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Kepler meets Einstein when a stellar skeleton bends space-time


Gravity-Bending Find Leads to Kepler Meeting Einstein

This is a press release by my postdoc, Dr. Phil Muirhead. Last summer he compiled a list of all of the planet candidates around the M dwarfs (red dwarfs) targeted by the NASA Kepler mission. One of our summer students, Andrew Vanderburg, noticed that the light curve of one of the candidate transiting Jupiters looked very strange. If a hot Jupiter transits a star, it should take about 20 minutes for the planet to move across the limb of the star, causing the light to go from the full, out-of-transit level, to the minium level during a full transit (eclipse). Here's what the light curve of Kepler Object of Interest number 256 looks like (KOI-256):

The light curve of KOI-256, along with the all-star cast of Muirhead et al. (2013)

Where the light level first decreases is called "ingress," and for KOI-256 the ingress time is about a minute, instead of 20 minutes. Weird! After pondering this a bit, Andrew and Phil realized that the ingress time implies an Earth-sized object. But why does an Earth-sized object block 2.6% of the light?

The next clue came when another undergraduate researcher, Juliette Becker, stepped in. She applied for time on the TripleSpec spectrometer on the 200-inch telescope at Palomar. The Caltech Optical Observatories director, Shri Kulkarni, allows undergrads to apply for 2 hour blocks of time during the summer for a research project of their own. Juliette won two hours of time with her proposal, and she started measuring the Doppler shift of the star. What she found was surprising: The star is getting yanked around by something that is (mostly) unseen, yet it is actually more massive than the star. Here are Juliette's radial velocities, which she measured from her spectroscopic observations (gotta love Caltech undergrads!):

Hot Jupiters tug on their stars and cause them to move by hundreds of meters per second but this star is getting yanked around by hundreds of kilometers per second! But remember, the ingress time implies that the object is the size of the Earth. The size of a small planet, but the mass of a star? Well, that's a pretty good description of a white dwarf. When stars like our Sun die, they leave behind "skeletons" in the form of tiny, super-hot yet faint white dwarf stars, which then cool down over time.

KOI-256 is orbited by a white dwarf on a 1.38-day orbit. When the white dwarf goes behind the red-dwarf star, the star blocks the white dwarf's light, causing a 2.6% dip in the total light from the system. When the white dwarf passes in front of the red dwarf, there is a tiny decrement of light:


The dip was evident when Caltech postdoc Avi Shporer and Phil looked carefully a half-period away from the main eclipses. However, the transit (passage of WD in front of RD) was 2x shallower than expected. The dashed line above shows the depth expected when an Earth-sized object blocks light from a red dwarf. The solid line shows the actual transit depth. Why the shallow transit? 

The answer is provided by Einstein's theory of general relativity. Massive objects can warp space time, causing light that is traveling through that space to be bent. The white dwarf around KOI-256 bends light rays that would normally miss our telescopes into our path, causing the system to appear brighter, thereby filling in the transit dip. Here's a really cool movie showing the effect of the WD warping space-time:


This effect is known as gravitational lensing, and it has been observed for stars near the Sun during a total solar eclipse, for stars lensing other stars in the Galaxy, and for galaxies. KOI-256 is the first time a transiting white dwarf has been observed to lens light from the star it orbits. Kepler meets Einstein!

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