The Ever More Puzzling, And Intriguing, “Tabby’s Star.”

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Star debris illustration
Did Tabby’s star going through periodic and deep dimmings because of dust and debris clouds that pass edbetween it and the mirror of the Kepler Space Telescope?  That was an earlier explanation for the highly unusual behavior of the star, but new research makes that answer less likely. Artist drawing by NASA/JPL-Caltech/T. Pyle

Substantial, sun-like stars are not supposed to dim.  They start with gravity and pressure induced nuclear reactions, and then they burn brighter and brighter until they either explode (go supernova) or burn all their fuel and become small, enormously dense, and not very bright “white dwarfs.”

Of course, the transit technique of searching for exoplanets looks precisely for dimmings — of stars caused by the passage of an exoplanet.  But those are tiny reductions in the star’s brightness and short-lived.  So if a star is dimming significantly over a much longer period of time, something unusual is going on.

And that is apparently exactly what is happening with the current poster child for mysterious stars — KIC 8462852 or “Tabby’s star,” named after the Yale University postdoc who, with the help of citizen scientists, discovered it,  Tabetha Boyajian.

First written up last fall, the big news was data from the Kepler Space Telescope showed that the star had experienced two major and dissimilar dips in brightness — a highly unusual and perplexing phenomenon.  The dips appeared much too large to represent the passage of an exoplanet, so explanations tended towards the baroque — a swarm of comets, a vast dust cloud, even an alien megastructure (proposed as a last possible explanation.)  The observation was first identified by citizen planet hunters working with Boyajian, making it an even more compelling finding.

Now the mystery has grown stranger still.  A paper made public last week based on a different kind of Kepler imaging (full-frame imaging) found not two but one enormous dip in the light curve, as well as a surprising and significant dimming the of star over the four year observing period of the space telescope.  The paper has been submitted for publication in American Astronomical Society journals.

Benjamin Montet of Caltech and Joshua Simon of the Observatories of the Carnegie Institution of Washington, analyzed the full-field images taken by Kepler every three months (rather than the hourly images studied by Boyajian et al,) and concluded that something strange was indeed going on.

Their conclusion: “No known or proposed stellar phenomena can fully explain all aspects of the observed light curve.”

 

Photometry of KIC 8462852 as measured from the FFI data. The four colors and shapes (green squares, black circles, red diamonds, and blue triangles) represent measurements from the four separate channels the starlight reaches as the telescope rolls. The four subpanels show ux from each particular detector individually. The main gure combines all observations together; we apply three linear osets to the data from dierent channels to minimize the scatter to a linear t to the rst 1100 days of data. In all four channels, the photometry is consistent with a linear decrease in ux for the rst three years of the mission, followed by a rapid decrease in ux of  2:5% over the next six months. The light gray curve represents one possible Kepler long cadence light curve consistent with the FFI photometry created by tting a spline to the FFI photometry as described in Section 4. The large dips observed by Boyajian et al. (2016) are visible but narrow relative to the cadence of FFI observations. The long cadence data behind this gure are available online.
Photometry of KIC 8462852 as measured from the full-frame imaging (FFI) data. The four colors and shapes (green squares, black circles, red diamonds, and blue triangles) represent measurements from the four separate channels the starlight reaches as the telescope rolls.  In all four channels, the photometry is consistent with a decrease in starlight for the first three years of the mission, followed by a rapid decrease in flux of  2:5% over the next six months. The large dips observed by Boyajian et al. (2016) are visible
but less broad relative to the FFI observations. (B. Montet and J. Simon)

Expanding a bit, Montet told Gizmodo:  “We spent a long time trying to convince ourselves this wasn’t real. We just weren’t able to.”

A paper describing the results from these full-frame observations went up recently on the prior to printing site arXiv.    The site allows members of the astronomy world to offer critiques, and so the results as now released may not be final.

But the story line does seem pretty clear — that Tabby’s star had one very large period of light dimming and had a secular decline in the light it was sending out over the four years of the Kepler mission.

Boyajian, a newly-appointed Louisiana State University researcher and professor, said that she considers the original findings to be entirely compatible with the newest results, with differences based on how the light was being captured (the once-monthly full-frame Kepler images versus the continuous imaging done of more than 100,000 stars.)

