Monday, April 29, 2013

Today in History: First Player Out

Jason Collins, an 11-year veteran of the NBA, just became the first openly gay athlete in any of the major US sports. In his self-penned Sports Illustrated article, he writes:

I'm a 34-year-old NBA center. I'm black. And I'm gay. 
I didn't set out to be the first openly gay athlete playing in a major American team sport. But since I am, I'm happy to start the conversation. I wish I wasn't the kid in the classroom raising his hand and saying, "I'm different." If I had my way, someone else would have already done this. Nobody has, which is why I'm raising my hand.

Read More:

Sunday, April 28, 2013

MC Monte Carlo - Gridding It Up In the Likeli-Hood

At the end of the term in my Ay117 Astrostats course, the students gave 15-minute oral presentations or poster presentations describing their final projects. The morning of presentations was organized like a Keck Science Meeting, so that students not only learned the primary course material, but also gained valuable practice in giving scientific presentations.

Of the many highlights from this year's session was Scott Barenfeld's performance of his latest single from his upcoming Astrostats hip hop album. It was most certainly the best rap performance of the day.

Griddin' it up in the Likeli-hood
From the upcoming debut album 
Straight Outta Inverse Compton

Scott Barenfeld (CIT 1st year)
March 19, 2013

They call me MC Monte Carlo
Be runnin' my code 'till tomorrow
I take the random walk
So don't sit there and squawk
If you need my routines, you can borrow.
I doin' my Bayesian stat stu ff
Comin' up with posteriors off the cu ff
It's Bayes' Theorem yo
You frequentists can go
No root n error bars, son, I had enough.
Findin' your $\mu$ and $\sigma$
Don't have to be such an enigma
Just get the likelihood
You really really should
Then Aaron and John will dig ya!

Wednesday, April 17, 2013

Minerva update: The eagle has landed

On Fri, Apr 12, 2013 at 4:39 PM, Jon Swift wrote:
Status update:

Our CDK700 is safely in place now! Rick and Kevin are
hooking up all the cables and our camera in prep for
tonight. The telescope fit nicely on the mounting bolts
(after a few strikes with a mallet) and the clearance is to
spec. Skies are clear, and if all goes well we'll have a
pointing solution soon after dark.

Telescope 1 was delivered and installed successfully Friday afternoon! That we were able to get on the sky immediately is a testament to the amazing engineering of Planewave, and the wisdom of going with top-of-the-line, yet off-the-shelf telescopes for our project. Telescope 2 will soon be rolling down the assembly line.

For now, testing of Telescope 1, the science camera and the fiber acquisition unit (FAU) have begun in earnest. Kristina Hogstrom (CIT Aerospace Engineering second-year) will be teaching the telescope to operate robotically, while Phil Muirhead and Mike Bottom (CIT Astro third-year) will work on the FAU and telescope guiding. 

Some pics!
Unpacking Telescope 1
The Crane getting ready to lift the telescope into place
Carefully lowering the telescope onto the pier. The pier for Telescope 2 is in the foreground and will hopefully be occupied this Fall.
Jon Swift sawing off the end of a PVC pipe so the telescope will fit properly
The open Aqawan, complete with one of two telescopes. The sides of the enclosure will be lowered further during scientific operations.
The telescope in the home position, along with the control computer and desk.
Reed Riddle and Becky Jensen-Clem investigate the telescope.
Mike Bottom is one very happy grad student
Kristina Hogstrom (Aero-astro engineering grad student) sizes up her quarry. She will teach it to operate robotically before setting it loose in the wild next year.
Phil Muirhead's SolidWorks design of the Fiber Acquisition Unit (FAU), which couples the light from the telescope to the spectrometer via a fiber-optic cable. 

Proud papa


It turns out that all went well with installation and setup. The team was on-sky that same night.

Rick Hedrick of Planewave looks on as the telescope pointing solution is worked out.
Unofficial first-light target: The Trapesium
The M51 "Whirlpool Galaxy", from Pasadena with no filters in front of our Apogee U230 CCD camera...from Pasadena. Jon measured 2."7 seeing!
Pizza break. From left to right: Mike Bottom, Peter Plavchan, Phil Muirhead, Becky Jensen-Clem

Tuesday, April 16, 2013

What the IAU should have written

At least to Dr. Wright's mind:

An excerpt:
In the light of recent events, where the possibility of buying the rights to name nominating or voting on popular names for exoplanets has been advertised, the International Astronomical Union (IAU) wishes to inform the public that such schemes have no bearing on the official naming processastronomers do not use such names and the international astronomical community currently has no plans to do so. The IAU wholeheartedly welcomes the public's interest to be involved in recent discoveries, but would like to strongly stress the importance of having a unified naming procedure for official names and designations.

