Did an unstable ‘luminous blue variable’ star some 2.5 million times brighter than the Sun in the Kinman Dwarf Galaxy collapse into a black hole without producing a supernova. Or, did it become become partially obscured by dust, like the detection of the infamous Tabby’s “alien megastructure star” discovered in 2016 by a group of astronomers at Penn State University that went viral as “the most mysterious star in the universe.”
Revealed by ESO’s Very Large Telescope (VLT)
Between 2001 and 2011, various teams of astronomers using the European Southern Observatory’s Very Large Telescope (VLT) studied the mysterious object. Their observations indicated it was in a late stage of its evolution. Andrew Allan of Trinity College Dublin and his collaborators in Ireland, Chile and the US wanted to find out more about how very massive stars end their lives, and the object in the Kinman Dwarf seemed like the perfect target.
However when they aimed ESO’s VLT at the distant galaxy in 2019, the telltale signatures of the star had vanished. “Instead, we were surprised to find out that the star had disappeared!” says Allan, who led a study of the star published today in Monthly Notices of the Royal Astronomical Society.
The Kinman Dwarf Galaxy
Located some 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 consistently showed evidence that it hosted the unstable object with its occasional dramatic shifts in their spectra and brightness. Even with those shifts, luminous blue variables leave specific traces scientists can identify, but they were absent from the data the team collected in 2019, leaving them to wonder what had happened to the star.
“It would be highly unusual for such a massive star to disappear without producing a bright supernova explosion,” says Allan.
The group first turned the ESPRESSO instrument toward the star in August 2019, using the VLT’s four 8-meter telescopes simultaneously. But they were unable to find the signs that previously pointed to the presence of the luminous star. A few months later, the group tried the X-shooter instrument, also on ESO’s VLT, and again found no traces of the star.
Clues in Old Data
“We may have detected one of the most massive stars of the local Universe going gently into the night,” says team-member , physicist Jose Groh, also of Trinity College Dublin. “Our discovery would not have been made without using the powerful ESO 8-meter telescopes, and the prompt access to those capabilities following the recent agreement of Ireland to join ESO” in September 2018.
The team then turned to older data collected using X-shooter and the UVES instrument on ESO’s VLT, located in the Chilean Atacama Desert, and telescopes elsewhere. “The ESO Science Archive Facility enabled us to find and use data of the same object obtained in 2002 and 2009,” says Andrea Mehner, a staff astronomer at ESO 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 newest high-resolution spectrograph ESPRESSO was especially revealing, from both an astronomical and an instrumentation point of view.”
From Very Large to Extremely Large Telescope
The old data indicated that the star in the Kinman Dwarf could have been undergoing a strong outburst period that likely ended sometime after 2011. Luminous blue variable stars such as this one are prone to experiencing giant outbursts over the course of their life, causing the stars’ rate of mass loss to spike and their luminosity to increase dramatically.
Future studies are needed to confirm what fate befell this star. Planned to begin operations in 2025, ESO’s Extremely Large Telescope (ELT) will be capable of resolving stars in distant galaxies such as the Kinman Dwarf, helping to solve cosmic mysteries such as this one.
The Daily Galaxy, Jake Burba, via ESO