What has also become increasingly clear is that the dimming is not the result of an instrument glitch, and that the surrounding stars are not exhibiting the same unusual behavior.

“As far as we know, dimming is not something stars do; they get larger and brighter,” she said.  “Especially on these remarkably fast time scales, the dimmings are unprecedented for any kind of star.”

Boyajian had initially favored the theory that the light was being blocked by a large swarm of comets, but she said the new results make that more unlikely.  She said it is similarly unlikely that the dimmings are the result of some internal dynamics of the star.  So is it all the result of some alien megastructure, the “explanation” that initially brought a lot of attention to Tabby’s star.  I think we can assume it is not.

But given the data now available, it has become extremely difficult to find an explanation that checks all the boxes.  And that’s why Boyajian and her colleagues began a kickstarter campaign to raise $100,000 for another year of observing through the telescopes of the private Las Cumbres Observatory Global Telescope Network.

As she explained it, one of the telescopes will image the star at least two hours per night for the next year.  And if a significant dimming is observed, larger ground-based telescopes will be available to look more closely.

It’s a waiting game now, which is exciting itself,” she said. “It’s only a guess, but based on Kepler light curves, we might see something interesting next spring.”

(My earlier story on Tabby and her star can be found here:  Tabby’s Star)

Tabetha Boyajian was the driving force behind bringing the mysterious star xxxx to public attention. It had initially been identified as peculiar by the citizen scientists of xxx.
Tabetha Boyajian was the driving force behind bringing the mysterious star KIC 8462852 to public attention. It had initially been identified as peculiar by the citizen scientists of Planet Hunters, which is part of the Oxford University based “Zooniverse” Citizen Science Alliance.

 

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The Still Mysterious “Tabby’s Star”

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Artist rendering of star xxx, and the unexplain ed objects close to it. KNown as "Tabby's" star
Artist rendering of dusty comets approaching star KIC8462852, an interpretation of the mysterious objects that periodically block out substantial amounts of the star’s light. Known informally as “Tabby’s” star, it was discovered by citizen scientists using Kepler Space Telescope data, and they are looking for ways to continue their work. (NASA/JPL-Caltech)

It’s been eight months since citizen “Planet Hunters”  working with Yale postdoc Tabetha Boyajian announced the discovery of a most unusual star, or rather a star where something most unusual was intermittently and erratically happening.

The puzzle began with some light curve data, taken over a four year period, by the Kepler Space Telescope  The citizen planet hunters pored through reams of data sent back by Kepler looking for signals of planetary transits — the ever-so-slight dimmings of the star caused by the crossing or an orbiting exoplanet.

But the light curve for KIC 8462852 showed dimmings that were anything but slight, and anything but regular.  The Planet Hunters flagged the star for Boyajian’s groups attention, and the mystery star was born.

Theories on what was causing the very large dips ranged from a host of enormous comets, to a violently exploding planet, to an asteroid belt or the presence of close by stars, from an artifact of Kepler’s camera to, finally, an alien megastructure.  (The last was offered by Penn State astronomer Jason Wright as a kind of “Hail Mary” explanation if and when the others are found wanting.  But that’s what got the press.)

Despite years of concerted observing, theorizing and analyzing, Boyajian, Wright the citizen planet hunters and others intrigued by the mystery say they are no closer to an explanation for whatever is passing in front of the star (now informally called “Tabby’s star.”)  NASA has ruled out a technical glitch in the Kepler data, and a range of astronomers have found fault with all the explanations put forward.

But while the quite tantalizing mystery remains, efforts to learn more about the star may have to wind down soon.  The primary Kepler mission is over, so it will provide no more data for this star.  Other space telescopes will not be looking, nor will the major ground-based observatories.  And  the first SETI searches for signals coming from the star has found nothing unusual.

So with options dwindling to learn more, Boyajian, her citizen astronomers and others have begun a grassroots effort to raise $100,000 to buy time at a network of smaller ground-based telescopes around the world.

“All the models so far have major problem. So to go forward, we need new data ,” Boyajian said this week.  “There’s a huge amount of interest in this star, and we’re trying to use that interest to help solve a real mystery.”