Is Uwingu fishy? I really don't think so.

I've read/heard a lot of negative comments from astronomers regarding Uwingu. I suspect this is because they have bad associations for any concept involving money for naming rights of astronomical objects. There are a lot of shady sites out there that supposedly let the public buy names for stars, all for profit and with no scientific interest in mind. But I'd like to assure you, dear readers, that Uwingu is no such organization.

The Uwingu website makes their mission and methods abundantly clear. They are compiling a "baby name book" of unofficial designations for exoplanets that may become unofficial monickers, or even eventually official names if the IAU ever gets into the business of officially sanctioning exoplanet names. But nowhere on the site have I seen evidence that they are misleading the public in how all of this will actually work. Under "About us" they state:
Funding great science and science education doesn’t take a lot of money, but it requires someone to make the choice to fund science. 
At Uwingu, we are the men and women who’ve decided to turn our profits into understanding of our universe. 
We’ve designed software products that will help people relate better to the sky and to space. 
We will market these products globally and use the proceeds after expenses to create something we call “The Uwingu Fund” for space research and education. We hope sales on Uwingu’s products will raise millions—and perhaps even tens of millions—annually—for The Uwingu Fund.

Here's a list of projects funded by Uwingu so far. This all seems pretty clear, noble and aboveboard to me. Nothing shady there, especially since they have professional astronomers such as Geoff Marcy on their advisory board. They may actually have nefarious aims, but coming to this conclusion after reading what they explicitly state on their website starts edging pretty close to conspiracy-theory-land, IMHO.

On the other hand, the IAU issued a poorly-written press release that impugned efforts like Uwingu without conferring with the Commission 53 or with the public. The IAU claims to be a democratic organization, yet some faction within it acted unilaterally in issuing that press release. This is clear. I have no idea how people have jumped from that wrong to accusing Uwingu of wrongdoing. I am very confused on that point. I am not confused about the aims of Uwingu or the potential for it to do something positive in these times of decreased federal funding for science.

Here's an official response from Uwingu regarding the IAU hoopla. See also the article at here.

Uwingu affirms the IAU’s right to create naming systems for astronomers. But we know that the IAU has no purview — informal or official — to control popular naming of bodies in the sky or features on them, just as geographers have no purview to control people’s naming of features along hiking trails. People clearly enjoy connecting to the sky and having an input to common-use naming. We will continue to stand up for the public’s rights in this regard, and look forward to raising more grant funds for space researchers and educators this way.

We now take this opportunity to note to the public that, contrary to the IAU press release:
    • Informal names for astronomical objects are common (e.g., “The Milky Way”). And in fact, there is no such thing as a unified astronomical naming system, and there never has been. Claims to the contrary are simply incorrect, as an astronomical database search on a representative star, Polaris, reveals. This star is also known to astronomers and the public as the North Star, Alpha Ursae Minoris, HD 8890, HIP 11767, SAO 308, ADS 1477, FK5 907, and over a dozen more designations.
    • There are many instances where astronomers name things without going through the IAU’s internal process. There are many features on Mars, ranging from mountains to individual rocks, with names applied by Mars-mission scientists and never adopted by, or even considered by, the IAU. And Apollo astronauts did not seek IAU permission before naming features at their landing sites or from orbit.
    • Uwingu looks forward to continuing to help the general public to engage creatively in astronomy and to participate in the excitement of the exploration of the universe in which we all live.
In our Alpha Centauri People’s Choice naming contest, anyone can nominate a name to honor a friend, colleague, loved one, or to recognize a place name, an author, an artist, or a sports team, for example. The name getting the highest number of votes will be declared the public’s choice for Uwingu to use as the name for this mysterious new world. Never before has the public been asked to choose its favorite name for a planet.

Name nominations are $4.99; votes cost $0.99. Proceeds from naming and voting fuel new Uwingu grants to fund space education projects affected by sequestration cuts to NASA.

