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Asteroseismology

Today’s post is brought to you by guest blogger Charles Baldner, who will be writing a few blog posts this summer on topics related to stellar structure,  asteroseismology, and stelalr activity. Charles is a graduate student in the Astronomy Department at Yale University. In his research, he uses helioseismology to study links between the interior of the Sun and solar activity.

Kepler is, first and foremost, an instrument designed to discover and investigate planets around other stars. It will probably not surprise you, however, if I tell you that Kepler data also provides an astounding amount of information about the stars themselves. What the planet hunter sees as noise – that annoying scatter in the data that hides or confuses the telltale signs of a planet – is music to another scientist. I mean that almost literally: like drums, flutes, bagpipes, or guitar strings, stars ‘ring’ at a variety of specific pitches, encoding information about all sorts of stellar properties. Using these `sounds’ to study stars is the science called asteroseismology.

A star is, more or less, a giant sphere of hot gas. Just like in the Earth’s atmosphere or oceans, waves can propagate through a star’s interior. These waves can reflect at the surface, causing it to move up and down, or to brighten or dim. If you can measure the velocity of the surface of a star very precisely, or measure the changes in brightness at the surface, you can detect these waves. If you take enough measurements, you can perhaps see the star ringing just like a musical instrument. In many stars, in fact, the waves you are seeing are sound waves, bouncing back and forth in the stellar interior just as they do inside an organ pipe.

We have used this kind of study to probe the inside of the Sun for more than thirty years. This is called helioseismology, and we have used it to determine the structure of the Sun very precisely. We can measure to great accuracy, for example, exactly where the interior of the Sun changes from `radiative’ to `convective’ (to learn more about the structure of the Sun, you can of course start at Wikipedia: http://en.wikipedia.org/wiki/Sun). We can also see the effects of rotation — different layers and different latitudes of the Sun rotate at different speeds, and we can measure this with helioseismology. Today, I use the tools of helioseismology to probe the regions just beneath sunspots.

In stars, as you can perhaps imagine, measuring these oscillations is much more challenging than it is in the Sun. After all, for most asteroseismic pulsations, we’re talking about minute changes
in velocity or brightness. But that, of course, is precisely what planet search instruments are built to measure, and the Kepler mission is providing us with an immense trove of data with which to use
asteroseismology to study large numbers of stars. In a future post, I’ll go over a few of the sorts of things we hope to glean from Kepler’s asteroseismic measurements.

Image Credit: NASA/ESA/SOHO http://sohowww.nascom.nasa.gov/data/realtime/eit_304/512/

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Inverse Transits

I was talking to last week’s seminar speaker, and we were talking about Planet Hunters and some of the things that might be lurking in the Kepler data.  One cool thought is there might be inverse transits so instead of dimming events, instead the star actually appears brighter.

There are lots of eclipsing binaries that you’ve probably seen as you’ve been classified, but another interesting type of eclipsing binary might be a transiting white dwarf orbiting a main sequence star. White dwarfs are about the same size or a little bit bigger than the Earth about half as massive as the Sun. Depending on where the white dwarf orbits, there could be magnification causing a brightening as the white dwarf crosses in front it’s companion star. This magnification is caused by gravitational microlensing, where a massive object bends  light of a background source resulting in images of the source that are magnified and distorted. Transiting exoplanets are not massive enough to bend and distort the light of their companion stars significantly. For eclipsing binaries it looks white dwarfs are in the sweet spot, if they are orbiting extremely close to their partner main sequence star. Papers in 2003  by Sahu and Gilliland (2003) and Farmer and Agol predicted that Kepler might be able to detect such events. In these cases during the transiting event, the ligthcurve gets brighter rather than fainter. These events last as long as the transit does so only a few hours (if the white dwarf is orbiting at 1 AU the event is ~10 hours in duration).

Here’s some examples from a paper by Sahu and Gilliland (2003) .