 The unusual light curves for "Tabby's Star," which feature some extremely large dips and other smaller ones. The X-axis label “Kepler day” means days following the Kepler launch. (NASA/Kepler Space Telescope) The unusual light curves for "Tabby's Star," which feature some extremely large dips and other smaller ones. The X-axis label “Kepler day” means days following the Kepler launch. (NASA/Kepler Space Telescope)
The unusual light curves for “Tabby’s Star,” which feature some extremely large dips and other smaller ones. The X-axis label “Kepler day” means days following the Kepler launch. (NASA/Kepler Space Telescope)

KIC 8462852 is an otherwise ordinary F-type star, slightly larger and hotter than our sun . It burns some 1500  light-years away.  Of all the 150,000 stars monitored during the Kepler mission, it is the only one to show these kinds of highly unusual light curves and, presumably, to have such massive astrophysical objects (or fields or other phenomena) nearby.

During a TED talk, Boyajian described the recent history of observing the star.

The Planet Hunters, she said,  first detected something unusual in the star’s light curve in 2009 –a dip of 1 percent dip that lasted a week. This is roughly comparable to a sign produced by a Jupiter-sized planet transiting the star. Orbiting planets produce symmetric dips and the one they found was definitely asymmetric, like something that could be the result of the passing of an irregularly-shaped object like a comet.

The light from the star remained steady for two years, then it suddenly took a 15 percent plunge that lasted for a week.

Another two years passed without incident but in 2013 the star began flickering with a complex series of uneven, unnatural looking dips that lasted 100 days. During the deepest of these dips, the intensity of the light coming from the star dropped 20 percent. According to Boyajian it would take an object 1,000 times the area of the Earth transiting the distant star to produce such a dramatic effect.

What’s causing these unusual and strong signals.  The jury remains very much out.

The Kepler field of study, observed by the space telescope nonstop for almost five years. (NASA)
The Kepler field of study, observed by the space telescope nonstop for almost five years. (NASA)

But the process of applying for grants and space telescope time is both very slow and highly competitive. So the group has decided on a different, self-financing path.  This is how they described their current and future plans on their “www.wherestheflux.com” website:

We have initiated observations on the Las Cumbres Observatory Global Telescope Network (LCOGT). LCOGT is a privately run global telescope network specifically designed for time domain astronomy, meaning that their network of telescopes is positioned strategically around the globe to ensure continuous monitoring of an object.

Our observation plan is as follows. From the 4 years of Kepler data, we know that the dips in the light curve are not periodic, so we need continuous monitoring throughout the year since we cannot predict when it will dip again. We also know that how much the brightness drops is also variable from dip-to-dip. The LCOGT data will not have the precision Kepler had, but will have plenty of sensitivity to detect the observed dips in this star.

What’s more, since we are observing this star from the ground we are also able to tailor our observation plan to reveal detailed information on whatever object(s) are passing in front of the star to make the dips! One way this will be done is by observing the star at different wavelengths, or colors, of light. These new observations will monitor the star’s brightness at an assortment of colors!

In addition to this, the data from the LCOGT are space processed in real time, so when data are seen to pass below a brightness threshold, it will trigger more observations in the LCOGT network. Our science team will then alert for observations to be taken at larger facilities to get a better look.

The observatory has gifted this program 200 hours to begin the project on their new 0.4-meter telescope network, which will take us to the end of the summer.

Tabetha Boyajian, a postdoc at Yale and soon to be on the faculty of Louisiana State University.
Tabetha Boyajian, a postdoc at Yale and soon to be on the faculty of Louisiana State University. She has been working to unravel the mysteries of KIC 8462852 since

It’s not at all easy to apply for and win the stiff competition for observing time on a major public telescope, and that reality led to the outreach effort aimed those interested in collecting more Tabby’s Star data. The Planet Hunters citizen scientist group was brought together by Yale professor Debra Fischer, herself a professional planet hunter. The group is part of the Oxford University based “Zooniverse” Citizen Science Alliance.