So when discussing this issue, please try to draw two clear boxes around two distinct issues: 1) The IAU either overstepped its bounds or issued a terribly-worded press release, or both. 2) Private funding through activities like Uwingu is good or bad. Conflating these two discussions doesn't help anyone.

Monday, April 15, 2013

How far away is Mars?

This far (2-minute interactive tour, click arrow at bottom of page):

Sunday, April 14, 2013

Something strange going on in the IAU

Prof. Jason Wright has the story over at his blog. If you care about the actions of the IAU---in particular a potential abuse of procedure---please read the post in full and pass it along.

It all starts with a company called Uwingo that wants to sell the ability to propose exoplanet names (think of an exoplanet baby name book) in order to raise funding for exoplanetary science. As stated on the Uwingo site, "We’re asking the public to create a vast list of planet names for astronomers to choose from."

In a recent press release, the IAU responds:
In the light of recent events, where the possibility of buying the rights to name exoplanets has been advertised, the International Astronomical Union (IAU) wishes to inform the public that such schemes have no bearing on the official naming process. The IAU wholeheartedly welcomes the public’s interest to be involved in recent discoveries, but would like to strongly stress the importance of having a unified naming procedure.
Interestingly, they do not name Uwingo specifically, but passive-agressively link to a star-naming site, most of which are clearly schemes meant to fool the public. Uwingo, by contrats, is run by astronomers and makes its intentions very clear.

Jason writes:
The IAU has issued a statement regarding the naming of planets by a group called Uwingu that is misleading or inaccurate in several ways.  Reading it, one could be forgiven for coming away believing that the IAU has given official names to planets, that these names can be found at, and that the commission responsible for this process has refused to consider Uwingu's names.  All three implications are absolutely false.  
He goes on to list the problems with this statement
1) Contrary to the press release's implication, the IAU does not name planets.
2) Contrary to the press releases's assertion, there is no "official naming process."
3) Contrary to the press release's assertion, Commission 53 has not foreclosed the possibility of using Uwingu's names.
4) Contrary to the press release's implication, the press release does not and cannot describe official IAU policy.
5) Contrary to the press release's implication, Uwingu is not actually promising to give official names to or to sell naming rights to specific planets
Like I said, check out his full post for all of the details and implications. This is really strange...

What is also troubling is the "official" endorsement of one exoplanets site ( over another ( apparently without consultation of the community, nor the IAU Commission 53 on Exoplanets. As Geoff Marcy wrote in an email:
The lack of consultation or vote by IAU members is particularly eyebrow-raising, as the two IAU co-signatories of the press-release reside in Paris, and the press release names as the catalog for exoplanets which comes from Observatoire de Paris.   At the least, this coincidence of two authorities in Paris promoting a website in Pairs raises the appearance, if not reality, of political back-room motivations.   Further, contains exoplanets not vetted by the IAU nor even published in many cases!
Stay tuned!

Compilation of mental health posts

Several people have requested this list, so here you go. Enjoy! And point me to other sources, please.

Performance Enhancing Drugs
Impostor Syndrome
           - Not just in academia
Work-life balance through working efficiently
           - Part 1
           - Part 2
           - Part 3
Professing with Depression
           - In good company: With tenure but not without troubles
           - It's not you, it's a disease
Zen and the Art of Astronomy Research

Friday, April 12, 2013

Transiting Exoplanet Survey Satellite (TESS)

Here's an informative, high-production-value video explaining NASA's next big exoplanet mission:

Thursday, April 11, 2013

Email Charter (NNTR)

Email is eating all of our lives. Let's all agree to a few simple ground rules:

Wednesday, April 10, 2013

I'm a lucky teacher!

Last term I taught Ay117: Statistics and Data Analysis. I wasn't expecting to have to teach the class in the winter; I thought I was teaching in the Spring. So I scheduled a bunch of travel in the Winter, most of it related to my job decision. Fortunately, I had the best TA on campus last term, Aaron Wolf. Aaron really carried me, subbing for me about 1/4 of the lectures. He also did the grading and office hours, and the students absolutely loved him. 

Aaron is my favorite type of human: extraordinarily smart, yet humble and extremely personable. These characteristics shone through in his teaching last term. Oh, did I mention he did all this while writing his thesis?! As a last-year grad student, he didn't even have to TA.

