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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