As I will discuss in a later column, I have my doubts about some of the big-dollar, high-profile individual and foundation efforts to jump-start space travel and space science. They can be wonderful, but they sometimes feel like efforts to get the proverbial camel’s nose into the tent, and NASA and its budget are ultimately the tent.  (I’m not thinking here of commercial space efforts like resupplying the International Space Station, although they too depend on NASA to an important financial and technical degree.)

But grassroots private efforts like this one to learn more about Tabby’s star are, to me at least, quite  different.   This is hardly the first time a private group of enthusiasts has asked the interested public to help with their research and (hopefully) it will not be the last.  At the proper scale and with proper goals, they seem generally like a most valuable part of future space science.

And if this particular effort does end up solving the Tabby’s star puzzle in the months and years ahead — or at least giving some strong possible explanations — it will strenghten the case for public citizen science of all kinds.

 

 

 

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Shredding Exoplanets, And The Mysteries They May Unravel

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In this artist’s conception, a tiny rocky object vaporizes as it orbits a white dwarf star. Astronomers have detected the first planetary object transiting a white dwarf using data from the K2 mission. Slowly the object will disintegrate, leaving a dusting of metals on the surface of the star. (NASA)
In this artist’s conception, a small planet or planetesimal vaporizes as it orbits close to a white dwarf star. The detection of several of these disintegrating planets around a variety of stars has led some astronomers to propose intensive study of their ensuing dust clouds as a surprising new way to learn about the interiors of  exoplanet.  (NASA)

One of the seemingly quixotic goals of exoplanet scientists is to understand the chemical and geo-chemical compositions of the interiors of the distant planets they are finding.   Learning whether a planet is largely made up of silicon or magnesium or iron-based compounds is essential to some day determining how and where specific exoplanets were formed in their solar systems, which ones might have the compounds and minerals believed to be necessary for  life, and ultimately which might actually be hosting life.

Studying exoplanet interiors is a daunting challenge for sure, maybe even more difficult in principle than understanding the compositions of exoplanet atmospheres.  After all, there’s still a lot we don’t know about the make-up of planet interiors in our own solar system.

An intriguing pathway, however, has been proposed based on the recent discovery of exoplanets in the process of being shredded.  Generally orbiting very close to their suns, they appear to be disintegrating due to intense radiation and the forces of gravity.

And the result of their coming apart is that their interiors, or at least the dust clouds from their crusts and mantles, may well be on display and potentially measurable.

“We know very little for sure about these disintegrating planets, but they certainly seem to offer a real opportunity,” said Jason Wright, an astrophysicist at Pennsylvania State University with a specialty in stellar astrophysics.  No intensive study of the dusty innards of a distant, falling-apart exoplanet has been done so far,  he said, but in theory at least it seems to be possible.

Artist’s impression of disintegrating exoplanet KIC 12255 (C.U Keller, Leiden University)
Artist’s impression of disintegrating exoplanet KIC 12557548, the first of its kind ever detected. (C.U Keller, Leiden University)

And if successful, the approach could prove broadly useful since astronomers have already found at least four of disintegrating planets and predict that there are many more out there.  The prediction is based on, among other things, the relative speed with which the planets fall apart.  Since the disintegration has been determined to take only tens of thousands to a million years (a very short time in astronomical terms) then scientists conclude that the shreddings must be pretty common  –based on the number already caught in the act.

Saul Rappaport, professor emeritus of physics at MIT, led the team that first identified a disintegrating planet around KIC 12557548, using data from transit light curves collected by the Kepler Space Telescope.  The transits clearly did not indicate the usual small but detectable blockage by a solid body planet,  but were nonetheless intriguing because they were showing that something interesting was crossing (or occulting) the star and trailing an orbiting object.

Rappaport said he was definitely not searching for a dust trail from a disintegrating planet.

“Nobody had suggested that and we weren’t looking for it,” he said. “It took us completely by surprise.  Actually, after we found it, we spent many weeks trying to model it as a collection of solid bodies or something other than a disintegrating planet.  But ultimately we had to face up to what it is – occultation by dust emanating from a planet.”

Four years after his first paper was published, Rappaport said he is now 99 percent certain that KIC 12557548 is a close-in planet slowly disintegrating via the emission of dusty materials, as are three other similar objects subsequently detected.

