Stars that change their brightness cyclically are not unusual in the cosmos. However, astronomers have discovered a star that does not seem to fit any of the common classes of variables. The star VVV-WIT-08, about 25,000 light years away, shone steadily for years without fluctuations, but then lost 97 percent of its luminosity for about six months – only to continue to shine as if nothing had happened. Although this behavior indicates an occultation by a companion, none of the conceivable objects and scenarios can explain all the features of this darkening.
Even the ancient Egyptians observed that the brightness of some stars fluctuated regularly. The cause of this pulsation, however, is very different. In some cases it is a question of double stars whose partners alternately cover each other or whose gravity influence causes the partner’s luminosity to fluctuate. But there are also internal processes that regularly make a star brighter and darker. In such Cepheids, temperature and brightness change because they alternate between phases of more intense nuclear fusion and bloating. The changes in 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 cosmos as a whole. This is one of the reasons why astronomers are constantly on the lookout for new variable stars. One of the projects for this is the VISTA Variables in the Via Lactea survey (VVV). For almost a decade, researchers have regularly been mapping around a billion stars in the Milky Way for changes in brightness in the infrared range. For this they use the VISTA telescope of the European Southern Observatory ESO in Chile. “In doing so, we sometimes come across variable stars that don’t fit into any established category. We then call it WIT for ‘What is This’, ”explains co-author Philip Lucas from the University of Hertfordshire.
Almost disappeared for 200 days
The team led by Lucas and first author Leigh Smith from the University of Cambridge has now discovered a particularly unusual “WIT” star around 25,000 light-years away from us in one of the star-dense areas near the center of the Milky Way. The star VVV-WIT-08 has about the mass of the sun, but is around 100 times larger than it. “Its spectrum suggests that it is a rather cool giant with an effective temperature of around 3600 Kelvin,” explain the astronomers. So far, so normal. But in the spring of 2012, the previously stable and evenly shining star suddenly changed: It lost 97 percent of its luminosity. “This single, smooth and almost symmetrical blackout event lasted around 200 days,” said Leigh and his team. The star then increased in brightness and has shone with its original luminosity without any further fluctuations.
According to the researchers, the long duration of this darkening and the obviously long interval between such events are unusual – it only occurred once in the 17 years of observation. So far, astronomers only know of two stars that show return periods of eclipses for such a long time. 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. But none of these stars shows such a drastic loss of brightness, which also affects all wave ranges of visible and infrared light equally, as Leigh and his colleagues explain. This raises the question of what could be the cause of this darkening of star VVV-WIT-08. From their observations, the astronomers conclude that the reason is probably not inside the star: “The behavior of the light curve of VVV-WIT-08 does not correspond to any known stellar variability,” the researchers say. The symmetry of the light curve and the otherwise fluctuation-free glow of the star do not suit either a red giant or a young star.
Who is the Occultator?
The astronomers therefore suspect that there is an occultation – the obscuration of the star by an object passing in front of it. “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. You have checked all possible candidates with the help of a model. At first glance, the most likely occultator would be a companion star surrounded by a large disk of dust in a wide orbit. However, none of the common circumstellar disks match the characteristics of the light curve, as the astronomers report. The disks around young stars are too short-lived and also leave behind a clearly recognizable infrared signature. 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.
A companion star that extracts material from the giant star and thereby forms an extended gas disk would also be conceivable. “The mass transfer from the giant star to the gravity area of a companion could explain the presence of an obscuring disk,” explain Smith and his team. “This scenario is tempting because it also provides a convenient explanation of why the star is smaller than expected.” However, the companion’s orbit would then have to 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.
According to astronomers, none of the scenarios can fully explain why the giant star VVV-WIT-08 was obscured for 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: Leigh Smith (University of Cambridge, UK) et al., Monthly Notices of the Royal Astronomical Society, doi: 10.1093 / mnras / stab1211