Debate Over Strange Star KIC 846852 Continues –“Comet Swarm or Alien Artifact?” (A ‘Galaxy’ Update)





It has been described as “the most mysterious star in the universe” by Yale astronomer Tabetha Boyajian who first described it in a TED talk she gave last February. “The dips found by Kepler are real. Something seems to be transiting in front of this star and we still have no idea what it is!” says German astronomer Michael Hippke.

The Kepler satellite was designed to search for Earth-sized planets in the habitable zone of stars by measuring dips in a star’s brightness as orbiting planets move across the stellar disc (transits). Its sensitive camera stares at more than 150,000 stars towards the constellations of Cygnus and Lyrae, and so far has found over 5000 exoplanet candidates. But Kepler also monitors the light fluctuations in all the other stars, even dips not caused by transits, and has found some bizarre situations.


Perhaps the strangest is the case of KIC 846852, an otherwise normal star slightly larger than the Sun that has exhibited significant, irregular dips in the flux that last as short as a few days or as long as eighty days, and are as deep as 20%. The source is so far unique in the Kepler database. The irregular and extreme nature of the episodes excludes planetary transits, and other suggestions have ranged from a catastrophic collision between planets that released a cloud of obscuring debris, to the presence of a huge alien artifact like a so-called “Dyson sphere!”

The graphic below shows a portion of a curve plotting light flux versus time (in days) for the strange star KIC 846852. The dips are irregular and therefore not due to orbiting planetary transits; the actual cause is not well understood. A new measurement of the dust in this system concludes there is too little to satisfy most suggested explanations, but is possibly consistent with the breakup of a cluster of Halley-like comets.




CfA astronomers Mike Dunham, Glen Petitpas, and Lars Kristensen, and their colleagues, realized that if a cloud of dust particles were present in the stellar system, it should be detectable at submillimeter and millimeter wavelengths because of its warm temperature. They used the Submillimeter Array and the James Clerk Maxwell Telescope to search for any such dust.

They found no signs of it.

They can therefore limit the amount of material to less than about one tenth of the Moon’s mass (at least in the regions mostly likely to host dust) and fewer than about eight Earth-masses throughout the entire stellar system. According to the scientists, such small amounts of dust make the catastrophic planetary collision scenario very unlikely, but might be consistent with the picture of the complete breakup of a cluster of about thirty Halley-like comets.




The cause of such a dramatic event, however, is not understood, and meanwhile other imaginable scenarios are still allowable, but the new results put a firm limit on the amount of dusty material around this strange and unique star.

Let’s take a deeper look at the events that preceded the Harvard CfA study. Last week we reported that the new Harvard study makes it far less likely that KIC 8462852, popularly known as Tabby’s star, is the home of industrious aliens who are gradually enclosing it in a vast shell called a Dyson sphere. Media interest went viral last October when a group of astronomers from Pennsylvania State University released a preprint that cited KIC 8462852’s “bizarre light curve” as “consistent with” a swarm of alien-constructed megastructures.

Public interest in the star, which sits about 1,480 light-years away in the constellation Cygnus, began last fall(“Tabby”) Boyajian and colleagues posted a paper on an astronomy preprint server reporting that “planet hunters” – a citizen science group formed to search data from the Kepler space telescope for evidence of exoplanets – had found unusual fluctuations in the light coming from the otherwise ordinary F-type star (slightly larger and hotter than the sun).

The most remarkable of these fluctuations consisted of dozens of uneven, unnatural-looking dips that appeared over a 100-day period indicating that a large number of irregularly shaped objects had passed across the face of the star and temporarily blocked some of the light coming from it.

The top panel of the graphic below shows four years of Kepler observations of the 12th-magnitude star KIC 8462852 in Cygnus. Several sporadic dips in its light output (normalized to 100%) hint that something is partially blocking its light. The portion highlighted in yellow, recorded in February to April 2013, is shown at three different scales along the bottom. The random, irregular shape of each dip could not be caused by a transiting exoplanet. (T. Boyajian & others / MNRAS).