Rappaport said that speaking generally, measurements of the size of the dust particles coming from those decaying planets would provide very valuable information to scientists, as would any insights into their chemical composition.  But he said that good data will be challenging to collect and equally difficult to interpret.

When an Earth-size planet passes in front of a star, it creates a symmetric dip in the star's light that's shaped like the red curve here. But astronomers detected the strange-looking, blue dip in light from the white dwarf 1145+017. The team suspects the signal comes from a tiny disintegrating planet or asteroid and its comet-like dusty tail. The black dots are measurements recorded by the Kepler spacecraft during its K2 mission. CfA / A. Vanderburg - See more at: http://www.skyandtelescope.com/astronomy-news/white-dwarf-eats-planet2610201523/#sthash.p9521Fxi.dpuf
When an Earth-size planet passes in front of a star, it creates a symmetric dip in the star’s light that’s shaped like the red curve here. But astronomers detected the strange-looking, blue dip in light from the white dwarf 1145+017. The team suspects the signal comes from a tiny disintegrating planet or asteroid and its comet-like dusty tail. (CfA /A. Vanderburg)

Unrelated to Rappaport’s work, Wright and a Penn State team, although with from the Arizona State University astrophysicist Steve Desch and others, have just sent a proposal into NASA to fund  disintegrating exoplanet research using ground-based telescopes and the Hubble Space Telescope.

The collaboration originated at a meeting of the Nexus for Exoplanet Systems Science (NExSS), a five-year NASA initiative to bring together exoplanet scientists from a variety of disciplines with the goal of having them work together across disciplines.  Organized by Mary Voytek, NASA’s senior scientist for astrobiology, it aims to bring the highly interdisciplinary model of astrobiology to the field of characterizing exoplanets.

“This is a project that really calls for, in fact requires, an interdisciplinary approach,” Desch said.  “This is where astronomy and astrophysics meet planetary science and geology, and that should be a very fruitful place.”

Is a measure of the interdisciplinary effort, their team also includes Casey Lisse at the Johns Hopkins University Applied Physics Laboratory.  He’s a comet scientist with a specialty in planet formation and astromineralogy.

Jason Wright, associate professor at Penn State University, initiated the collaboration to use disintegrating planets as a pathway to understanding exoplanet interiors. (Gudmundur Stefansson)
Jason Wright, associate professor at Penn State University, initiated the collaboration to use disintegrating planets as a pathway to understanding exoplanet interiors. (Gudmundur Stefansson)

Wright and Desch want to focus on the unusual transit signals from five stars — three M dwarf identified by Kepler, one a burned-out but super-dense white dwarf and other made famous last fall when a substantial and currently impossible-to-explain dust cloud was detected nearby it.  All the known explanations to explain it were deemed inadequate, which led to (last option) suggestions that perhaps it was an alien “megastructure” or Dyson swarm built by intelligent beings.

Wright was part of the group trying to explain the vast cloud around the star — KIC 8462852 or “Tabby’s star,” named after Yale University post-doc and co-founder Tabetha Boyajian) and now suspects that a disintegrating planet could be a source (though he says that Desch was the first to make the case.)

KIC 8462852, informally known as Tabby’s Star, is a magnitude +11.7 F-type main-sequence star located in the constellation Cygnus approximately 1,480 light-years from Earth. Data from NASA’s Kepler space telescope shows that the star displays aperiodic dimming of 20 percent and more. KIC 8462852 is shown here in infrared (2MASS survey, left) and ultraviolet (GALEX). Image credit: IPAC/NASA (infrared); STScI/NASA (ultraviolet).
KIC 8462852, informally known as Tabby’s Star, is a magnitude +11.7 F-type main-sequence star located in the constellation Cygnus approximately 1,480 light-years from Earth. Data from NASA’s Kepler space telescope shows that the star displays unexplained periodic dimming of 20 percent and more. KIC 8462852 is shown here in infrared (2MASS survey, left) and ultraviolet (GALEX) IPAC/NASA (infrared); STScI/NASA (ultraviolet)

The object that orbits a white dwarf star at a distance about the same as between Earth and the moon.  When its discovery was announced last year by Andrew Vanderburg of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, he said that something unique had been found:  “We’re watching a solar system get destroyed.”

