Thursday, April 4, 2013

Gravitational microlensing with...Kepler?

Here's a random cool result from the Kepler mission.

Kepler, which I have mentioned before as a dedicated mission for finding exoplanets, simultaneously measures the brightnesses of 160,000 stars to try to find exoplanets that transit, or pass in front of, their host stars. Because Kepler was designed to detect planets like Earth transiting stars like our Sun (which results in a very small signal), Kepler is made to be excruciatingly sensitive to very small changes in a star's brightness. As such, it has been able to see some very strange things, including planets orbiting binary star systems (Kepler-16, -34, -35, -38, -47) and even one planet orbiting a binary star system that is itself part of a quadruple star system!

Earlier today, in a press release, NASA announced that Kepler had detected the gravitational lensing of a red dwarf star by its binary white dwarf companion. White dwarfs are a type of stellar remnant, consisting of the leftover core of a star that was once of similar size to our Sun or a bit more massive. White dwarfs squeeze an amount of mass near that of the Sun into a sphere that is about the size of Earth, so it's no surprise that they would have strong gravitational fields.

Gravitational lensing is an interesting effect predicted by Albert Einstein's general theory of relativity. In general relativity, massive bodies can actually cause light to bend around them as a result of the mass of the object warping spacetime. The general analogy is a heavy ball placed on a rubber sheet, where the rubber sheet represents spacetime. Of course, this is only a two-dimensional analog, but it works for visualization purposes.

There are also different types of gravitational lensing that depend on the mass of the object warping spacetime and the amount of distortion you get in the images of background objects whose light is getting bent. Strong and weak lensing both occur on the scale of entire clusters of galaxies. Strong lensing produces multiple images of a background object and what have come to be called Einstein rings, where the light from the background object becomes smeared out into a circular pattern around the lensing cluster. Weak lensing is far more common, and causes a small distortion in the appearance of the background object.

Gravitational microlensing works on the scale of individual stars, and can even be used as a way of detecting exoplanets! Microlensing doesn't produce the same spectacular images as strong and weak lensing do. Rather, it re-directs some of the light emitted by the background object that would otherwise completely miss us towards Earth. This causes the background object to appear temporarily brighter when the alignment of the two objects is just right.

We've detected white dwarf/red dwarf binary systems before, so we know pretty well what to expect from the observationally. Kepler, however, is sensitive enough to observe that the light curve (the graph of the system's brightness over time) was a bit wonky from what we expected. Sadly, because the paper hasn't been published yet, I can't show you what this actually looks like, but I will post an update when the data become available. I can however, show you this nifty video representation of what is going on in this system. The main observable distortion occurs when the white dwarf is first moving in front of the star and the edge of the star becomes distorted. Here's a frame from that video that shows the beginning of the transit.
Image credit: NASA, JPL, Kepler Team
This completely interrupted the other post I was working on today, but oh well. Results like this are just some of those cool, totally unexpected things that happen when you're studying astronomy that you have to just sit back and admire for a bit. As we like to say, one man's signal is another man's noise.

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