Starspots on Kepler 186

Today there were two amazing discoveries announced from Kepler (everybody's favorite planet-hunting telescope):

KOI-3278: A Self-Lensing Binary Star System
(by my friend and fellow UW grad student Ethan Kruse!)

and

An Earth-Sized Planet in the Habitable Zone of a Cool Star

The latter paper is important to me, not because of the very neat planet that was discovered, but because I study cool stars!

Kepler 186 - an Interesting Star

Kepler 186 is an M1 dwarf star, about 50% the mass of our Sun (a G2 dwarf). These M-dwarfs are the most common stars in our Galaxy, making up about 75% of the Milky Way's 400 billion stars! They are also famous for having dramatic levels of magnetic activity, which in turn can generate large starspots, as well as stellar flares thousands of times more energetic than those on our Sun.

In the discovery paper, as well as the public telecon that happened this morning, the authors discussed a lack of flares in the 4 years of Kepler data. This is not unexpected for a higher-mass M dwarf, as their "active lifetime" is relatively short (maybe a billion years) compared to the ages of stars in the Milky Way -- though still much longer than the active lifetime of the Sun! However, just because this star does not have obvious flares in it's light curve, it still shows signs of powerful magnetic activity on it's surface!

Searching for Starspots

All the data from the Kepler primary mission is public. I used the Kepler ID number for this star to go download all the "long cadence" (30-min exposure) data for Kepler 186, about 4 years worth! I immediately could see dramatic starspots were present in the data, which the Kepler team worked hard to remove to search for signs of a planet! (One scientists's trash is another scientist's treasure, after all)

The starspots are like dark patches on the surface of the star. As the M dwarf spins, the starspots rotate in and out of view. On the Sun we can see this happen as well. I used a simple signal-processing technique to search for the rotation period, which essentially folds the data at lots of different periods and finds the most correlation power. Check it out:

The spike at around 34 days is the rotation period we're looking for! Interestingly, this is longer than the rotation period of the Sun. Also of note, some of the smaller amplitude peaks around 10-11 days are due to one of the planets in the system.

So Kepler 186 definitely does have starspots, and it's a pretty darn slow rotator for an M dwarf!

Starspot Evolution

Here's where it gets more interesting. These starspots are transient features. On the Sun, spots decay within a few rotations, maybe a few hundred days at the longest usually. On lower mass stars like M dwarfs we don't have a good census for how long these features last, or even how large they typically are. M dwarfs are faint, and before Kepler this was very hard to measure!

Here's a kind of complex figure that tells us a lot about the starspots:

The top panel is the actual light curve (flux over time). Here I have normalized every quarter of data to have the same peak flux. You can see the sinusoidal dips: these are the starspot(s) rotating in and out of view! Just from the top panel you can see the change in size quite a bit. They on average about 1% variations in flux, about 5-10x larger than seen on the Sun. This means if you lived on the planet Kepler 186f, you likely be able to see these spots by eye (hard to do for the Sun)

The bottom panel is a brightness map over time. The vertical axis is the phase of the stellar rotation (which goes from 0 to 1), which I show folded twice. The horizontal axis is time. Pixel shade indicates the brightness as this time and phase. We can use this to map the phase (or longitude) evolution of the features over time.

The starspots are the dark regions, and have a range of lifetimes. Near the beginning of the data they seem to only live for a few rotations, similar to the Sun. At around time=800 a very large starspot (or spot group) emerges! This group seems to live for over 600 days, almost 2 years. More interestingly, the spot(s) appear to move in phase, almost making a linear streak in longitude over time. My interpretation of this (as we've seen it in other data from Kepler as well) is that the star is probably differentially rotating, and the starspots are moving on the surface.

Conclusions

Kepler 186 is a neat system, and we now have a ton of great data from Kepler on it! This is just a morning's worth of work to make these plots. My thesis work involves making figures like this and fitting them with models to study the evolution (in size and phase) of the starspots over time. One thing that makes me very excited about Kepler 186 is the possibility that the planets could be used to trace out finer details of the starspots! This would require follow-up from the ground during many planet transits, but could be used to break some fundamental degeneracies about starspot temperature, size, and position.