Forget about alien mega-structures: new observations explain the behavior of the star Tabbi with dust alone

Original author: Ethan Siegel
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The artistic image of KIC 8462852, whose brightness over the past few years has changed in an unusual way. The

scientific hunt for the planets in the 21st century has really unfolded, and the transit method is in the first place . When the planet passes in front of its parent star, if we look from our point of view, the part of the star’s light disappears for a while. Such transits provide planetary hunters with a fruitful method of searching for worlds in orbits alongside other stars. Today we already know thousands of stars with their own planets, and most of them have been discovered by the transit method.

When developing a mission that focuses on the discovery of planets, we can expect that with its help we will discover many strange things. However, astronomers were not at all ready to detect such a curiosity as the star Tabbi , whose luminosity is muffled very strongly and completely irregularly. After several years of speculation on this topic, during which versions from comet storms to alien megastructures were expressed, scientists finally solved this riddle . The culprit was assigned dust, although it behaves in a somewhat unusual way.

On the left - infrared, on the right - the ultraviolet radiation of the star Tabbi. No evidence of a multitude of explanations for luminosity failures for “natural” reasons.

For several years, the Kepler mission has managed to explore more than 100,000 stars. Of the hundreds of thousands of stars alone has stood out for its interesting behavior. KIC 8462852, known as Tabbi's star, or Boyadzyan's star (in honor of the discoverer of this behavior), or as WTF? (where is the flux? [The wordplay is the same abbreviation for where is the flux, “where the glow is,” and what the fuck is “what the fuck” / comment perev.]) has a completely unique combination of properties. She is at the same time:

  • demonstrates a serious drop in luminosity, up to 22% (for most stars on the planet, their luminosity decreases by no more than 1%),
  • tarnishes slowly on periods of several decades, but sometimes it becomes brighter (which no other similar stars do),
  • the overall brightness fluctuates during diminution of luminosity (and not just smoothly decreases, and then increases, as is the case with planets),
  • does not give infrared radiation (which is shown by all other stars with a serious drop in luminosity).

This gave rise to a complex puzzle.

Already received a large number of images of protoplanetary systems. The SPHERE infrared telescope is considered a work of art in this area, regularly issuing images with a resolution of about 10 ", or less than 0.003 degrees per pixel. KIC 8462852 does not have the usual properties or infrared radiation.

It could not be a planet, because the planet cannot be so large to block so many stars of light. Even if you imagine a planet with a huge system of rings, such as giant Saturn, the dips in luminosity will be periodic and smooth, with a certain plateau on the graph. But it is contrary to the typed data.

An artistic depiction of the ring system of a young giant planet or brown dwarf J1407b orbiting another star. Worlds with extremely strongly developed rings can lead to a strong decrease in the visible light of a star, but such reductions should be periodic, which was not observed.

It could be a very young star with planetesimals , a protoplanetary disk and an extremely dusty environment. All the stars we have seen with a strong drop in luminosity fall into this category.

But Tabbi's star is too old to have a protoplanetary disk: it is many hundreds of millions of years old. And most importantly, it does not give infrared radiation, which should be a star with a protoplanetary disk. Therefore, this star was originally called WTF, "where is the glow."

Artistic image of a young star, surrounded by a protoplanetary disk. Many of the properties of protoplanetary disks around sun-like stars are unknown, but all of them have characteristic infrared radiation. Tabby has no such thing.

This could be several successive events related to the flight of comets, during which they emit a large amount of dust, falling into the interior of the star system. Relatively recently, it has been shown how this can explain the observed luminosity dips.

Illustration of a comet storm around a star close to us, This Crow . The comets version is one of the explanations for reducing the luminosity of the Tabbi star, which was also rejected after obtaining an astronomical high-quality spectrum.

But there is another phenomenon that does not explain the proposed solution: the long-term decrease in the luminosity of a star. A star is called a “Tabbi star” not because this particular scientist discovered it, but because it began a scientific investigation of interesting and important behavior that has not been seen before.

But this star has been known for more than a century, and we are seeing a long-term decrease in luminosity, which this model cannot explain. Planet dust blows away in a few months; in order to block the star's light for a hundred years, it would have been necessary to bombard it with comets almost continuously. It would have required the presence of many comets in similar orbits, and we do not imagine how to obtain such a configuration.

Harvard light curve for the star KIC 8462852, and a comparison with two other stars whose apparent brightness has not changed.

What possible explanations were left? One of the popular ideas was that around this star, some technologically advanced civilization builds a megastructure, which periodically (or aperiodically) covers most of the star's light. The structure eventually comes to an end, and blocks more and more light. The fact that over the past hundred years the light from the star has faded so badly could be explained by the progress of construction.

The idea is intriguing, albeit non-standard.

Partially obscure the light of the star could be an unfinished alien megastructure, which the Gaia space telescope could in principle detect. However, in the vicinity of KIC 8462852 it is not at all this - this option rejects the study of the spectrum.

But, thanks to the huge amount of follow-up observations, we know that this is not the case. The reason is that the megastructure of aliens would be completely opaque to light: he could not pass through it. How could not pass through planets, moons, or any other solid objects.

Of the 19,000 images obtained in the last three years, made in four wavelength ranges, from blue to infrared light, we learned that in all cases of dimming of the glow, blue light is first of all blocked: from short-term recessions to long-term fading. And only one thing can block blue light while passing red: particles of a certain size of dust.