The planet (or planetesimal) orbits its white dwarf, WD 1145+017, once every 4.5 hours. This orbital period places it extremely close to the super-dense star, and that speeds the shredding and evaporating of the planet. But makes it a theoretically easier target to observe.  Each time it orbits is a potentially detectable transit to be captured and studied.

White dwarf stars have also served as an earlier destination for those looking for information about potential insides of planets, but via a more indirect approach.  Because of their greatly heightened gravity, white dwarfs have surfaces covered only with light elements of helium and hydrogen. For years, researchers have found evidence that some white dwarf atmospheres are polluted with traces of heavier elements such as calcium, silicon, magnesium and iron. Scientists have long suspected that the source of this pollution has been asteroids or, what was then theoretical, a small planet being torn apart.

Steven Desch, an astrophysicist at ASU, sees a frequent gap between the work of astronomers and planetary scientists, and hopes to help bridge it.
Steve Desch, a theoretical astrophysicist at ASU, sees a frequent gap between in the exoplanet work of astronomers and of planetary scientists, and hopes to help bridge it. (ASU News)

Another prime target for disintegrating-planet research is the first one identified,  KIC 12557548 b.  Because it is so small — no bigger than Mercury — it’s an object that would never be detected by telescopes looking for transits across a star.  It is, after all, 1500 light years away.  But the dust cloud is much bigger and blocks as much as 1 percent of the light from the star every time it orbits.  To compare, our Jupiter would block about the same amount of the sun’s light in a similar scenario seen from afar.

The team leaders said that while their goal is to collect data that will help them understand the grain size and chemical composition of the dusty planetary remains, they also aim to refine the observing and spectrographic techniques for future observations — most especially on the James Webb Space Telescope.

The JWST, which launches in 2018, will have the capacity to collect information about the disintegrating planets that current instruments cannot.  But time on the telescope will be very costly and competitive, so Wright said the team will be doing the groundwork needed to make disintegrating planets an appealing subject for research.

“A lot of the observational technique has to be invented,” said Wright.  “JWST will be prime time for new science, but before that we need a lot of ground-based pre-study to make the case.”

The proposal also calls for extensive modeling of the dynamics of how dust grains would be released under the pressure of intense gravity and radiation pressure.

Coincidentally, a paper that models exoplanetary interiors authored by Li Zeng of the Harvard-Smithsonian Center for Astrophysics (CfA) and others, has been accepted for publication by The Astrophysical Journal.

Making sure it first could reproduce the Preliminary Reference Earth Model (PREM) — the standard model for Earth’s interior — Zeng and his team modified their planetary interior code to predict the structure of exoplanets with different masses and compositions, and applied it to six known rocky exoplanets with well-measured masses and radii.

They found that the other planets, despite their different masses and presumably different chemical makeup, nevertheless all appear to have a iron/nickel cores containing about 30% of the planet’s mass, very similar to the 32% of the Earth’s mass found in the Earth’s core. The remainder of each planet would be mantle and crust, just as with Earth.

The model, however, does not add new information about the observed make-up of exoplanet interiors.  That’s where the disintegration of close-in exoplanets just might come in.

In this Chandra image of ngc6388, researchers have found evidence that a white dwarf star may have ripped apart a planet as it came too close. When a star reaches its white dwarf stage, nearly all of the material from the star is packed inside a radius one hundredth that of the original star. Using several telescopes, including NASA’s Chandra X-ray Observatory, researchers have found evidence that a white dwarf star – the dense core of a star like the Sun that has run out of nuclear fuel – may have ripped apart a planet as it came too close. ( NASA)
In this Chandra image of globular cluster NGC 6388, researchers have found evidence that another white dwarf star may have ripped apart a planet as it came too close. When a star runs out of nuclear fuel and reaches its white dwarf stage, nearly all of its material from the star is packed inside a radius one hundredth that of the original star. The images was made with from images taken by several telescopes, including NASA’s Chandra X-ray Observatory. (NASA)
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