From: Registrar's Office REGIS
Sent: Tuesday, April 09, 2013 9:00 AM
To: Wolf, Aaron S.
Subject: Aaron, thank you for being an excellent TA!
Dear Aaron,

It has come to our attention that you were one of the highest rated TAs in the TQFR for the past term. I hope you have read the wonderful comments that the students made about you. The Office of Graduate Studies and the Registrar’s Office would like to thank you for your dedication and caring of the students in Ay 117. As a small token of our appreciation, we are sending you a gift card from!

Keep up the good work!


Joe E. Shepherd
Dean of Graduate Studies

Tuesday, April 9, 2013

Planets and planetesimals around kappa CrB

My collaborators and I have just published a paper announcing a newly detected planet and dust disk around the subgiant kappa Coronea Borealis. I've written about kappa CrB previously here. Back then I only knew of one planet orbiting the star. With additional RV measurements, we discovered a second planet in a long-period orbit. The period of the second planet is so long that we only see a portion of the orbit, which looks like a linear RV "trend," or constant acceleration (scientists: think first-order Taylor expansion of a Keplerian orbit).
Radial velocity (RV) measurements of kappa CrB made with the Lick and Keck telescopes. The first measurement was made when I was a fourth-year graduate student at Berkeley. Other key events are labeled. The solid line shows the best-fitting two-planet solution, and the dashed line shows the acceleration due to the second planet.

Additionally, my collaborators Amy Bonsor and Grant Kennedy used the Herschel space telescope to observe the star in the far infrared. At these long wavelengths, the star is very faint, but any warm material around the star will be bright. Amy detected extended emission around the star consistent with a flattened disk of warm dust grains, similar to our Kuiper belt, only much larger and more massive.

The dust disk around kappa CrB (not to be confused with a space eyeball)

Here's the link to the press release (also reprinted below), and the paper.

Retired Star Found With Planets and a Debris Disk

ESA’s Herschel space observatory has provided the first images of a dust belt – produced by colliding comets or asteroids – orbiting a subgiant star known to host a planetary system.
After billions of years steadily burning hydrogen in their cores, stars like our Sun exhaust this central fuel reserve and start burning it in shells around the core. They swell to become subgiant stars, before later becoming red giants.
At least during the subgiant phase, planets, asteroids and comet belts around these ‘retired’ stars are expected to survive, but observations are needed to measure their properties. One approach is to search for discs of dust around the stars, generated by collisions between populations of asteroids or comets.
Thanks to the sensitive far-infrared detection capabilities of the Herschel space observatory, astronomers have been able to resolve bright emission around Kappa Coronae Borealis (κ CrB, or Kappa Cor Bor), indicating the presence of a dusty debris disc.
The star is a little heavier than our own Sun at 1.5 solar masses, is around 2.5 billion years old and lies at a distance of roughly 100 light years.
From ground-based observations, it is known to host one giant planet roughly twice the mass of Jupiter orbiting at a distance equivalent to the Asteroid Belt in our own Solar System. A second planet is suspected, but its mass is not well constrained.
Herschel’s detection provides rare insight into the life of planetary systems orbiting subgiant stars, and enables a detailed study of the architecture of its planet and disc system.
“This is the first ‘retired’ star that we have found with a debris disc and one or more planets,” says Amy Bonsor of the Institute de Planétologie et d’Astrophysique de Grenoble, and lead author of the study.
“The disc has survived the star’s entire lifetime without being destroyed. That’s very different to our own Solar System, where most of the debris was cleared away in a phase called the Late Heavy Bombardment era, around 600 million years after the Sun formed.”
Dr Bonsor’s team used models to propose three possible configurations for the disc and planets that fit Herschel’s observations of Kappa Cor Bor.
The first model has just one continuous dust belt extending from 20 AU to 220 AU (where 1 AU, or Astronomical Unit, is the distance between Earth and Sun).
By comparison, the icy debris disc in our Solar System – known as the Kuiper Belt – spans a narrower range of distances, 30–50 AU from the Sun.
In this model, one of the planets orbits at a distance of greater than 7 AU from the star, and its gravitational influence may sculpt the inner edge of the disc.

