Mysterious darkening of a star – Wissenschaft.de

Okkultation

Stars that change their brightness cyclically are not uncommon in the universe. However, astronomers have discovered a star that does not fit into any of the general classes of variables. The star VVV-WIT-08, about 25,000 light-years away, shone continuously for years without any fluctuations, but then lost 97 percent of its brightness for about six months—only to continue to shine as if some It didn’t happen either. Although this behavior indicates a spy by a partner, no conceivable object and scenario can explain all the characteristics of this darkness.

Even the ancient Egyptians noticed that the brightness of some stars fluctuated regularly. However, the reason for this beating is very different. In some cases it is a question of double stars whose companions alternately overlap each other or whose gravitational influence causes the companion’s brightness to fluctuate. But there are also internal processes that regularly make a star brighter and darker. In such Cepheids, changes in temperature and brightness occur as they alternate between phases of more rapid nuclear fusion and swelling. The changes in the brightness of these stars are so regular that astronomers can use them as cosmic rangefinders.

Overall, variable stars are therefore important tools for gaining deeper insights into the physics and evolution of stars, but also into the dynamics of the universe as a whole. This is one reason why astronomers are constantly on the lookout for new variable stars. One of the projects for this is the Vista Variable in Via Lactea Survey (VVV). For nearly a decade, researchers have been regularly mapping the nearly one billion stars in the Milky Way to look for changes in brightness in the infrared range. For this they use the European Southern Observatory ESO’s VISTA telescope in Chile. “In doing so, we sometimes come across variable stars that don’t fit any established category. Then we call it WIT for ‘what is it,'” explains co-author Philip Lucas of the University of Hertfordshire Huh.

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disappeared for almost 200 days

A team led by Lucas and first author Leigh Smith from the University of Cambridge has now discovered a particularly unusual “WIT” star some 25,000 light-years away from us in one of the star-dense regions near the center of the Milky Way. The VVV-WIT-08 star has almost the mass of the Sun, but it is about 100 times larger than that. “Its spectrum suggests that it is a cool giant with an effective temperature of about 3600 Kelvin,” explain the astronomers. So far, so normal. But in the spring of 2012, the previously stable and uniformly bright star suddenly changed: It lost 97 percent of its brightness. “This single, smooth and nearly symmetrical blackout event lasted about 200 days,” Leigh and his team said. The star again increased in brightness and continues to shine with its original brightness without any further fluctuations.

According to the researchers, this long period of darkness and the apparently long interval between such events is particularly unusual – it has happened only once in 17 years of observation. So far, astronomers know of only two stars that show a return period of eclipses for such a long time. These include the star Epsilon Aurigae, which is covered by its companion’s dust disk every 27 years, and the red giant TYC 2505-672-1, which also has 69 years between events. But none of these stars shows such a great loss of brightness, which affects all wave ranges of visible and infrared light equally, as Leigh and colleagues explained. This raises the question as to what could be the reason for this darkening of the VVV-WIT-08 star. From their observations, the astronomers have concluded that the reason is probably not inside the star: “The behavior of VVV-WIT-08’s light curve is not consistent with any known stellar variability,” say the researchers. The symmetry of the light curve and the otherwise fluctuation-free brightness of the star do not correspond to that of a red giant or young star.

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Astronomers therefore suspect there is an enigma – the star being obscured by an object passing in front of it. Explain Smith and his colleagues, “the occult object must have a few key features: it must be gravitationally bound to the giant star, must be very faint, must have a radius greater than 50 Earth’s and appear elliptical.” You have checked all the possible candidates with the help of a model. At first glance, the most likely detectable would be a companion star surrounded by a large disk of dust in a wide orbit. However, none of the normal circumstellar disks match the characteristics of the light curve, as astronomers report. The disk around young stars lasts for a very short time and leaves behind a clearly recognizable infrared signature. A disk of debris around an old star would not be dense and opaque enough, while around a white dwarf would not be large enough to explain a long and nearly total eclipse.

A companion star that ejects material from the massive star and thus forms an expanding gas disk would also be conceivable. “Mass transfer from the giant star to a companion’s gravitational field may explain the presence of an obscure disk,” explain Smith and his team. “This scenario is fascinating because it also provides a convenient explanation for why the star is smaller than expected.” However, the companion’s orbit would then have to be huge. Then it would be too far from the star to suck up the material. However, it is conceivable that the companion is an object with a particularly high gravity, such as a neutron star or black hole. Their accretion discs typically emit X-rays, but there are exceptions, as the researchers point out.

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According to astronomers, no single scenario can fully explain why the giant star VVV-WIT-08 was so long and completely hidden. “The duration, depth and achromaticity of the blackout make this phenomenon extraordinarily unusual – its mystery has yet to be revealed,” Smith and colleagues said. But he is hopeful that more such incidents will come to the fore in the future. They may have already tracked down two candidates as part of the Vista survey.

Quayle: Leigh Smith (University of Cambridge, UK) et al., Monthly Notices of the Royal Astronomical Society, doi: 10.1093/manras/chhura 1211

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