The Road to Characterizing PH1: Stellar Evolution Models
Today we have a guest post from Willie Torres. Willie is an astronomer the Harvard-Smithsonian Center for Astrophysics (Cambridge, MA), and member of the Kepler team. His work for Kepler includes the statistical “validation” of transiting planet candidates that cannot be confirmed in the usual way, that is, by measuring their mass and showing that it is small enough to be a planet. He also works on determining fundamental parameters of stars in eclipsing or astrometric binaries, for testing models of stellar evolution.
Transiting circumbinary planets are interesting because they show us that planets can form in environments that are very different from our Solar System. Instead of being a single star, the central object is actually a pair of stars orbiting each other, and in these systems the planet can occasionally pass in front of one or even both stars, producing transit signals. For circumbinary planets such as those the Kepler Mission has announced (Kepler-16, Kepler-34, Kepler-35, Kepler-38, and most recently Kepler-47), the orbit of the two stars is such that they eclipse each other periodically, and these typically deep eclipses are what calls attention to them in the first place, in the light curves produced by Kepler. The neat thing about these transiting circumbinary systems is that they can also provide a wealth of information about the stars that is normally not available in regular transiting planet systems with a single host star. Two important stellar properties one can often measure are the masses and radii, from knowledge of the orbit of the stars around each other.
Masses and radii can of course also be determined in favorable eclipsing systems that don’t have planets, but when there is a transiting circumbinary planet, it’s even better. This is not hard to understand: as the planet passes in front of one or both stars, it is actually chasing a moving target because the two stars are revolving around each other. Each time the planet transits, the stars are in a different place in their orbit. This means that by measuring the precise times of these transits, we are actually mapping the orbit of the binary in a different way than would normally be done for a regular eclipsing binary. This provides extra information about the motion of the stars, and in particular it constrains the ratio of the masses between the two stars very well. It also helps to determine their sizes. Combining this with additional observations such as radial velocities measured from spectra of one or both stars, their masses and radii can be measured to high precision.
Astronomers care about the masses and radii of stars because these measurements allow them to test their models of how stars form and evolve. Theorists have come up with a fairly detailed prescription for how a star of a given mass and chemical composition changes its properties (radius, temperature, luminosity, etc.) as time goes by. But without real observations against which to check those predictions, we can’t be sure they’re right. This is important because astronomers often use those same models to infer properties of single stars that are much more difficult to measure directly. Or they may be interested in knowing the age of a star, which also relies on theoretical models. As it turns out, observations have shown that models for low mass stars (such as the cool M dwarfs) are not quite right: real stars tend to be a little bit larger and cooler than the models predict.
Circumbinary planets in which the eclipsing binary at the center contains an M dwarf are particularly interesting, because they allow us to test theory in this problematic low-mass regime. That happens to be the case for the recent exciting Planet Hunters discovery of KIC 4862625. The primary component in the eclipsing binary is an F star of about 1.3 solar masses, and the secondary is an mid M dwarf a little under 0.4 solar masses. They orbit each other every 20 days. The circumbinary planet goes around every 138 days. With other colleagues I’ve been working on determining the stellar properties of both stars as accurately as possible, and comparing them with several sets of stellar evolution models (since models are not all created equal). For getting the stellar properties we use not only high-quality spectra taken with the 10-meter Keck telescope in Hawaii, but also results from a very sophisticated modeling of the Kepler light curve that can reproduce all the binary eclipses as well as the transits of the circumbinary planet nearly perfectly. This tells us that we at least understand the dynamics of the system pretty well (i.e., how all the objects move).
But there are always complications. In this case, we took a high-resolution image of the system and discovered that there’s another star right next to eclipsing binary that (we realize now) is introducing contamination in the Kepler light curve. It’s only about 0.7 arcsec away from the eclipsing binary, and we believe it is physically associated. But wait, there’s more! The images show that this companion is actually a close binary itself! At the time of this writing we are still trying to figure out exactly how much extra light these new objects are contributing to the Kepler photometry, so that we can take that into account in order not to bias the measured properties of the eclipsing binary stars, or of the circumbinary planet.
The Road to Characterizing PH1: Transits and Initial Modeling
Today we have a blog post from Dirk Terrell. Dirk is an astrophysicist and the manager of the Astronomy and Computer Systems section at the Southwest Research Institute in Boulder, CO. His hobbies include coaching football, racing motorcycles and space art. He is a Fellow and former President of the International Association of Astronomical Artists.