A variation on this model has the disc being stirred by the gravitational influence of both companions, mixing it up such that the rate of dust production in the disc peaks at around 70–80 AU from the star.
In another interesting scenario, the dust disc is divided into two narrow belts, centred on 40 AU and 165 AU, respectively. Here, the outermost companion may orbit between the two belts between a distance of about 7 AU and 70 AU, opening the possibility of it being rather more massive than a planet, possibly a substellar brown dwarf.
“It is a mysterious and intriguing system: is there a planet or even two planets sculpting one wide disc, or does the star have a brown dwarf companion that has split the disc in two?” says Dr Bonsor.
As this is the first known example of a subgiant star with planets and a debris disc orbiting it, more examples are needed to determine whether Kappa Cor Bor is unusual or not.
 “Thanks to Herschel’s sensitive far-infrared capabilities and its rich dataset, we already have hints of other subgiant stars that may also have dusty discs. More work will be needed to see if they also have planets,” says Göran Pilbratt, ESA’s Herschel project scientist.

Sunday, April 7, 2013

This Week's Astro Nutshell: It's full of stars!

Each week I work with first-year grad students Marta and Becky on "order of magnitude" problems at the blackboard. I put that in quotes because we tend to do many more scaling arguments than true OoM. The idea is for them to draw on what they've picked up in class and apply it to common problems that arrise in astronomy.

Several weeks ago we asked

Suppose you have a magnitude-limited survey such that all stars have magnitudes $m < m_{\rm max}$. What will be the most common type (mass) of star in your survey?

This question is pretty much the same as "What types of stars visible in the night sky are most numerous?" This type of problem was first addressed by Swedish astronomer Gunnar Malmquist back in the 20's, which led to what we now refer to as the Malmquist Bias.

Initially, one might thing: well red dwarfs are the most common stars in the Galaxy, so M dwarfs will be the most common in our survey (or sky). However, M dwarfs are very faint (low luminosities). If the Sun is a 1000 Watt lightbulb then a typical M dwarf would be a Christmas tree light (thanks to Dave Charbonneau for the analogy). Since we're magnitude-limited (brightness-limited), we might not see many M dwarfs.

If we denote the number of stars in our survey as a function of mass $N(M)$, then
$N(M) \sim $(density of stars) $\times$ Volume
Where the "Volume" is characterized by the distance $d_{\rm max}$ out to which you can see a star of a given mass ($V_{\rm max} = d_{\rm max}^3$). Let's denote the density of stars by $\phi$, which is the number of stars per unit volume. This results in
$N(M) \sim \phi \times d_{\rm max}^3$   (1)
The density of stars in a given volume is given by the present-day mass function. Note that this is different from the initial mass function (IMF) because the stars in our survey will not be newly born, but  will instead represent a well-mixed sample of stars of all ages. Since massive stars die young, there will be even fewer massive stars than predicted by the IMF. The PDMF has the form $\phi \sim M^\alpha$, where $\alpha = -1.35$ for stars less massive than the Sun (the standard Salpeter IMF), and $\alpha = -5.2$ for stars more massive than the Sun. Plugging into Eqn 1 gives:
$N(M) \sim M^\alpha \times d_{\rm max}^3$   (1)
As for $d_{\rm max}$, we can use the handy equation that we derived a couple weeks ago (I'll blog about it later), which gives the scaling of the flux received from a star at the peak of its spectral energy distribution. The peak shifts to longer wavelengths for cooler stars, and shorter wavelengths for hotter stars. This all encompassed by the simple scaling relationship
$F \sim T^2 R^2 d^{-2}$ (1)
As the temperature $T$ increases, the flux increases. The same as when the star's radius $R$ increases. Move the star further away, the flux drops. Since our survey is sensitive only up to a limiting magnitude, we can only observe stars with $F < F_{\rm min}$. This means
$d_{\rm max} \sim T R F_{\rm min}^{-1/2}$    (2)
 From stellar structure, we recall that $R \sim M$, and $T \sim M^{1/2}$. Subbing into Eqn 2, we get
$d_{\rm max} \sim M^{1/2} M F_{\rm min}^{-1/2}$ 
And since our flux (magnitude) limit is fixed, there is a maximum distance out to which we can see a star of a given mass, given by
$d_{\rm max} \sim M^{3/2}$
We can now evaluate Equation 1 in terms of stellar mass, $M$:
$N(M) \sim M^\alpha \times M^{9/2}$   

