When observing AR Scorpii, researchers noticed that its brightness varied over a 3.5-hour period. So they labeled it a periodic variable and paid no further attention to it. Now, however, a large international team of astronomers has returned to take a closer look at the star. The astronomers found that AR Scorpii is much more variable than first thought, with 400 percent changes in brightness in just 30 seconds. The reason for this? AR Scorpii is actually two stars, one launching relativistic electrons at the other.
The article describing these results was published this week in Nature.
The researchers were drawn to AR Scorpii because of seven years of archival footage that revealed a lot of additional variability over and above the well-described 3.5-hour period. Instead of peaking at a similar level every time, the output can vary by as much as a factor of four.
This caused the astronomers to look more closely at the output of the M-class star. They found that the light alternately shifted red and blue during the same 3.5-hour period in which the brightness varies. This usually means that the star is being pulled around by something orbiting the star, causing the star to accelerate towards and away from Earth, which explains the Doppler shift. “The period of 3.56 hours is therefore the orbital period of a nearby binary star,” the authors conclude.
Based on the strength of the red and blue shifts, that companion should be about a third the mass of the sun, which puts it squarely in the territory of the white dwarf.
Because the changes in brightness match the trajectory so nicely, the authors took a closer look at that too. And again something strange happened. We know the types of radiation that M dwarfs and white dwarfs produce, but AR Scorpii produces more than that. “Especially in the infrared and radio,” the authors write, “are orders of magnitude brighter than the thermal emission of the constituent stars.” While the combined brightness of the two stars should be about 1024 Watts, the maximum luminosity of the system is more than 1025 watts.
That’s a lot of watts to consider. But the authors determined that the white dwarf spins incredibly fast, with a “day” on it lasting just under two minutes, based on pulses in its light output. And if that rotation slows down, it can easily provide enough power to make up for the extra energy.
But how is that energy converted into light? The broad spectrum of light produced by the white dwarf suggests that it is produced by accelerated particles, which lose energy at different wavelengths as they are rotated through a curved path. In addition, most of the emissions at visible and UV wavelengths seem to come from the face of the M dwarf, suggesting that the electrons eventually bombard it.
So we now have a rough idea of why the system behaves the way it does. But we don’t know many of the finer details about the mechanics of it all, like where the electrons come from, how they’re accelerated, or how they ultimately produce these bursts of radiation. As the researchers put it, “the exact emission mechanism operating in AR Scorpii is perhaps its most mysterious feature.” That’s understandable, since they also note that we’ve never seen anything like it before.
Nature2016. DOI: 10.1038/nature18620 (About DOIs).