The discovery of PH1, a planet orbiting an eclipsing binary with another pair of stars about 1000 AU away, was exciting for everyone involved. As someone who has worked with amateur astronomers for over twenty years, I was particularly happy to see that the initial discovery of the transits was made by two amateur scientists, Kian Jek and Robert Gagliano. My involvement came shortly thereafter when Meg Schwamb was visiting my institution , Southwest Research Institute in Boulder, Colorado, last spring to give a talk on another topic. She was meeting with Hal Levision, a dynamicist here at SwRI, and was describing the rather complicated Kepler data on the system. That’s when Hal gave me a call.
I get calls from Hal all the time, almost always on some computer-related topic. But this time he said “Do you have time to talk science?” My area of expertise is eclipsing binary stars, and here at SwRI I am surrounded mostly by scientists who do research on topics in planetary science, so I don’t often get phone calls from people here to talk about science. Curious, I walked down the hall to Hal’s office.
When I walked in, he introduced me to Meg and she began to describe this eclipsing binary system that had been observed by the Kepler satellite. She pulled up the light curve on the Planet Hunters web site. Having been doing the analysis of eclipsing binary light curves for nearly a quarter century, I immediately began to develop an idea of what the system looked like. It had an eccentric orbit and the stars were reasonably small compared to their separation, so they would not be very distorted by each other’s gravity. It was also clear that there were regular variations in the light curve outside the eclipses of the stars, probably pulsations or spots on one of the stars. It was a very busy looking light curve! Then she showed me the dips that Kian and Robert had noticed. With all of the activity in the light curve, it wasn’t quite as obvious as it is in simpler systems that we had transits by a planet. She asked me if I could help clear things up enough to warrant turning very expensive big telescopes like Keck on the system to get complementary data. I told her I’d be happy to look at it.
When working with data, I like to start as closely as possible to what the instruments produce and then do what’s needed to get the data into the form needed for an analysis. That way I know exactly what’s been done. Kepler data, like those from any instrument, have various effects that have nothing to do with the object you are observing. For example, the spacecraft has to make periodic maneuvers to keep its solar panels properly aligned. These rolls show up clearly in the data because the stars being observed are in different places on the imaging detectors before and after the rolls. Kepler, as you probably know, measures the brightness of stars very, very precisely with CCD arrays, the same device inside your digital camera. The pixels that make up the CCDs are not all uniformly sensitive, so when a star is moved from one set of pixels to another, the response to the same brightness level from the star will be a little different. But even a relatively small 1% difference in sensitivity will show up as a big jump with detectors as precise as those on Kepler. To give you an idea of what these raw data look like, here is a plot of the raw Kepler data for PH1 from quarter four:
You can see five deep eclipses of the hotter star, the higher frequency pulsations or spot modulation, as well as large instrumental effects and gaps. And, as we later showed, there is indeed a transit by the planet in there too, but it certainly doesn’t jump out at you because of everything else that is going on. So, my job was to clean up the light curve so that we could isolate the potential transit signatures. While we will later be very much interested in the stellar eclipses and the spots/pulsations, at this stage of the game we just needed to get rid of them. My approach was pretty simple: break the light curve up into parts to which I could fit a combination of linear and periodic terms, and then subtract those fits to get the residuals (i.e. what was left). Since spot/pulsation modulations happened at a much higher frequency (period of about 2.6 days respectively) than the suspected transits (period of about 138 days), removing the high frequency terms would leave the transits and the stellar eclipses (20 day period) intact and the linear term would remove most of the instrumental effects. Then I could fit the stellar eclipses with the Wilson-Devinney light curve program and remove the eclipse signatures leaving only the transits. This approach worked well enough to answer our big question at the time: are there transits? Lurking in the residuals were things like this:
and plotting them all together:
Yes, we had discovered transits and at that point decided it was worth turning Keck towards this system to further characterize it.
The Road to Characterizing PH1: The Overview
It’s been an exciting week for exoplanets with the discovery of PH1 and the discovery of an Earth-mass planet around the closest star to Earth (Alpa Centauri B). In the coming days and weeks, you’ll hear more about the specifics of how we came out confirming and validating the discovery of PH1 and solidifying that it was a planet orbiting the two central stars in a four star system, but I wanted to give a brief summary of the data and results.