For the different mass regimes, the present-day mass function has different values of $\alpha$:
$N(M) \sim M^{3.2}$      for  $M < 1~M_{\rm sun}$
$N(M) \sim M^{-0.7}$  for  $M > 1~M_{\rm sun}$
Given that $N(M)$ has different slopes on either side of 1 $M_{\rm sun}$, then it's clear that stars like our Sun will dominate your stellar sample. Even though M dwarfs are 75% of the stars in the Galaxy, you won't see many of them. This is why so many of the planets found in wide-field transit surveys such as HAT and WASP show up around Sun-like stars. The visible sky is full of G2-F8 stars!

At higher stellar masses, there aren't many stars formed, and those that do form die young because stellar lifetimes scale as $M^{-3}$ or so. But the effect isn't as severe as on the low-mass side. It's a gentle fall-off toward A and B dwarfs.

Friday, April 5, 2013


The Transiting Exoplanet Sky Survey (TESS) has been selected as NASA's next next Explorer mission. TESS 
is like an all-sky Kepler. While the Kepler telescope stares at hundreds of thousands of faint stars in one patch of the sky, TESS will look at an order-of-magnitude more stars (2.5 million!), and it will focus on those much closer to home. This mission is a big part of my future science plans on a ten-year time scale, so I'm extremely excited that it was selected. Go NASA!

WASHINGTON -- NASA's Astrophysics Explorer Program has selected two missions for launch in 2017: a planet-hunting satellite and an International Space Station instrument to observe X-rays from stars. 

The Transiting Exoplanet Survey Satellite (TESS) and Neutron Star Interior Composition Explorer (NICER) were  among four concept studies submitted in September 2012. NASA determined these two offer the best scientific value and most feasible development plans. 

TESS will use an array of telescopes to perform an all-sky survey to discover transiting exoplanets ranging from Earth-sized to gas giants, in orbit around the nearest and brightest stars in the sky. Its goal is to identify terrestrial planets in the habitable zones of nearby stars. Its principal investigator is George Ricker of the Massachusetts Institute of Technology in Cambridge. 

NICER will be mounted on the space station and measure the variability of cosmic X-ray sources, a process called X-ray timing, to explore the exotic states of matter within neutron stars and reveal their interior and surface compositions. The principal investigator is Keith Gendreau of NASA's Goddard Space Flight Center in Greenbelt, MD. 

"The Explorer Program has a long and stellar history of deploying truly innovative missions to study some of the most exciting questions in space science," said John Grunsfeld, NASA's associate administrator for science in Washington. "With these missions we will learn about the most extreme states of matter by studying neutron 
stars and we will identify many nearby star systems with rocky planets in the habitable zone for further study by telescopes such as the James Webb Space Telescope." 

NASA's Explorer program is the agency's oldest continuous program and is designed to provide frequent, low-cost access to space using principal investigator-led space science investigations relevant to the Science Mission Directorate's astrophysics and heliophysics programs. Satellite mission costs are capped at $200 million and space station mission costs are capped at $55 million. 

The program has launched more than 90 missions. It began in 1958 with the Explorer 1, which discovered the Earth's radiation belts. Another Explorer mission, the Cosmic Background Explorer, led to a Nobel prize. NASA's Goddard Space Flight Center manages the program for the agency's Science Mission Directorate in Washington. 

For more information about the Explorer program, visit: 

For information about NASA, visit: 

Thursday, April 4, 2013

Quick-twitch muscles: theory and practice

Here are the muscles that help humans jump. Most important are the quick-twitch muscle cells. This is theory.

The video below shows the theory in practice, as seen in last night's Clippers game. I was in the crowd, up near the rafters, but it was still a plenty good view of DeAndre Jordan tearing up the Suns. Poor Jermaine O'Neal at the 1:50 mark. Not quite Mosgov'd, but close...

NOTE: Cover the kids' ears near the 0:30 mark. We heard it in the stadium, too, when the basket mic picked up O'Neal exclaiming "Oh sh*t!" when he saw the lob going up over his head. Guard yo' grill!

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!

Monday, April 1, 2013

New disorder identified

Four-oh-phobia - The persistant, irrational fear that your page cannot be found.