The question of precisely what happens as stars and planets condense from vast clouds of gas is still very much an unanswered one. We have some good ideas of how things work, but really it’s difficult to be certain. Newly forming stars are shy, hiding away in dense veils of gas and dust which form the stuff which planets will be made from. So shy in fact that they’re barely visible in optical light.
This image shows an assortment of such young stellar objects in the Orion nebula. Newborn stars still swaddled in their gassy cocoons, shining with the light of their first fusion, encircled by an protoplanetary disk which is feeding the formation of both the star itself and any planets which will one day revolve around it.
One concept which has become quite widespread in astronomy is that of the so-called “frost line”. Sometimes referred to as the snow line, this is the region in the star’s disk where the temperature becomes cold enough that light, volatile molecules can condense into ices. In much the same way that in the solar system today, we see icy objects further out, the same process is expected to occur in disks as stars form.
This paper, in essence, adds another simple factor to the mix. Previous models considered only rocky minerals like silicates and metals like iron, alongside light molecules which form into ices. Monika Kress et al, very simply, add another component to the mixture – polycyclic aromatic hydrocarbons, aka PAHs.
Which makes sense, really. Up to 10% of all interstellar carbon – which will eventually be incorporated into stars – exists in the form of PAHs. They’re tough little molecules which are hardy enough to survive the intense ultraviolet of interstellar space until they get caught by the gravitational pull of a newly collapsing star. We know this because infrared emission from PAHs has been seen in protostars, and they almost certainly didn’t form in situ. Essentially, though some simple modelling of PAH chemistry (most of which I agree with… most of which), this tidy little paper finds that there should be another line inwards of the Frost Line, which they’ve dubbed the Soot Line. Basically, inwards of the soot line, PAHs and other condensible carbon are pyrolysed, breaking down into light molecules. They find a magic temperature of around 1000 K. Above 1000 K, PAHs are hot enough to rapidly tear themselves apart. Below 1000 K, they survive long enough that they can be expected to end up as a part of any planets forming.
This image illustrates the point quite nicely. The Frost Line for a star like the Sun is estimated at around 2.7 AU. Interestingly though, this paper estimates the Soot Line to be quite far out – at around 2 AU. While the authors note that this isn’t a sharp transition and the line itself is likely to be quite blurry, my intuition tells me that this estimate seems a bit too distant from the central star (consider that the surface of the Moon is only around 400 K – rather short of 1000 K). It seems off somehow. I have to wonder if something in their calculations has caused the soot line to appear so far out.
Mind you, it seems the authors themselves were a little uncertain of this. They actually caution against using this particular model and note that it needs some work in order to be more comprehensive. And I respect that. It’s nice that they’ve published this material in order to make it known to the community, while acknowledging that it’s still incomplete.
Personally, I can’t wait to see what further amendments to this model are published next. Seeing as this is quite firmly within my own research field, perhaps, time permitting, I should spend a while looking into it myself…
Kress, Monika E., Tielens, Alexander G. G. M., & Frenklach, Michael (2010). The ‘soot line’: Destruction of presolar polycyclic aromatic hydrocarbons in the terrestrial planet-forming region of disks Advances in Space Research, 46 (1), 44-49 DOI: 10.1016/j.asr.2010.02.004