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Astronomers discover a star whose behavior contradicts all current models

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Mysterious phenomenon: astronomers have discovered a star whose behavior contradicts all current models. The giant star VVV-WIT-08 appeared steadily for decades, then it suddenly lost 97 percent of its luminosity and remained dark for a good six months before it appeared again. 

This behavior cannot be explained by the obscuration of a companion star – and all other theories do not fit either, as the researchers explain. What is going on there remains inexplicable for the time being.

Stars that change their brightness cyclically are not unusual in the cosmos. Often these are double stars whose partners take turns covering up. But there are also internal processes that make a star blink regularly. 

In such Cepheids, the temperature and brightness change because they alternate between phases of more intense nuclear fusion and bloating. Even stars that only pulsate on one side have already been observed.

“What is this?”

But now astronomers working with Dr. Leigh Smith from the University of Cambridge have discovered a star that does not fit into any of the previously known classes of variables. The mysterious object was tracked down as part of a sky survey with the VISTA telescope from ESO in Chile, which has been monitoring around a billion stars for changes in brightness in the infrared range for almost a decade.

“In doing so, we might learn something new about how these kinds of systems evolve,” explains co-author Philip Lucas from the University of Hertfordshire. 

One of these WIT stars is now the giant star VVV-WIT-08, some 25,000 light-years away from us. Although it has the mass of the sun, it is around 100 times larger than it. 

“Its spectrum suggests that it is a rather cool giant with an effective temperature of around 3,600 Kelvin,” said the team.

Darkened for 200 days

The strange thing, however: after this giant star showed no changes for many years, it suddenly darkened in spring 2012 and lost 97 percent of its luminosity. 

“This single, smooth and almost symmetrical darkening event lasted around 200 days,” report the astronomers. 

The star then increased in brightness again and has been shining with its original luminosity without any further fluctuations.

Such dimming duration and intensity is extremely unusual and rare. Because telescope observations only go back 17 years, it is not even clear whether it is a one-off or a regular event. 

So far, astronomers only know two stars that show extremely long return periods of eclipses. These include the star Epsilon Aurigae, which is covered by the dust disc of its companion every 27 years, and the red giant TYC 2505-672-1, which has even 69 years between the events.

No internal cause can be determined

But VVV-WIT-08 is different: none of the known variables shows such a drastic loss of brightness, which also affects all wave ranges of visible and infrared light equally. Corresponding data was provided to the team by images from the OGLE telescope at the University of Warsaw, which had also observed the darkening of this star. In order to get to the bottom of the phenomenon, Smith and his team systematically checked all possible causes – without finding a clear explanation.

It seems clear that the cause is probably not inside the star:

“The behavior of the light curve of VVV-WIT-08 does not correspond to any known stellar variability,” write the astronomers. 

The symmetry of the light curve does not match the fluctuations of a red giant and for a young star, the object outside of this one event shines too evenly and stably.

Dense disk in front of the star

The darkening must therefore come from outside – through a dark, large object that has covered the star. But what one? 

“The occult object must have a few key features: it must be bound to the giant star by gravity, be very faint, have a radius greater than 50 Earth radii and appear elliptical,” explain Smith and his colleagues. They have checked all possible candidates with the help of a model.

The most likely occultator would be a companion star surrounded by a dense disk of dust. However, the typical features of such circumstellar disks do not match the observations: the disks around young stars are too short-lived and also leave behind a clearly recognizable infrared signature, as the team explains. A disk of debris around an older star would not be dense and opaque enough, while that around a white dwarf would not be large enough to explain the long and almost complete eclipse.

Too far away and too dark

Another possibility would be a companion star that sucks material from the giant star and thereby forms an extended gas disk. 

“The mass transfer from the giant star to the gravity area of ​​a companion could explain the presence of an obscuring disk,” said Smith and his team. 

“This scenario is tempting because it also provides a convenient explanation of why the star is smaller than expected.”

The problem, however, is that if the occultation was a one-time event, but part of a long-period cycle, then the companion’s orbit must be huge. It would then be too far away to suck up material from the star. It would be conceivable, however, that the companion is an object with particularly high gravity, such as a neutron star or a black hole. Their accretion disks usually emit X-rays, but there are exceptions, as the researchers explain.

“The secret has not yet been revealed”

The conclusion of the astronomers: none of the scenarios can fully explain what the giant star VVV-WIT-08 was covered by so long and completely. 

“The duration, depth, and achromaticity of the blackout make this event extraordinarily unusual – its secret has not yet been revealed,” said Smith and his colleagues. But they hope to discover more cases of such events in the future. They may have already tracked down two candidates as part of the VISTA survey.

Source: University of Cambridge

Photo by Amanda Smith / University of Cambridge

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