This effort has taken months and months from obtaining the telescope observations, to modeling the light curves, combining Kepler data with radial velocity observations, and applying stellar evolution models. Robert Gagliano and Kian Jek get enormous credit for the discovery and starting this process off and recognizing and spotting the planet transits.
Many collaborators have worked hard to confirm PH1 and study its properties, especially: Jerry Orosz, Josh Carter, Willie Torres, and Debra Fischer who have put tremendous effort (particularly in the past few weeks) to get us from so we found some transits to bona fide planet.
Everyone in this collaboration involved in the paper (including Kian and Robert who were coauthors on the discovery paper) are listed below:
To summarize PH1’s confirmation story, I thought I’d share my press talk slides:
Robert and Kian identified three transits in the light curve of an eclipsing binary . In the binary, there are two stars, one larger and hotter and one smaller and cooler. When the smaller cooler star crosses the face of the larger hotter star, you get some of the larger star’s blocked out and we call that a primary eclipse. When the smaller cooler star crosses behind the larger hotter star, we get a secondary eclipse where the light from the smaller cooler star is blocked out. So we see this big dip+ small dip pattern. Robert and Kian noticed three transits separated by ~137 days in Quarters 1-5 data superimposed on the light curve indicating a third small body in the system, and notified the science team of a possible circumbinary planet.
There’s a small chance that we’re seeing a false positive, where on the sky our Kepler eclipsing binary is aligned with a faint background eclipsing binary giving rise to the transit-like signal. If the transiting body is truly orbiting both stars, we have a way of checking. A body in a circumbinary orbit (orbiting around both stars in a stellar binary), is orbiting a moving target, so the positions and velocities of the two stars are different each transit. So this means that at each transit, there are slightly different gravitational tugs on the planet causing the timing and the duration of the transit to change. We see these changes and this gives us high confidence the planet is really orbiting the Kepler eclipsing binary and not some background source. If you look below at the 7 planet transits across the larger bigger star in the binary, you can convince yourself that the widths of the transits are changing. So bingo, planet!
We got radial velocity observations from the Keck telescopes on Mauna Kea. The radial velocity observations measure the wobble of the larger star in the binary that the planet orbits. With the precision of the observations and time duration we have on the observations, we cannot measure the wobble caused by the gravitational pull of the planet. What we are measuring is the wobble due to the gravitational tug of the smaller star as it orbits the larger star in the binary.
To our surprise we found two velocity contributions in the radial velocity observations. One is from the larger star in the eclipsing binary (solid points) with the model fit for the velocity observations shown in red. The 2nd component is stationary over the roughly 5 months we were taking radial velocity observations. This 2nd component is coming from a source that is providing some light in the spectra slit we place across the Kepler target when observing on Keck. It has the same value as the average or systemic velocity of the binary. So if you take the average value of the black points that’s the velocity of the eclipsing binary (and planet host stars) moving towards or away form us. This second component has the same velocity and random field stars have velocities in the galaxy ranging from ~20-60 km/s so to have a source that has to be very close to the eclipsing binary on the sky that we see it in the Keck observations and have the same average velocity as the eclipsing binary tells us that this source is bound to the eclipsing binary.
We used adaptive optics observations from the Keck II telescope to zoom in and look around our eclipsing binary for other stars that would be contamination the Kepler photometric aperture. We also used deep optical imaging to look for slightly further contamination stars as Bill Keel described in his blog post. As Bill discussed in his post, if there are stars providing extra light to the aperture that is summed up to make the Kepler light curve, it will give us the wrong planet radius. This is because the extra light will decrease the observed transit dips causing us to underestimate the size of the planet. We go these adaptive optics observations while we getting the radial velocity observations.
In the adaptive optics observations there is a source 0.7” away (or about 1000 AU) from the eclipsing binary, and we knew about its existence when we were analyzing the Keck radial velocity observations. There is was the ah-ha moment, where we went oh, this source in the adaptive optics observations must be that second velocity source we see in the radial velocity observations, that we think is gravitationally bound to the eclipsing binary orbiting well outside the planet’s orbit. To our surprise the adaptive optics observations revealed that this source is elongated in one direction (which you can see in the slide below). What we think this means is that we’re just barely resolving the source as a visual binary (2 stars!). So that we have a pair of stars, getting 2 for the price of 1 – (getting us to four stars in the system!) orbiting the eclipsing binary. Our best guess from the Keck observations is that the 2 stars in the distant binary are separated by no more than 40-50 AU.
