Whyfore flares?

If you've been hanging around this blog for some time, you may remember that, shortly after I started this, I spent a summer in San Diego working on some research at SDSU with one of the professors here. The target of investigation was a cute little cluster named NGC 7142 and the program was a pretty straightforward photometric analysis (find the color-magnitude diagram, determine distance, age, identify variables...).

That's been over and done with for awhile. But this semester I've begun some new work with Dr. Melott here at KU. Melott's primary interests lie in Astrobiology, namely asking how astronomical events would impact life here on Earth, or even possibly elsewhere, if it exists. His most well known work is implicating a Gamma Ray Burst as a cause for the Ordovician extinction and has received some pretty large coverage. Melott has also identified a link between position in the galaxy and more minor extinctions.

Meanwhile, what I'm working on has less implications for life here on Earth. At least hopefully not.

Most people are aware that the Sun undergoes occasional flares. These flares are eruptions of ionized plasma that are flung out into the solar system. For the most part, they're not all that dangerous to us. We're pretty far away so they will spread out and come pretty diffuse before they reach us, and we have a nice magnetic field to protect us. About the most that typically happens is that we get a nice auroral display.

However, not all flares are created equal. Just before Halloween in 2003, a somewhat larger flare occurred that caused damage to some satellites and ionized the upper atmosphere, disrupting radio communication. And this is only the second largest flare recorded since we started watching a scant 40 years ago.

In September of 1859 a significantly larger flare erupted. It was so strong that it caused power lines to catch fire. The induced current in telegraph wires allowed telegraph operators to send messages even without power for several minutes. The aurora were reported as far South as Florida. It was an exceptional event.

At least, as far as we know. But could there be bigger?

The typical consensus is that main sequence stars with similar mass to our Sun are pretty stable and flares shouldn't get much bigger. There have been superflares observed very young stars and stars with close companions, but only a fistful of superflare events have ever been discovered on solar type stars without some sort of odd characteristic.

In a 1999 ApJ paper, Schaefer, King, and Deliyannis identified 9 stars which were within half a spectral class of the Sun, similar in metallicity, without unusual magnetic fields, and without any sort of interfering binary companion. The question, they asked, is what could be causing these flares?

A possible solution followed that paper: Rubenstein and Schaefer noted that the appearance of these flares had similar characteristics to flares caused by a class of stars known as RS Canum Venaticorum (RS CVn). These stars are close binaries. Since their magnetic fields interact, the magnetic fields get tangled up. When they break and untangle themselves, this results in a flare. This is essentially the same mechanism that causes flares on the Sun. Since the Sun rotates faster at the equator than at the poles, the magnetic field gets stretched and torn.

But the stars that Schaefer and his group discovered didn't have any companions. At least, not visible ones. The question then became, could there be a hidden companion, and if so, what could possibly hide that well and still have a strong enough effect to cause such massive flares?

The answer they proposed was a close in super Jovian planet. Fortunately, that's just the kind of planet that we're getting good at detecting.

So my research this semester is trying to tease out any possible connections between known planetary systems and flare activity.

Going through the list of stars that Schaefer found, only one of them was observed for planetary systems (κ Ceti) and was not found to have any. Of course, that doesn't really mean that it doesn't have one since the method they used (radial velocity measurements) would only detect planets that are orbiting in a direction in which the plane was near the line of sight to the Earth. If the plane the system was aligned with the plane of the sky, we'd never see it.

So we're now trying to approach the problem the other way around: do stars with close in Jovian planets have excited photospheres? This problem is a bit harder to tease out answers to and I'll post more on that topic as our research progresses.

---

Schaefer, B.E., King, J.R., Deliyannis, C.P. (2000). Superflares on Ordinary Solar‐Type Stars. The Astrophysical Journal, 529(2), 1026-1030. DOI: 10.1086/308325

Rubenstein, E.P., Schaefer, B.E. (2000). Are Superflares on Solar Analogues Caused by Extrasolar Planets?. The Astrophysical Journal, 529(2), 1031-1033. DOI: 10.1086/308326