A transiting 0.6 solar mass white dwarf orbiting at 1 AU

 

0.6 solar mass white dwarf at different orbital radii from a solar-type star

There are some estimates of how many might be there ranging from a few to a about a hundred or so events in the Kepler monitored stars, but we really don’t know.  No one has detected them, and there could be 1 or none but with so many eyeballs staring at the data, we might uncover them if they’re there. Anyone seen anything like this in the light curves you’ve classified? It would be very exciting if we found one, it would be the first such discovery – if you see an inverse transit like the examples above, please share on Talk and let us know about your discovery!

Cheers,

~Meg

Q2 Data Release and Site Upgrades

We wanted to talk more about the changes to the site and give you all an update on the addition of Quarter 2 data. John’s already talked about the candidates page and some of the new features associated with that, so I wanted to focus on the changes specific to Q2 data release.

NASA and the Kepler team released Quarter 2 on Feb 1st and on Feb 2nd the latest results from the Kepler mission including a complete list of planet candidates and false positives for the first 2 quarters of data. You can read the paper detailing all of this here as well as the Kepler press conference site

The second data release is 90 days so we now have the first approximately 120 days of the Kepler science mission to go through. Q1 was about 35 days, we have chosen to show chunks of the lightcurve in the same size as we were for Q1. So Q2 is broken into three sections. Our aim was to have 5 days worth of overlap in each section, so that we don’t miss any transits that happen at the starts and ends of where we separated the lightcurves. We’re also uploading the Q1 data from the ~400 stars originally withheld and released on Feb 1st. We’ll keep you all posted on the progress.

We have been uploading the new data in batches to make the transition as smooth and seamless as possible. Occasionally the Talk links lag behind because we’re trying to upload as fast as you’re all going through the data. And sometimes you beat us to it

so we’ve increased how fast we’re uploading the Q2 data to keep up with your pace. We’ve appreciated all your patience during this process.

You can tell which part of the lightcurve you are looking at by the APH#. The first two numbers are quarter and section so APH22332480 is section 2 of Quarter 2. We use APH for the lightcurve sections and SPH for referring to the star itself. For the SPH numbers the first two numbers refer to what quarter the star first appeared in the public data set. so SPH21332480 first appeared is Quarter 2 Section 1.

The star source pages (like http://www.planethunters.org/sources/SPH10129795) contain all the sections of lightcurve for you to review and the x-axis is the days from the first observation, so you can look for repeat transits in other sections of the lightcurve easily. Also the downloadable CSV file now contains all the available lightcurve data. We have also updated the gap question (the first question asked) in the classify interface, so now you will now be asked the variability questions regardless of how your answer the gap question (before the variability questions were skipped if you answered yes to their being a data glitch or gap in the lightcurve)

We’ve made some changes to Talk to accommodate the Q2 data. The new planet candidates list and false positive list from the Kepler team are now identified. We’re planning in the near future of marking Planet Hunters planet candidates as well. Each lightcurve section has it’s own object page (ie http://talk.planethunters.org/objects/APH22332480). We now have group pages that gather all the available lightcurve object pages for the star (http://talk.planethunters.org/groups/SPH21332480) which you can access through the “View Star” link on any of the object pages. The “Examine Star” link will take you directly to the star’s source page.

As always we welcome feedback on the new changes, and we are listening to your comments and suggestions on Talk and in your emails. We can’t wait to see what we find in the Quarter 2 data.

Happy Hunting,

~Meg

More star info…

Thanks again for your amazing work and feedback.  We are working to keep up with you! There is now a data-download button (thanks to Chris, Arfon, Michael, and Stuart!) on the star pages. We are also integrating information about stars that are known eclipsing binaries (EB), Kepler planet candidates (PC) and false positives (FP). Here is an ascii list of light curves with this information. On this list, the APH number is given, followed by the Kepler ID and a flag (EB, PC, FP). For EB objects, D indicates detached binaries, SD is semi-detached, OC is an overcontact binary. Kepler PC stars include columns with the prospective period and planet radius (in Jupiter radii units).

