The Corpse of a Dead Star Zaps Its Companion With a Death Ray

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Jul 28, 2016
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Three hundred eighty light-years from Earth lies a very bizarre binary star.

Called AR Scorpii (or AR Sco for short), it’s been known for decades but was masquerading as a relatively humdrum variable, a single star that changes its brightness due to pulsations, literally an expansion and contraction of its size.

But there was something odd about it. Instead of a nice, smooth brightening and dimming, a team of astronomers noticed that it has a lot of scatter in its brightness, a lot of apparently random noise on top of the smooth variations. Not only that, but the noise was only apparent in one part of the cycle, which didn’t make sense.

They decided to take a closer look. And what they found is that AR Sco is not a simple star at all. It’s a binary, with one of the stars being the corpse of a normal star. And not only that, the corpse is feisty, and well-armed: It’s zapping the other star with the stellar equivalent of a death ray.

Man, I love stories like this.

Here’s the deal. AR Sco is two stars orbiting each other. One, the brighter of the two, is a red dwarf with less than half the mass of the Sun. These stars are generally pretty faint and cool.

When the astronomers took a spectrum of the star, they got some surprises. A spectrum is when you break the light up into individual colors, sometimes thousands of them. The result can tell you quite a bit about the object giving off the light. The first interesting bit is that the red dwarf has a lovely, smooth Doppler shift in its spectrum, which means that it’s in orbit around another star. The period of that orbit is just 3.56 hours.

Next, no hint of light from that other star is found. That means it’s very faint. It also has a heckuva gravitational pull, if it can toss around another star in an orbit that’s just a bit longer than the length of a typical Marvel movie.

Also, the red dwarf spectrum is lousy with emission lines. These are bright lines in the spectrum usually associated with fairly hot material, and you don’t usually see them with red dwarfs. These are booming out, though, which is consistent with a very hot nearby source of energy.

That strongly points to the other star being a white dwarf or a neutron star. The first is the core of a star like the Sun after its “normal” life has ended, and it’s blown off all its outer layers. The remaining object is small, about the size of the Earth, but can have from half to about 1.4 times the mass of the entire Sun squeezed down in it!

A neutron star is the core of a more massive star that exploded. It’s even smaller and more massive than a white dwarf. Both are very hot, and could heat the side of the red dwarf facing them, causing it to have emission lines.

Figuring out which one of these two bizarre objects is the key to understanding this system. Happily, there’s more data! And this one’s a doozy.

The entire system pulses in brightness on a very short timescale. The visible light (the kind we see) increases by a factor of 20 times every two minutes. Similar pulses are seen in ultraviolet, infrared, and radio light as well!

The kicker: No X-rays are seen from the system at all. So what does all this mean?

The most likely culprit is that the unseen star is a white dwarf. The pulse period of two minutes is consistent with the spin of a white dwarf, but neutron stars tend to spin much faster. Also, the only way for a neutron star to heat the red dwarf would be for it to be drawing material off the dwarf, which would impact the neutron star and glow. If that were the case, this system would be blasting off X-rays, but none are seen.

So it’s a spinning white dwarf. The fact that we see light emitted from the system across all wavelengths is a pretty strong piece of evidence that it’s coming from what’s called synchrotron radiation: Light emitted by electrons in a very strong magnetic field. That’s consistent with a white dwarf as well, which are known to be pretty strong magnetically.

And now we have all the pieces to the puzzle. The red dwarf is orbiting the white dwarf. The white dwarf’s magnetic field is accelerating electrons up to very nearly the speed of light, and likely emitting them in a beam, like a lighthouse. This whips around the star every two minutes as it spins, and when it passes over the red dwarf it heats the side of the red dwarf facing the white dwarf. When this happens the system blasts out light at much higher rates.

Remember the odd scatter in the light from the red dwarf that started this whole investigation? What we’re seeing there is the inflamed side of the red dwarf facing the white dwarf, the part under the withering blast of the electron beam*! We only see this part of the star when it’s facing us, when the red dwarf is on the other side of the white dwarf (it’s like the phases of the Moon that way). When we see the “back” side of the red dwarf, things look normal. But when the hot part slides into view as the red dwarf circles around the white dwarf, the light we see gets brighter overall but also fluctuates wildly as the star gets zapped by the electron beam.

Yeah, I know. This system is weird. In fact, we’ve never seen anything quite like it. Other binaries with a red and white dwarf don’t behave this way, so we have something special here. And the cool news is it’s fairly close, so we can study it more easily.

And the part that leaves me smiling about all this? This truly peculiar star has been sitting there in our back yard all this time, and astronomers only took a closer look because they saw something odd in the light it was giving off. When they looked closer, they found all sorts of lovely treasures.

What else is out there we haven’t stumbled on yet?

* There is another possibility: The red dwarf is plowing through the white dwarf’s magnetic field, and that’s what’s causing the glow. But that doesn’t explain the two-minute pulse period, so I’m leaning toward the electron death ray hypothesis.