Skip to main content

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!

Comments

Popular posts from this blog

back-talk begins

me: "owen, come here. it's time to get a new diaper" him, sprinting down the hall with no pants on: "forget about it!" he's quoting benny the rabbit, a short-lived sesame street character who happens to be in his favorite "count with me" video. i'm turning my head, trying not to let him see me laugh, because his use and tone with the phrase are so spot-on.

The Long Con

Hiding in Plain Sight ESPN has a series of sports documentaries called 30 For 30. One of my favorites is called Broke  which is about how professional athletes often make tens of millions of dollars in their careers yet retire with nothing. One of the major "leaks" turns out to be con artists, who lure athletes into elaborate real estate schemes or business ventures. This naturally raises the question: In a tightly-knit social structure that is a sports team, how can con artists operate so effectively and extensively? The answer is quite simple: very few people taken in by con artists ever tell anyone what happened. Thus, con artists can operate out in the open with little fear of consequences because they are shielded by the collective silence of their victims. I can empathize with this. I've lost money in two different con schemes. One was when I was in college, and I received a phone call that I had won an all-expenses-paid trip to the Bahamas. All I needed to d

Reader Feedback: Whither Kanake in (white) Astronomy?

Watching the way that the debate about the TMT has come into our field has angered and saddened me so much. Outward blatant racism and then deflecting and defending. I don't want to post this because I am a chicken and fairly vulnerable given my status as a postdoc (Editor's note: How sad is it that our young astronomers feel afraid to speak out on this issue? This should make clear the power dynamics at play in this debate) .  But I thought the number crunching I did might be useful for those on the fence. I wanted to see how badly astronomy itself is failing Native Hawaiians. I'm not trying to get into all of the racist infrastructure that has created an underclass on Hawaii, but if we are going to argue about "well it wasn't astronomers who did it," we should be able to back that assertion with numbers. Having tried to do so, well I think the argument has no standing. At all.  Based on my research, it looks like there are about 1400 jobs in Hawaii r