One note about false positives: There are light curves that masquerade as transiting planets. For example, light from a bright foreground star is spread out over several pixels on the CCD detector.  The halo of starlight is swept up into a single brightness measurement by the Kepler team’s software. However, in some cases a more distant eclipsing binary (EB) star system blends into the edges of the foreground star.  Since the EB is more distant, it is fainter and contributes a smaller fraction of the light. In this case, the background eclipse produces a diluted signal that looks very much like a transiting planet.  There are a couple of ways to eliminate these imposters:

  1. the Kepler team has software that looks for pixel contamination and identifies the star as a false positive (FP). When available, we are listing this information on the light curve and star pages.
  2. Follow up radial velocity measurements of the bright star will also include the background blended eclipsing binary.  A large velocity signal can be a give away sign that the light curve does not arise from a transiting planet.

This follow-up is a critical effort, required to move an object from a transit candidate to a planet.

A word from the Kepler Team

Dear Planet Hunters: Dr. Natalie Batalha, Deputy Science Team Lead for the Kepler Mission, asked us to post the following message:

Welcome! We are so glad you’re here!

I’m sure I speak for the entire Kepler team when I say how happy we are that Zooniverse is being applied to the Kepler data. For some time now, I’ve watched the public actively work with archived data from other missions. The folks at Unmanned Spaceflight, for example, regularly share the latest images they’ve doctored up from Solar System missions like MER and Cassini. And the SOHO mission recently hit a milestone, discovering its 2000th comet on December 26th, 2010. The discoverer was not part of any formal SOHO science team but rather an astronomy student at Jagiellonian University in Krakow, Poland. I’ve added “Citizen Scientist” to my urban dictionary and appreciate its tremendous potential.

That’s all well and fine when it comes to Martian landscapes, comets, and sunlight glinting off the surface of methane lakes millions of miles away. But how in the world could we entice the public to look at boring old lightcurves? PlanetHunters.org has done exactly that. Not only are thousands of people looking at light curves, they are getting just as hooked on their variety as we are! Welcome to the ranks of those who love light curves.

The Kepler spacecraft is a new piece of technology. Never before have humans stared at stars with such unwavering precision and patience. And whenever humanity does something new, there are sure to be surprises. One of the biggest surprises to me so far is the impact that Kepler is having on stellar astrophysics. Who knew, for example, that a star like RR Lyrae — one of the brightest and well-studied objects in the sky — would blow the dust off textbooks written on this class of star? Who knew we’d see such a symphony of variability occurring just below the noise levels typical of ground-based telescopes?

But the name of the game here is planet hunting. I’ve heard people wonder why they should bother to hunt for planets when the Kepler team has spent years designing savvy computer algorithms to do exactly that — algorithms that can tease signals out of the noise that the human eyes cannot even see. The answer is simple.

Kepler relies, in large part, on automation. We are a relatively small team. There are currently less than 15 scientists working in the Kepler Science Office here at Ames. In the early days, there were only 5 of us! Let’s say we divided up the 150,000 stars we are monitoring amongst the 15 scientists at Ames. We’d each be responsible for 10,000 stars. If we spent only 60 seconds looking at each star, it’d take us over 160 hours to finish out allotment. That’s a solid month of doing nothing else but looking at light curves. Just in time since more data comes down from the spacecraft each month and the process would have to start all over again. Such a plan would never have earned taxpayer dollars. We need our scientists doing other things — like monitoring the instrument and optimizing the software and vetting out the false positives and interpreting the results. And so we write computer software that combs through the data searching for transit-like features.

It’s a challenge to design a one-size-fits-all approach to transit detection. The transit are buried in the light curves of stars with widely different properties and behaviors. You’d build one kind of tool for finding a needle in a haystack but a different kind of tool for finding a needle in a swamp. We don’t even yet know what all the possibilities are because we’ve never looked at stars with this kind of precision.

