ASTRONOMY y

Unusual Stars

Researchers puzzled by recent discoveries of stars with variable brightness

Published in December 2018

razilian astronomer Roberto Kal- WIT-01, which vanished after emitting a eight years, this star—which appears to busch Saito was searching for stars burst of light, possibly having suffered an be older than the Sun and has an un- that brighten at regular intervals— explosive death. However, VVV-WIT-07 known mass—suffered at least four major Bwhich helps scientists calculate behaved differently than the other six dips in luminosity of varying intensities galactic distances—when he encountered objects. Instead of increasing in bright- and durations. The longest reduction in an object behaving strangely that he has ness, this star experienced several dips in brightness occurred in 2012. been unable to explain. Described in an luminosity, as if a dense body was pass- In late May of that year, the star slowly article published in the journal Monthly ing in front of the star and preventing a began to lose brightness over a period Notices of the Royal Astronomical Soci- portion of its light from reaching Earth. of 48 days. In July, the luminosity of the ety in November, the enigmatic star was Saito and his colleagues noted that star abruptly decreased by 80% in an named VVV-WIT-07. WIT stands for these eclipses were highly irregular. The event that lasted 11 days. At the end of "what is this?" The number indicates that reductions in brightness were variable, the eclipse, the star returned to its pre- the star is the seventh object to display and the frequency of the reductions was vious brightness within just a few days. this unexpected behavior from the ap- inconsistent. Stellar eclipses, caused "VVV-WIT-07 does not fit into any exist- proximately 800 million that have been when the light from a star is obscured by ing category of variable star," says Saito. observed between 2010 and 2018 by the a planet or a smaller star, usually occur He is still unable to offer a detailed ex- Vista telescope in Chile, which is creating at regular intervals and can be mapped planation for the eclipses. The variations a three-dimensional, high-resolution map on light curve graphs, which show how could be caused by remnants of a newly of the center of the Milky Way. the brightness varies over time. Each formed planetary system, clouds of dust Of the seven objects detected by Vista eclipse causes a decrease in the amount released by a low-mass star, or fragments whose brightness fluctuates abnormally, of light reaching Earth, followed by an of a planet that suffered a recent colli- six showed an initial increase in light increase in brightness, always with the sion. "We do not know what we're see- emission that then faded. This behavior same duration and intensity. "When a ing," he says. led the astronomer of the Federal Uni- spherical body like a planet passes in To date, only two other stars have ex- versity of Santa Catarina (UFSC) and his front of a star, it generates a U-shaped hibited similar behavior. The first star colleagues to speculate that these ob- graph," explains Saito. was described in 2012 by American as- jects are stars that have experienced sur- This effect was not the case for VVV- tronomer Eric Mamajek, a professor face eruptions. The exception was VVV- WIT-07. Observed 85 times in the last at the University of Rochester, USA.

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Artistic representations of Tabby's Star, orbited by a thick ring of dust that would explain the irregular eclipses of the star (left), and VVV-WIT-07 (above, in the center of the image)

Working in Chile at the time, Mamajek 3,000 amateur astronomers manually "Although many stars vary in bright- analyzed data from the SuperWASP, a reviewed the light curves produced by ness, I do not know of any others that telescope array that can detect extraso- Kepler. One curve in particular caught vary in the way these three stars do," says lar planets, and identified the irregular her attention. A star similar to the Sun, Brazilian astronomer Márcio Catelan, a eclipse of a star with a mass similar to located in the Cygnus constellation 1,470 professor at the Pontifical Catholic Uni- that of the Sun but much younger than light-years from Earth, had experienced versity of Chile and coauthor of the pa- the Sun: Swasp J140747.93-394542.6, 10 occultations. per on VVV-WIT-07. "I believe these are or simply J1407. At 16 million years old The eclipses of this star—called KIC just the first of a new class of variable and 420 light-years from Earth, the star 8462852 and nicknamed Tabby's Star— objects, which could be called Tabby- showed an unusual dip in brightness also varied in duration and reduced the Bobby-Mamajek stars." Other objects are that lasted several days in early 2007, brightness of the star by anywhere from expected to be found in surveys planned culminating in April with a 95% reduc- 0.5% to 22%. If the most significant of for the next decade. tion. In an article describing the event these events was caused by a planet, the For now, researchers do not know how in The Astronomical Journal, Mamajek planet would have to be a thousand times different VVV-WIT-07 is from the other and his colleagues suggested that the larger than Earth, said Boyajian, now two stars. "There is still very little informa- event may have been an eclipse caused a professor at Louisiana State Univer- tion on VVV-WIT-07," Boyajian told Pesqui- . 2019 as by a planet with rings much larger and sity, USA, in a TED lecture in 2016. In sa FAPESP via email. "It seems to be hot-

mnr denser than those of Saturn. the article describing the occultations, ter and larger in mass than the other two Three years later, American astrono- published in the Monthly Notices of the cases, but we do not yet know if it is young-

. K. et al. mer Tabetha Boyajian reported another Royal Astronomical Society in 2015, the er, older, or the same age," said Mamajek, , r to i unusual star. During her postdoctoral authors presented a number of possible also via email. n Ricardo Zorzetto sa fellowship at Yale University, USA, she explanations for the phenomenon. The ech 2 t had access to four years of data from NA- most plausible, the authors said, is that l

a Scientific article

l-c SA's Kepler satellite, which was search- the light from Tabby's Star was blocked p SAITO, R. K. et al. VVV-WIT-07: Another Boyajian’s star or ing for Earth-like planets in 150,000 star by a swarm of hundreds or thousands a Mamajek’s object? Monthly Notices of the Royal Astro- nasa / j systems. While the data were being auto- of comets or fragments released by the nomical Society. v. 482, i. 4, p. 5000–09. Feb. 1, 2019. s 1 matically analyzed, Boyajian, research- rupture of a rocky body at least 100 ki- Other cited articles are listed in the online version of this article. photo ers from the US and Europe and almost lometers in diameter.

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