“We have speculated that the most likely explanation is that we observed a disk of gas-dust around another star,” Sergey Koposov from the University of Edinburgh told The Daily Galaxy about the detection of a giant blinking star –VVV-WIT-08– 100 times the size of the sun at the center of our Milky Way Galaxy that dims by 97% then slowly returns to former brightness. “That star itself is invisible, hidden by the disk. We think that the ‘star’ with that gas-dust disk is itself orbiting around the giant star with a period of tens of years.”
Mystery Object at Center of the Disk
“What kind of object is at the center of the disk is really not known at this point,” Koposov told The Daily Galaxy. “ It pretty much could be anything, from a completely dark object, like a black hole or a very faint object like white dwarf star to a quite massive star.”
“A further mystery is how the object came to have a disc around it,” astrophysicist Philip Lucas at the University of Hertfordshire wrote in an email to The Daily Galaxy. “ Sometimes matter can be transferred from a giant star to a smaller star, creating a disc around the little one. But here the disc is so far from the giant star that this is hard to explain. We hope to answer these questions by gathering more data, e.g. an X-ray observation has been approved with the NASA Swift satellite and over time we can learn about the mass of the companion and the size of the orbit through measurement of the “Doppler wobble”, method that is used to find planets.”
“It appeared to come out of nowhere,” said Dr Leigh Smith at Cambridge University’s Institute of Astronomy, on the sudden dimming of the star. It began to fade in early 2012 and almost vanished by April that year before recovering over the next 100 days.
Astronomers spotted the giant ‘blinking’ star towards the center of the Milky Way, more than 25,000 light years away. The star is decreasing in brightness by a factor of 30, so that it nearly disappeared from the sky. While many stars change in brightness because they pulsate or are eclipsed by another star in a binary system, it’s exceptionally rare for a star to become fainter over a period of several months and then brighten again.
New class of ‘blinking giant’ binary star system?
The researchers believe that VVV-WIT-08 may belong to a new class of ‘blinking giant’ binary star system, where a giant star 100 times larger than the Sun is eclipsed once every few decades by an as-yet unseen orbital companion. The companion, which may be another star or a planet, is surrounded by an opaque disc, which covers the giant star, causing it to disappear and reappear in the sky. The study is published in Monthly Notices of the Royal Astronomical Society.
The discovery was led by Leigh Smith working with scientists at the University of Edinburgh, the University of Hertfordshire, the University of Warsaw in Poland and Universidad Andres Bello in Chile.
“It’s amazing that we just observed a dark, large and elongated object pass between us and the distant star and we can only speculate what its origin is,” said co-author Koposov. “And the reason for the elongated shape is that we observe that disk at an angle,” he told The Daily Galaxy.
Some Unknown Dark Object?
Since the star is located in a dense region of the Milky Way, the researchers considered whether some unknown dark object could have simply drifted in front of the giant star by chance. However, simulations showed that there would have to be an implausibly large number of dark bodies floating around the Galaxy for this scenario to be likely.
One other star system of this sort has been known for a long time. The giant star Epsilon Aurigae is partly eclipsed by a huge disc of dust every 27 years, but only dims by about 50%. A second example, TYC 2505-672-1, was found a few years ago, and holds the current record for the eclipsing binary star system with the longest orbital period ? 69 years ? a record for which VVV-WIT-08 is currently a contender.
The UK-based team has also found two more of these peculiar giant stars in addition to VVV-WIT-08, suggesting that these may be a new class of ‘blinking giant’ stars for astronomers to investigate.
VVV-WIT-08 was found by the VISTA Variables in the Via Lactea survey (VVV), a project using the British-built VISTA telescope in Chile and operated by the European Southern Observatory, that has been observing the same one billion stars for nearly a decade to search for examples with varying brightness in the infrared part of the spectrum.
A ‘What-is-This?’ Object
“Occasionally we find variable stars that don’t fit into any established category, which we call ‘what-is-this?’, or ‘WIT’ objects,” said Philip Lucas. “We really don’t know how these blinking giants came to be. It’s exciting to see such discoveries from VVV after so many years planning and gathering the data.”
While VVV-WIT-08 was discovered using VVV data, the dimming of the star was also observed by the Optical Gravitational Lensing Experiment (OGLE), a long-running observation campaign run by the University of Warsaw. OGLE makes more frequent observations, but closer to the visible part of the spectrum. These frequent observations were key for modelling VVV-WIT-08, and they showed that the giant star dimmed by the same amount in both the visible and infrared light.
“There are certainly more to be found, but the challenge now is in figuring out what the hidden companions are, and how they came to be surrounded by discs, despite orbiting so far from the giant star,” said Smith referring to the half a dozen potential known star systems of this type, containing giant stars and large opaque discs.. “In doing so, we might learn something new about how these kinds of systems evolve.”
Avi Shporer, formerly a NASA Sagan Fellow at the Jet Propulsion Laboratory (JPL) currently with the MIT Kavli Institute for Astrophysics and Space Research via Cambridge University Institute of Astronomy, Philip Lucas and Sergy Koposov