An unstable ‘luminous blue variable’ star, 2.5 million times brighter than the Sun in the dwarf galaxy Kinman, collapsed into a black hole without producing a supernova. Or was it partially obscured by dust, such as the infamous Tabby’s “alien megastructure star” detection discovered in 2016 by a group of astronomers at Penn State University who went viral as “the most mysterious star in the universe “
Revealed by ESO’s Very Large Telescope (VLT)
Between 2001 and 2011, several teams of astronomers who used the Very Large Telescope (VLT) of the European Southern Observatory studied the mysterious object. His observations indicated that he was at a late stage in his evolution. Andrew Allan of Trinity College Dublin and his collaborators in Ireland, Chile and the USA wanted to know more about how very massive stars end their lives, and the object on the Dwarf Kinman seemed like the perfect target.
However, when they targeted ESO’s VLT at the distant galaxy in 2019, the star’s telltale signatures had vanished. “Instead, we were surprised to discover that the star had disappeared.” says Allan, who led a study of the star published today in Monthly Notices of the Royal Astronomical Society.
Kinman’s Dwarf Galaxy
Located about 75 million light-years from Earth, the dwarf galaxy is too far away for astronomers to see its individual stars, but they can detect the signatures of some of them. From 2001 to 2011, the light from the galaxy constantly showed evidence that it harbored the unstable object with its occasional dramatic changes in its spectra and brightness. Even with those changes, the luminous blue variables leave specific footprints that scientists can identify, but they were missing from the data the team collected in 2019, leaving them wondering what happened to the star.
“It would be very unusual for such a massive star to disappear without producing a brilliant supernova explosion,” says Allan.
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The group first rotated the ESPRESSO instrument toward the star in August 2019, using the VLT’s four 8-meter telescopes simultaneously. But they could not find the signs that previously pointed to the presence of the luminous star. A few months later, the group tested the X-shooter instrument, also on ESO’s VLT, and again found no trace of the star.
Clues on old data
“We may have detected that one of the most massive stars in the local Universe goes smoothly into the night,” says team member physicist Joseph Groh, also from Trinity College Dublin. “Our discovery would not have been made without the use of the powerful 8-meter ESO telescopes and rapid access to those capabilities following Ireland’s recent agreement to join ESO” in September 2018.
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The team then turned to older data collected with X-shooter and the UVES instrument on ESO’s VLT, located in the Chilean Atacama Desert, and telescopes elsewhere. “The ESO Science Archive Center allowed us to find and use data from the same object obtained in 2002 and 2009,” says Andrea Mehner, an astronomer on ESO staff in Chile who participated in the study. “The comparison of the 2002 high-resolution UVES spectra with our observations obtained in 2019 with ESO’s new ESPRESSO high-resolution spectrograph was especially revealing, both from an astronomical and instrumentation point of view.”
From very large to extremely large telescope
Ancient data indicated that the star at Kinman Dwarf may have been experiencing a strong explosion period that probably ended sometime after 2011. Variable luminous blue stars like this are prone to experiencing giant explosions over the course of their lifetime, causing the stars’ mass loss rate to peak and their luminosity to increase dramatically.
Future studies are needed to confirm what fate happened to this star. Planned to start operating in 2025, ESO’s Extremely Large Telescope (ELT) will be able to resolve stars in distant galaxies like the Dwarf Kinman, helping to solve cosmic mysteries like this one.
The Daily Galaxy, Jake Burba, via ESO
