CHRIS LINTOTT
Dust choking far flung stars may be behind their unusual brightness - but where does it come from?
Something odd and mysterious is happening around many of our nearest stars. Surveys of otherwise normal main sequence stars have shown that at least 10 per cent and maybe as many as a third of them are glowing more brightly in the infrared than they should be. The likely culprit is dust close to the star, which can absorb its light and re-emit it in the infrared, but the source of large amounts of hot dust is a mystery.
Such an infrared excess was first detected around Vega, providing a nice in-joke in the 1997 film
Contact when, having received what appears to be a signal from the star, a control-room astronomer points out that any alien civilisation there would have trouble with the debris around the star. Instead of worrying about hazards to alien shipping, a new paper by a team led by Florian Kirchschlager from Kiel University in Germany stacks the evidence to find out what might be going on.
In our Solar System, dust left behind by shortperiod comets is sometimes revealed by reflected sunlight, causing a faint glow known as zodiacal light. Observations of these other stars points to exozodiacal light that is typically three times brighter than our own.
While we could always imagine stars other than the Sun to be more pestered by comets, there are two problems. One is that such a large amount of dust would be quickly dispersed by the stellar wind, amongst other processes, and the other is that these effects are greatest closest to the star, just where we need the dust to be for it to be hot enough to generate a nice bright infrared excess.
The Kiel team decide that the solution is to pack the necessary dust into a narrow ring around the star, an arrangement that might be more stable than an even distribution. At least for those stars with the brightest infrared excess, these rings must be spectacular: in order to explain what we see their radius can’t be any farther out than Earth is, and in at least one case it must skirt just a few million kilometres above the surface of the star.
But this closeness creates a problem. In the Solar System, dust grains are often like sand grains, made of silicates and incapable of resisting the temperatures they’d encounter in such orbits. Graphite would work – it can survive temperatures up to several thousand degrees, as would other carbonaceous dust – but this makes the problem of the dust’s origins worse. You’d need a very special set of comets to deliver just the right kind of dust, for starters. There’s an old joke that astronomers turn to blaming magnetic fields for any unexplained phenomenon when they become desperate, and this problem is no different. If the particles are small, they can be charged by the interstellar wind, and once that happens the star’s magnetic field might be able to trap them in a ring.
It’s a model that might just work, but really we need more observations. Only a few dozen of the nearest stars have yet been surveyed, and only when we have more data will we be sure of what’s going on - and what the implications for any alien around Vega will be.
CHRIS LINTOTT was reading… Constrains on the structure of hot exozodiacal dust belts by Florian Kirchschlager et al. Read it online at https://arxiv.org/abs/1701.07271
“There’s an old joke that astronomers turn to blaming magnetic fields when desperate, and this problem is no different”