The attention caused scientists at the SETI Institute to train its Alien Telescope Array on the star to see if they could detect any radio signals indicating the presence of an alien civilization. In November it reported finding “no such evidence” of signals with an artificial origin.

Then a study released in January by a Louisiana State University astronomer threw even more fuel on the fire of alien speculation by announcing that the brightness of Tabby’s star had dimmed by 20 percent over the last century: a finding particularly difficult to explain by natural means but consistent with the idea that aliens were gradually converting the material in the star’s planetary system into giant megastructures that have been absorbing increasing amounts of energy from the star for more than a century. That study has now been accepted for publication in the peer reviewed Astrophysical Journal.

However, a new study – also accepted for publication in the Astrophysical Journal – has taken a detailed look at the observations on which the LSU study was based and concluded there is no credible evidence that the brightness of the star been steadily changing over this period.

When the LSU study was posted on the physics preprint server ArXiv, it caught the attention of Vanderbilt doctoral student Michael Lund because it was based on data from a unique resource: Digital Access to a Sky Century @ Harvard. DASCH consists of more than 500,000 photographic glass plates taken by Harvard astronomers between 1885 and 1993, which the university is digitizing. Lund was concerned that the apparent 100-year dimming of Tabby’s star might just be the result of observations having been made by a number of different telescopes and cameras that were used during the past century.

Lund convinced his advisor, Professor of Physics and Astronomy Keivan Stassun, and a frequent collaborator, Lehigh University astronomer Joshua Pepper, that the question was worth pursuing. After they began the study, the Vanderbilt/Lehigh group discovered that another team – German amateur astronomer Michael Hippke and NASA Postdoctoral Fellow Daniel Angerhausen – were conducting research along similar lines. So the two teams decided to collaborate on the analysis, which they wrote up and submitted to the Astrophysical Journal.

“Whenever you are doing archival research that combines information from a number of different sources, there are bound to be data precision limits that you must take into account,” said Stassun. “In this case, we looked at variations in the brightness of a number of comparable stars in the DASCH database and found that many of them experienced a similar drop in intensity in the 1960’s. That indicates the drops were caused by changes in the instrumentation not by changes in the stars’ brightness.”

The planet hunters first detected something unusual in the star’s light curve in 2009. They found a 1 percent dip that lasted a week. This is comparable to the signal that would be produced by a Jupiter-sized planet passing in front of the star. But planets produce symmetric dips and the one they found was decidedly asymmetric, like something that would be produced by an irregular-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.

“The Kepler data contains other cases of irregular dips like these, but never in a swarm like this,” said Stassun.

Boyajian and her colleagues considered a number of possible explanations, including variations in the star’s output, the aftermath of an Earth/Moon type planetary collision, interstellar clumps of dust passing between the star and earth, and some kind of disruption by the star’s apparent dwarf companion. However, none of their scenarios could explain all of the observations. Their best explanation was a giant comet that fragmented into a cascade of thousands of smaller comets. (This hypothesis took a hit when the LSU study was announced because it could not explain a century-long dimming.)

The Kepler telescope is no longer collecting data in the Cygnus region, but Hippke reports that the mystery has captured the imagination of amateur astronomers around the world so thousands of them are pointing their telescopes at Tabby’s star, snapping images and sending them to the American Association of Variable Star Observers in hopes of detecting further dips that will shed new light on this celestial mystery.

Reference(s): “Constraints on the Circumstellar Dust around KIC 8462852,” M. A. Thompson, P. Scicluna, F. Kemper, J. E. Geach, M. M. Dunham, O. Morata, S. Ertel, P. T. P. Ho, J. Dempsey, I. Coulson, G. Petitpas and L. E. Kristensen, MNRAS 458, L39.

The Daily Galaxy via and Vanderbilt University


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