Combining all of these observations, we went after obtaining the properties of the planet and the stars in the PH1 system. Here below are the properties that come out of the combined photometric-dynamical model that uses the radial velocity observations and Kepler light curve for both PH1 and the stars,. The age we estimate for the entire system from spectral modeling. We estimate the mass of the planet by the fact that it not massive enough to pull on its parents stars sufficiently for us to see slight changes in the timings of when the stars eclipse each other (so when the smaller cooler star crosses the face of the larger hotter star and when the smaller cooler star crosses behind the larger hotter star).
We confirmed with modeling the eclipsing binary properties with just the Kepler light curve, and we’re confident in our estimation for the planet and host star properties. The planet is a gas giant, a bit bigger than Neptune and slightly smaller than Saturn. PH1 orbits its parent stars at a distance between Mercury and Venus in our own Solar System. The planetary system is stable. The planet happily orbits the eclipsing binary every ~138 days not really noticing there’s a second pair of stars out at 1000 AU .
PH1 is the 7th circumbinary planet, and the 6th circumbinary system. Below are the orbits of all the circumbianry Kepler planets and PH1 (not depicting the second distant pair of stars).So why do we care? Well, circumbinary planets are the extremes of planet formation, and we need to understand how they form if we are really understand how we form planets in our own solar system. Planet formation models need to be able to explain both environments, and each of the systems detected gives us another puzzle piece to this picture.
Planet Hunters’s First Circumbinary Planet- A True Team Effort
Today we have a guest post by Planet Hunters Robert Gagliano and Kian Jek, the discoverers of PH1, our first confirmed planet and first circumbinary planet.
Kian Jek found an anomalous dip in APH10421275 in May 2011 which turned out to be KIC 12644769 (Kepler-16b) the Kepler team’s first circumbinary planet discovery. He documented it on Talk in his thread “Strange transit in an EB”. He subsequently started a thread in the forums called “Finally-an EB with a planet?” Meg Schwamb then added a list of all known Kepler Eclipsing Binaries (EBs) with links to the light curves to this thread in November 2011.
Robert Gagliano did a systematic search of the ~ 1500 known Kepler EB’s, looking for possible planets in February 2012. He initially spotted a possible transit in Q4.1 at day 244 in SPH10052872 and subsequently a possible 2nd matching transit at day 106 in Q2.3. Interestingly, the day 106 transit had been detected previously by JKD and commented on by Kian in the thread “Potential TERNARY System“. Robert also noted a possible 3rd transit in Q5.3 at day 379 but didn’t comment on it because it was distorted and he wasn’t sure whether this was a real 3rd transit. This Q5.3 transit was subsequently predicted by an seo company and officially confirmed by Kian.
Kian decided to check the Skyview image to be sure it wasn’t contaminated from other background stars and did an analysis to determine if the transit period, depth, and duration were consistent with a planet. He detrended the light curve with a modified smoothing filter that removed the EB eclipses, leaving the suspected planetary transits in place, and then folded the curve to confirm that the profile of the transits were similar in depth and duration. His analysis was confirmatory. Meg then assembled an outstanding science team of 10 professionals to conduct extensive follow-up observations and data analysis. Eureka! KIC 4862625 was Planet Hunters’s “Tatooine”….we bagged our first circumbinary planet!
Today’s post is by one of our undergrads, Farris Gillman ——
I am a junior at Yale, and just beginning a project to follow up and model the EB’s that you all have discovered! Prof Debra Fischer and I took the 7am train from New Haven to Villanova University on Friday 11/11/11, where we met with Prof Andrej Prsa, an expert in modeling eclisping binary systems. It was a beautiful Fall day in Philadelphia! We returned on the 5am train Saturday morning, a little painful for me, but I got to sleep on the train since I didn’t down gallons of green coffee the night before!
Prof Prsa showed us how to use his program to model the periods and phases of eclipsing binary star systems, and we began working through the list of unique EB’s compiled by kianjin. It was this list of ~150 eclipsing binaries prompted our trip, and I will be working on this data set for my senior thesis project as well. I’m hoping to improve the modeling software and begin looking at some of the statistics of EB’s. Planet Hunters has been particularly helpful in finding eclipsing binary systems with long periods, which preliminary searches had missed. Dr. Prsa also gave us an overview of his publicly available program PHOEBE (http://phoebe.fiz.uni-lj.si/), which you can download too, to model binary systems. Thanks so much for all of your help – I’m really excited to start modeling some of these systems!