On the left - a photo in visible light, on the right - in infrared; the image shows a dust-rich globuleBarnard 68. Infrared light is blocked less, because small particles of dust cannot interact with light with a long wavelength.

So it must be dust. Whatever the reason for reducing the amount of transmitted light, as well as long-term tarnishing, dust must form the basis of this phenomenon. The dips of luminosity, obtained by Kepler, and the “secular dimming” are caused by the same phenomenon. As stated in the new job :

This color attenuation indicates the presence of dust particles up to ~ 0.1 microns in size. Such dust should have been blown off by the star's radiation pressure rather quickly, therefore dust clouds should have formed within a few months. Modern infrared observations were made with a dust coating of 12.4% ± 1.3%, and they correspond to the muting of the light by the star-colored dust.

Here is what the evidence points to: dust. But still the situation remains somewhat mysterious.

An illustration of a complex, dust-filled region around a star that has been superimposed with recent data from Tabota Boyajyan, demonstrating recent dips in the glow. Dust can not be on the surface of the star as shown in the picture.

As a result, the star Tabbi combines features, the simultaneous presence of which we could not expect:

  • The picture coincides with that which should be in the presence of a large amount of circumstellar dust, which usually speaks of the extreme youth of a star that is at the stages of formation.
  • The star itself is brighter, hotter and more massive than the sun: it emits four times more light than the sun.
  • The star is old: it is hundreds of millions of years old, and in all respects it stably burns, being in the main sequence.

In other words, the dust visible by us should remain only a few months, taking into account the properties of the star. So, the star has some way to replenish dust. As far as we know, there are two sensible possibilities: either the star has an outer dust ring with dense dust clouds or is constantly bombarded by asteroids, or the star has some external object blocking its light.

Currently, the leading idea is the existence of a disk of dust and fragments around the star. Then an incredible coincidence was to occur, in which its plane would ideally coincide with our line of sight so perfectly - this would be a remarkable, but unlikely event. Even with a probability of 1%, it would be strange that we did not observe this in the remaining 99% of stars.

The decrease in the apparent brightness of the star since the 1890s, apparently, continues today, in 2018, but it goes unevenly. In addition, there are constantly periods of temporary decrease in luminosity, lasting for months, as well as shorter dips, lasting no more than a day. This is definitely due to the presence of dust particles, probably about 100 nm in size. The percentage of blocked light at different wavelengths confirms this and rejects other hypotheses.

But where does this dust come from? To reduce the number of options, scientists have calculated the amounts of dust needed to explain the gradual dimming of the visible light of a star over the past 100 years, as well as for short-term dips. To block, based on the intersection of the dust plane of our line of sight, you need a mass of dust equivalent to the moon.

Initially, to explain what was happening with Tabby's star, a scenario with fragments of a comet was considered. But instead, several comet-like objects with long periods of revolution and massive dust halos can lead to these temporary transitive luminosity drops, but this would require a huge mass of matter that is not in the form of opaque objects.

But that's not all. Previous studiesassumed that in space there may be a large amount of interstellar dust, which is confirmed by the data.

This theory can replace or supplement the variant with circumstellar dust. The disk material around the star is the minimum necessary thing to explain. A huge amount of dust may not be in the plane that we observe, but outside of it, possibly forming a halo. We do not know this, but we know that if it exists, then it should not be too close to the star, otherwise it would radiate in the infrared. Comets must also emit infrared radiation; James Webb's telescope is likely to be able to find out if comets are responsible for temporal reductions in the luminosity of a star.

A disk of dust and debris that revolves around either the star itself or around its planets should emit infrared radiation — but this is not observed. However, our observations can be explained by the presence of a disk (or halo) located further from the star.

Finally, there is another theory candidate for explaining observations: dust can be the result of stellar "indigestion."

If this star tried to digest the planet, a gas giant of size, say, with Uranus, this could explain everything. A long-term fall on a star in a spiral of a planet or several planetary bodies, which occurred, perhaps hundreds or thousands of years ago, could have led to a temporary increase in brightness, from which the star returns to its normal, stable state. And the observed dips in luminosity can be explained by planetary fragments of previous cataclysms, or by evaporation and emission of gas from smaller bodies.

Artistic image HD 189733 b, a hot Jupiter, so close to his star that its atmosphere boils and breaks out into space. If KIC 8462852 had recently swallowed the gas giant, it could, in principle, "burp" dust particles, which could cause the observed muting of the light.

Regardless of what mechanism works here, we can be sure of one thing: the reason for the tarnishing of the star Tabby lies in the dust. This is a normal particulate dust containing particles up to 100 nm in size, or less than the wavelength of visible light. The same dust, which causes short, day-long or less drops in brightness, also causes tarnishing that lasts for months, and has led to a general fading of the star over the past hundred years. All this is due to normal, normal dust.

The following question remains the big and open question: where did this dust come from? It is not there because this star is young or is still forming, but the existence of an invisible companion of the star is seriously constrained. It cannot fully be interstellar. Was some planet digested by a star? Is there something else out of the ordinary? The only way to find out is to conduct more scientific research, and better quality. One thing is clear: even if somewhere there are alien megastructures, they do not exist there.

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