however, still lingers on. You see, the less luminous a star is, the longer it lives. Although fainter stars tend to be less-massive as well, their nuclear burning rate drops off through the floor. The theoretical life expectancy of a red dwarf of (say) 0.1 Sun-masses is on the order of 6,000 billion years(!).[1] Pop II red dwarfs should still be visible – the universe is only 13.7 billion years old, so they should barely even have changed in that time. (In fact, we can still see Pop II stars to spectral types as hot as F – these are the so-called ‘cool subdwarfs’.)[2]

And, if they exist, we should also be able to see Population II brown dwarfs. Now, I said something about brown dwarf formation earlier. Pop II T-dwarfs will have formed under very different conditions from ‘modern’ objects. The structure of the Galaxy was different and so was its chemical composition. One might suppose this would affect formation processes. So, if we can find a robust sample of Pop II brown dwarfs, we can compare their properties to modern ones and see if there are any differences. If we can find differences, then we just be able to shed some light on the actual physics of star formation. Also, it may clue us in as to which processes are active in brown dwarf formation.

…But first we need a decent sample. It’s only in the last few years that the instrumentation has become advanced enough to allow for a survey for such faint objects. Finding and following up candidates is a lot of work. That’s where I come in. Creating that sample is the science aim of my PhD.

So, now you have an idea what it is I do, and why. Next post: why I like T-dwarfs. (Answer: they’re rather Lovecraftian…)

[1] Laughlin et al 1997
[2] Yes, there is an elephant in the room – where are the Pop III red dwarfs?


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