Another consideration is that the software pipeline requires 3 transits for complete modeling and pipeline generation of they key statistics that are used to vet out the false positives — astrophysical signals masquerading as planet transits. It’s certainly true that we’ve gone back and cherry-picked some of the more compelling light curves displaying less than 3 transits — especially those of the brightest stars. However, many such signals are still lurking in the archive.

So what else did our algorithms miss? Ah, let’s find out, shall we? We’re here with you, ready to help. Come stand here in the crow’s nest and experience the thrill of discovery with us. We welcome your keen eyes!

A huge thank you to the folks at planethunters.org for putting this together.

Natalie Batalha

Deputy Science Team Lead

Kepler Mission

Variable stars (examples)

The reasons for changes in the brightness of a star can be divided into two categories: (1) orbiting companions or (2) stellar astrophysics.

(1) In principle, the variability from orbiting companions (this includes eclipsing binaries or transiting planets) should be as regular as clockwork.  In practice, the variability can deviate from clockwork regularity if stellar binaries get too close together, if there are multiple transiting planets, if there is additional background electronic noise or astrophysical noise.

(2) Brightness variations caused by physical processes internal to the star (stellar astrophysics) can arise from pulsations of the star, starspots or flares. Flares are random spikes in the light curve brightness.  Pulsations from stars (like RR Lyraes) are quasi-periodic: they can appear to be regular for a while and the cycles are relatively short (generally hours to a day or so). The Figure below shows two variable stars with short periods that might be best classified as “variable” and “pulsating.” These could be short period binary systems – this could quickly be verified with follow-up observations.

puls1 puls2

Starspots produce complex variations. As the star spins, the spots rotate in and out of view with a   periodicity of a day or two (for the most rapidly spinning stars) to several days for slowly rotating stars (the Sun has a rotation period of 25 days).  Starspots can form at different latitudes on the star.  Since some latitudes rotate faster, spots can show multi-cyclical variations. The light curves below might be best classified as variable and irregular.  However, a case could be made for classifying the light curve in the figure below (and left) as variable and regular. Even though the amplitude of the curves changes, the time from one peak to the next is about the same.

irreg1

irreg2

Eclipsing Binaries

Figure 1.  Eclipsing Binary (detached, Algol type)

Figure 1. Eclipsing Binary (detached, Algol type)

I’m Debra Fischer, a Professor of Astronomy at Yale University. Many of you have already discovered some amazing eclipsing binary light curves, and we wanted to provide you with some information. The Figures here show examples that you have put into collections. Some great additional examples are shown in a paper from the Kepler team (Prsá et al. 2010 http://arxiv.org/abs/1006.2815).

The Kepler light curves show how the brightness of the star changes with time.  In Figure 1 (APH10135736 = KID 6449358) above, there are two stars orbiting each other.  Similar to transiting planets, these stars cross in front of each other. The light curve shows the brightness level of the star, plotted vs time in days.  Most of the time, both the larger, hotter star and the smaller cooler star yield a combined brightness measurement for the light curve. When the deep dip in brightness (the primary minimum) occurs it’s because the smaller cooler star is eclipsing the hotter star, which contributes most of the light; when the smaller dip (secondary minimum) occurs, it’s because the larger hotter star is eclipsing the smaller star, which contributes less light to the combined brightness. Stars with flat regions punctuated by relatively sharp dips (e.g. Figure 1) are known as Algol binaries.

A key indicator of eclipsing (or transiting planet) light curves is repeatability.

  • you can count the number of days between the large dips to determine the orbital period (about 5 days) of this binary star system in Figure 1
  • you can determine how long it takes the stars to cross by the duration of the transit dip (hours for Figure 1)
  • you know that one star is larger than the other if the transits don’t have equal dips

Notice that the depth of the brightness dips for an eclipsing binary star can be similar to those for a transiting planet. The transit depth tells us the ratio of the size of the transiting (or eclipsing) object relative to the size of the primary star and the smallest stars have diameters that are similar to Jupiter (stars are gas and the increased gravity from the larger mass star compresses the structure).

Figure 2. Contact eclipsing binary stars

Figure 2. Contact eclipsing binary stars

Sometimes binary stars are so close that the surfaces are distorted into an elliptical shape and the light curve between the eclipses is rounded, as in the left image of Figure 2 (APH10039007 = KID 9357275), where the orbital period is a little more than one day. You can see both the primary and secondary transit dip in this light curve. The most bizarre eclipsing binary light curves are those where the stars are even closer together, called over-contact binaries. An example of this is shown in the right image of Figure 2 (APH10102932 = KID 4633285). These stars can be so close together that they share a common envelope. The eclipse depth is variable, the light curve looks irregular, and there can be mass transfer between the stars.

Gwiazdy zmienne (przykłady)

Powody zmian jasności gwiazd można podzielić na dwie kategorie: (1) orbitujący towarzysze oraz (2) astrofizyka gwiazd.

(1) W teorii zmienność będąca efektem działania orbitujących towarzyszy (w tym gwiazd i planet) powinna cechować się regularnością szwajcarskiego zegarka. W praktyce mogą wystąpić pewnie odchylenia spowodowane zbytnim zbliżeniem się do siebie gwiazd w układzie podwójnym, jednoczesnym tranzytem kilku planet lub dodatkowymi szumami elektronicznymi bądź astrofizycznymi w tle.

(2) Zmiany jasności gwiazd wynikające z ich wewnętrznych procesów fizycznych (czyli astrofizyki gwiazd) mogą być spowodowane pulsacjami, plamami lub rozbłyskami. Rozbłyski to losowo pojawiające się skoki na krzywej blasku. Pulsacje (np. gwiazd typu RR Lyrae) mają charakter quasiokresowy: przez pewien czas mogą pojawiać się regularnie, a ich cykle są dość krótkie (zwykle trwają od kilku godzin do mniej więcej jednego dnia). Zdjęcie poniżej przedstawia dwie gwiazdy zmienne krótkookresowe, które można określić jako “zmienne” i “pulsujące”. Mogą to być krótkookresowe układy podwójne, co łatwo zweryfikować za pomocą kolejnych badań.

puls1 puls2

Efektem plam gwiazdowych są złożone wariacje. Kiedy gwiazda się obraca, plamy na zmianę pojawiają się i znikają z pola widzenia w okresach od 1-2 dni (w przypadku najszybciej obracających się gwiazd) do kilku dni w przypadku wolniej obracających się gwiazd (okres obrotu Słońca wynosi 25 dni). Plamy mogą powstawać na różnych szerokościach geograficznych gwiazdy. Ponieważ niektóre szerokości geograficzne obracają się szybciej, a inne wolniej, pomiędzy cyklami poszczególnych plam mogą występować znaczne różnice. Krzywe blasku przedstawione poniżej najlepiej sklasyfikować jako zmienne, nieregularne. Jednak wykres po lewej można by również określić jako zmienny i regularny. Mimo że amplituda krzywych ulega zmianie, czas pomiędzy poszczególnymi szczytami pozostaje taki sam.

irreg1

irreg2

Transiting Planets

The effects of 3 different types of transiting planets on a Kepler light curve. (Illustration: H. & M. Giguere)

Hi I’m Matt, a graduate student at Yale University and a member of the Science Team. We’re really impressed with the turnout so far on planethunters.org and users have already pointed out some really amazing objects! Quite a few people have asked for some clarification on what transits look like, so I’ll address that in this post.

In the figure above, we’ve taken a Kepler light curve from a star that’s about the same size as the Sun and have simulated what the effects would be if a few different types of planets were to transit.

The white dots show the amount of light from the star measured with Kepler with no planets transiting. The blue points show what we would see if a planet just like Jupiter orbiting this star were to transit. This Jupiter-size planet, at about 11.2 times the size of the Earth and one tenth the size of the star, is shown to scale transiting its parent star in the top left blue box.

The green dots show what a planet just like Neptune would look like transiting. Since it is much further away from the star than Jupiter, it would have a slower orbital speed so it would take longer to transit the disk of its parent star, which is what explains the longer duration, or wider width, of the transit event. With Neptune’s much smaller size than Jupiter, at 3.9 times the radius of the Earth, it doesn’t block out as much light, which is why the depth is much shallower.

Both of these events are very noticeable, compared to the effects of an Earth-size planet. The tiny speck on the star in the far right red box shows, to scale, what a transiting Earth-size planet would look like if we could see it. Now you get an idea of how difficult finding Earth-size planets is going to be! If that transiting planet had an orbital period of 1 year just like the Earth, then the dip in light observed from the parent star as the planet transits would be similar to the red points in the light curve. Since the Earth is much closer to the star, it has a much faster orbital speed, which then makes the duration of transit much shorter than the duration of either Jupiter or Neptune. Because the Earth-size planet is much smaller than either Jupiter or Neptune, it also blocks out less light making the dip in light we receive here on Earth barely discernible from no transit at all.

We don’t expect people to see these events all the time, so don’t worry about missing them. That’s why we’ve introduced fake planets into the mix. The fake, or synthetic, planets will help us determine the completeness of Planet Hunters, or how likely we are to detect planets of different sizes and with different orbital periods if they exist.

Stellar Variability

Greetings from Kevin Schawinski and Meg Schwamb, postdoctoral fellows at Yale and members of the Science Team.

Wow, we’ve been blown away by how enthusiastic everyone has been about the project. In this post, we wanted to talk more about another goal of Planet Hunters, which is to study and better understand stellar variability.  The public release Kepler data set is unprecedented, both in observing cadence and in the photometric precision. The lightcurves reveal subtle variability that has never before been documented.

The Kepler lightcurves are complex  many exhibiting significant structure including multiple oscillations imposed on top of each other as well as short-lived variations. Most of this variability is due by starspots or stellar pulsations.With Planet Hunters we will not only be looking for stars harboring planets outside of our solar system, but we will be able to study and classify stellar variability in ways that automated routines cannot. Unlike a machine learning approach, human classifiers recognize the unusual and have a remarkable ability to recognize archetypes and assemble groups of similar objects.

Users have the ability to identify strange or unusual lightcurves as well as tag similar curves and come up with their own classes or  ”collections”  of variability with  Planet Hunters Talk. You can add a comment and  use the #hashtag like in Twitter to mark an interesting lightcurve and alert others including the science team. Every light curve, or collection of curves has a short-message thread (140 characters) associated with it for general comments. You also can start discussions if you want to chat in a more in-depth fashion.

Mining the Kepler data set will inevitably lead to unexpected discoveries, showcased by the successes of Galaxy Zoo. The prime examples are the discoveries of  ”Hanny’s Voorwerp” and the ”green peas” by Galaxy Zoo users. Hanny’s Voorwerp is a cloud of ionized gas in the Sloan Digital Sky Survey image of the nearby galaxy IC 2497. Unlike an automatic classification routine, citizen scientist Hanny van Arkel spotted a blue smudge next to IC 2497, recognized it as unusual, and alerted the Galaxy Zoo team and the other users. Since then, Hanny’s Voorwerp has been identified as a light echo from a recent quasar phase in IC 2497, making it the Rosetta Stone of quasars. The Galaxy Zoo participants started noticing a very rare class of objects of point sources showed as green in the SDSS color scheme. Dubbing them the ”green peas,” the citizen scientists scoured the SDSS database, and assembled a list of these ”pea galaxies.”  The ”peas” were revealed to be ultra-compact, powerful starburst galaxies whose properties are highly unusual in the present day universe, but resemble those of primordial galaxies in the early universe. The citizen scientists found veritable fossils living in the present-day universe.

With so many eyes looking at the lightcurves, we are bound to find new variability types! We’re hoping that Planet Hunters, like Galaxy Zoo, will yield exciting new results that we can’t even attempt to speculate or imagine! We can’t wait to see what turns up.